imc 12p guide

Allied IMC ConduitSpecification Guide

Electrical Infrustructure Solutions™

www .alliedeg.com

• Features

• How to specify IMC

• Comparing IMC & Rigid

• Test Data

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What Allied IMC IsIMC is lighter weight rigid-type steel

electrical conduit. It was originated and

introduced by Allied Tube & Conduit

Corporation and is manufactured in

accordance with Underwriters’ Laboratories

safety standard 1242 and ANSI C80.6.

The Allied Intermediate Metal Conduit (IMC)

raceway system is an optimized system

matching product to requirement – it was

designed specifically to protect insulated

electrical conductors and cables, eliminating

the need for a heavier wall product. Allied IMC

is lighter in weight and has a larger interior

diameter than Galvanized Rigid Conduit (GRC)

and is covered by NEC article 342.

Allied IMC has been subjected to more

extensive testing than any other conduit

product to validate its electrical and

mechanical performance characteristics. It

meets or exceeds the service requirements of

modern electrical systems. And since its

introduction, it has been successfully installed

in commercial and industrial applications

involving millions of feet.

Why Allied IMC Is BetterAllied IMC is lighter in weight than

GRC. It is easy to handle, and can result in

sizable economies in shipping and final

installation.

Allied IMC has a larger interiordiameter. Compared to the same GRC trade

size, its larger and smoother interior makes

wire pulling significantly easier.

Allied IMC is made to more precisetolerances than GRC. Greater uniformity in

electrical and mechanical performance

characteristics is assured. It is completely

interchangeable with Rigid conduit because its

precision tolerances fall well within the wider

tolerance range established for Rigid.

Allied IMC is more “rigid” than Rigid.In many applications, such as service masts,

IMC provides better protection for wiring

than GRC.

Where IMC Is UsedIMC is recognized by the NEC for the same

uses as Rigid conduit, including all hazardous

locations. It can be installed above or below

grade, in concrete or earth, and used in high-

voltage installation (over 600 volts). IMC may

be used under all atmospheric conditions, in all

occupancies, and under the same conditions

as Rigid.

The NEC recognizes IMC as an equipment

grounding conductor and permits use

of U.L. listed set-screw and compression

no-thread fittings.

Features of Allied IMCAllied IMC is manufactured from sheet steel,

work hardened through the forming process to

provide exceptional strength for its weight.

(The steel used in Allied IMC meets the general

requirements for carbon and high-strength low-

alloy steel ASTM-A568).

Allied IMC is protected from corrosion by a

patented hot galvanizing Flo-Coat ® process

which combines galvanizing and other special

protective coating steps with the mill process.

In addition, a specially formulated, highly

corrosion-resistant lubricating coating is

applied to the interior surfaces. (Both interior

and exterior coatings meet U.L.1242). This

coating, together with IMC’s larger interior

diameter in all trade sizes, greatly reduces wire

pulling effort.

Allied IMC is available in 10 trade sizes from

1/2 (16 mm) through 4 (103 mm).

Each length is identified at 21⁄2 ft. intervals

with the letters IMC 1/4 in. (6.35 mm) high

and carries the U.L. label. A coupling is

supplied with each 10 ft. (3.048 m) length.

Further identification is provided by

IMC color-coded end-cap thread protectors

as follows:

� Even Trade Sizes (1, 2, 3, 4): Orange

� 1/2 Trade Sizes

(1/2, 11⁄2, 21⁄2, 31⁄2): Yellow

� 1/4 Trade Sizes (3/4, 11⁄4): Green

IMC threads are full-cut for tight-fitting

connections. The standard threads

accommodate all the threaded GRC couplings,

junction boxes, outlet boxes, and other

electrical fittings. The threads are galvanized

for protection against corrosion.

IMC is easily cut and field-threaded using

standard tools. Bending can be done in

the field or shop using a variety of bending

equipment.

How To Specify IMCTo specify IMC, or any other electrical

conduit product, you can use the standard

Construction Specifications Institute (CSI)

format, Section 16. An additional reference to

cite is the Underwriters’ Laboratory General

Information card #DYBY.

Allied Intermediate Metal Conduit (IMC)

These IMC raceways are the secondary feeder and distribution lines leading from a basementswitchboard supplying power for an office building. This installation utilized 4200 feet of IMC in avariety of sizes.

Allied Electrical Group

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A Strength ComparisonThe results of the various beam strength and deflection tests, which follow later in this brochure, may create questions as to how this relative

strength of IMC is achieved.

Allied’s method of manufacture increases the average tensile strength of IMC to such an extent that its wall strength is over 50% greater per

unit of thickness than that of representative samples of GRC measured by Allied.

The wall strength characteristics are functions of both the yield strength of the material and the section modulus. The accompanying table

gives a comparison of IMC and GRC for various trade sizes.

Dimension ComparisonNominal 3/4 (21) trade size.

Weight Comparison of Trade SizesApproximate lb/kg per 100 ft. (30.48 m)

Comparison of Max. MomentsIMC vs. GRC Calculated fromSection Moduli &Yield Strengths

GRC IMC

* Weight specifications are for comparative purposes only.They are not a requirement of U.L.1242.

Average Yield StrengthsGRC 35,000 - 40,000 PSI (241 - 276 MPa) IMC 55,000 - 60,000 PSI (379 - 414 MPa):

trade size 3 (78) and underIMC 50,000 PSI (345 MPa): over trade size 31⁄2 (91)

Trade A B CSize * Nominal Outside Length of finished conduit ** Wall

Designator Diameter w/o Coupling Thickness

US Metric (in) (mm) (ft. & in.) (meters) (in) (mm)

1/2 16 0.815 20.70 9’ 111⁄4” 3.03 0.070 1.79

3/4 21 1.029 26.13 9’ 111⁄4” 3.03 0.075 1.90

1 27 1.290 32.76 9’ 11” 3.02 0.085 2.16

11⁄4 35 1.638 41.60 9’ 11” 3.02 0.085 2.16

11⁄2 41 1.883 47.82 9’ 11” 3.02 0.090 2.29

2 53 2.360 59.94 9’ 11” 3.02 0.095 2.41

21⁄2 63 2.857 72.56 9’ 101⁄2” 3.01 0.140 3.56

3 78 3.476 88.29 9’ 101⁄2” 3.01 0.140 3.56

31⁄2 91 3.971 100.86 9’ 101⁄4” 3.00 0.140 3.56

4 103 4.466 113.43 9’ 101⁄4” 3.00 0.140 3.56

Trade SizeDesignator IMC GRC

US Metric lb.in N.m lb.in N.m

1/2 16 1905 215 1424 161

3/4 21 3432 388 2763 312

1 27 6130 693 4405 498

11⁄4 35 10,423 1178 8915 1007

11⁄2 41 14,574 1647 12,070 1364

2 53 24,660 2786 19,622 2217

21⁄2 63 47,044 5315 37,241 4208

3 78 63,085 7128 60,345 6818

31⁄2 91 83,641 9450 83,785 9466

4 103 103,000 11,637 112,507 12,712

Allied IMC Dimensions

* Outside diameter tolerances:± .005 in (.13 mm) for trade sizes 1/2, 3/4, and 1 • ± .0075 in (.19 mm) for trade sizes 11⁄4 through 2 • ± .010 in. (.25 mm) for trade sizes 21⁄2 through 4.

** Wall thickness tolerances are + 0.15 in (.38 mm) and –.000 for IMC 1/2 through 2, and + 0.20 in (.51 mm) and –.000 for IMC 21⁄2 through 4. There is no specific wall thickness or tolerance for Rigid conduit.

B

CA

O.D. 1.050 in. O.D. 1.029 in.(26.67 mm) (26.1 mm)

I.D. 0.824 in. I.D. 0.863 in. (20.92 mm) (21.0 mm)

Wall 0.113 in. Wall 0.083 in.(2.87 mm) (2.11 mm)

Trade Size GRC IMCDesignator w/Coupling w/Coupling*

US Metric Lbs. Kg Lbs. Kg

1/2 16 80 36.29 60 27.22

3/4 21 109 49.44 82 37.29

1 27 165 74.84 116 52.62

11⁄4 35 215 97.52 150 68.04

11⁄2 41 258 117.03 182 82.55

2 53 352 159.67 242 109.77

21⁄2 63 567 257.19 428 194.14

3 78 714 323.87 526 238.59

31⁄2 91 860 390.10 612 277.60

4 103 1000 453.60 682 309.53

How Allied IMC Compares with Rigid

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ARC TestTest Objective: To determine the ability of

IMC and GRC to contain the heat generated by

high currents in conductors, or by arcing

between conductors, or between conductors

and conduit.

Test Description: Five-foot lengths of IMC

and GRC clamped to a grounded steel column

were wired with #4 3-conductor service-

entrance cable. About one foot of the cable

extended out the bottom of the conduit; the

other end was connected to a transformer with

a secondary rated at approximately 240V,

open circuit, which could deliver 800 or 200

amperes. The conductors were connected at

the bottom, allowing 800 amperes to flow

through the cable. The heating effect

destroyed the insulation, open-circuited the

conductors, and allowed an arc to form

between the conductors and the metal

sheathing, or between the conductors and

the grounded conduit. When the arc was

formed, transformer current was reduced to

200 amperes and the test continued until

arcing ceased. The conduit was then inspected

for damage.

Test Result Summary: It was observed

that significantly greater heating took place

at points along the conduit interior where

the cable came nearest the wall. The test

results indicated that IMC will contain heat

from over-current or arcing for a period of time

in excess of that required for the operation of

protective devices.

Electrical Conduit As A GroundReturn Conductor

Metal conduit of any type should have a

current carrying capacity for short-circuit faults

far in excess of the conductors supplying the

fault. When IMC was first introduced, a series

of tests determined that IMC was equal to

Rigid conduit for providing a path to carry fault

currents safely to ground. In 1994, the Georgia

Institute of Technology, under the sponsorship

of the Steel Conduit Section of NEMA,

conducted research on the grounding

capabilities of Rigid Metal Conduit, IMC and

EMT. This was the first update in many years to

information included in the Soares Book on

Grounding. A computer model was developed

based on the research and addresses

fundamental issues associated with the use of

steel conduit in secondary power distribution

systems, including the length of run that can

be used with specific steel conduit types and

sizes. The maximum run length for IMC is

actually slightly longer than the maximum

allowable run length for Rigid. Complete

information on the research study and the

GEMI (Grounding and ElectroMagnetic

Interference) analysis software is available from

Allied Tube or can be downloaded from the

Steel Tube Institute's Conduit Section website:

www.steelconduit.org

Electrical Resistance TestTest Objective: To determine resistance

across junctions in IMC and GRC in all

trade sizes.

Test Description: Two 18 in. (456.85 mm)

lengths of conduit threaded with Standard NPT

thread were cleaned, degreased and joined

with a cleaned standard coupling with straight

pipe threads, wrench tightened. Conduit

resistance, conduit-to-coupling resistance,

and joint resistance were measured, using a

Biddle “Ducter” Model 751 ohmeter.

Test Result Summary: Resistance values

for IMC junctions are not significantly different

from those across GRC junctions. The table

below shows resistance in micro-ohms as

measured between those points.

Impedance TestTest Objective: To determine actual

impedance value of test assembly at

low amperage/low voltage and high

amperage/high voltage levels stimulating

the use of conduit as an equipment

grounding conductor.

Test Description: Lengths of GRC and

IMC with elbows and threaded couplings

were assembled as illustrated, filled with a

conductor using a minimum #8 wire

(minimum size grounding conductor in NEC

Table 250-122) up to a maximum of 500

MCM, graduating in size for the conduit

diameter according to common usage.

The wire was fastened to a lug welded onto

the conduit at the far end of the assembly.

Measurements were made by feeding a low

current followed by a high current into the wire

and using the conduit as the return path.

Electrical Test Data

Trade Between PointsSize

Designator Conduit-to-Coupling Coupling-to-Conduit Across Coupling Across 12 in. Conduit Length

US Metric IMC GRC IMC GRC IMC GRC IMC GRC

1/2 16 21 12 18 19 50 53 280 300

3/4 21 15 19 17 14 42 40 240 180

1 27 17 15 20 14 41 44 200 150

11⁄4 35 11 19 20 11 41 45 182 105

11⁄2 41 15 20 21 15 41 41 149 105

2 53 20 18 18 18 35 32 122 90

21⁄2 63 15 15 21 19 32 31 104 89

3 78 13 15 13 12 39 31 98 66

31⁄2 91 18 15 18 19 25 30 82 62

4 103 11 17 15 18 28 30 80 60

Resistance in Micro-Ohms


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This approximated fault conditions in the field

where the conduit serves as the equipment

grounding conductor.

Low current impedances were measured at

30 amperes RMS, 60 Hz, feeding 30 amperes

into the test assembly and measuring the

voltage drop across the circuit. High currents

were approximately 48,000 amperes peak

at 250 volts or 36,000 amperes peak at

600 volts.

Test Result Summary: GRC showed

the greatest reduction in impedance at high

current because of its greater mass of

magnetic metal. IMC showed a smaller but

substantial drop. Impedance drops became

correspondingly greater as the size of the

conduit was increased. At low currents

GRC had the highest impedance; at high

currents, GRC and IMC impedance were

much closer together.

Heating TestsTest Objective: To measure the effects of

heating when excessive currents are fed

through the conduit system.

Test Description: Currents, about ten

times the wire rating, were fed directly through

the conduit assemblies, eliminating any wire,

since it was not possible to get significant

heating of the conduit before the wire was

damaged. In many cases, this copper wire

would have melted by the end of the test.

On the small sizes, full assemblies were

used. On trade sizes 2 (53) and larger, shorter

assemblies were used because the sheer mass

of metal to be heated was too great, requiring

high currents for too long a period in order to

get significant heating.

Measurements were recorded on an

oscillograph from thermocouples fastened to

the conduit.

Temperatures were checked in the middle

of the conduit, on the conduit near a coupling,

and on the coupling itself, to determine the

hottest spot.

Test Result Summary: Some of the

results of the tests were unexpected. Highest

temperatures in threaded conduits of all types

consistently occurred in the center of the

conduit, not at a threaded joint. This indicates

that the conduit joint is actually less a

generator of heat than the conduit itself,

although the greater mass to be heated at the

coupling is a contributing factor. These were

clean, tight joints. The effect of oil, dirt,

corrosion and poor assembly was not tested.

Short Circuit Tests On JointsTest Objective: To determine the effects of

fault current on IMC joints.

Test Description: The short circuit test on

joints was the same short circuit test described

previously to measure impedances at short

circuit currents. The fault current went into the

wire, returned along the conduit, and was

measured by oscillograph.

Since all the joints in any conduit run are in

series, all joints were subjected to the full peak

short circuit currents. During each test, the

conduit was observed for arcing or “splashing”

(a display of sparks from molten metal at a bad

joint). In addition, each oscillogram was

examined for evidence of arcing.

Test result summary: No arcing was

observed visually or detected in the recording

of any of the tests. These tests demonstrated

that clean, oil-and corrosion-free, tight joints did

not arc or splash as the current levels and

durations used.

5

Mechanical Test Data

Allied has conducted a variety of tests on physical performance characteristics of Allied

IMC compared to other approved metallic conduit. Evaluation of these studies establishes

that Allied IMC not only performs as well as Rigid steel conduit in most cases, but surpasses

Rigid in many cases. Significant representative results are included on the following pages.

More detailed comparative test data is presented in Allied’s Technical Report No. TR-691,

available upon request.

Wire Pull TestTest Objective: To compare the ease of pulling conductors into and out of IMC and GRC.

Test Description: Three samples each of10-foot lengths (3.04 m) of 3/4 (21) trade

size Rigid conduit and IMC were identically bent into approximately 21⁄2 in. (63.5 mm)

square configurations and tested for wire-pulling ease. Five #8 THWN stranded wire

insulated conductors bundled together (providing approximately a 40% “fill” of the cross

section – the maximum permitted by the NEC) were first drawn into the test samples and

then out, under identical conditions. Measurements of the maximum pulling forces required

in each phase of the test are tabulated on the next page.

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Impact StrengthTest Objective: To determine and

compare the degree of protection for

conductors offered by IMC and GRC against

certain possibly damaging impact forces.

Test Description: A 2 in. (50mm)

diameter cylindrical 25 lb. (11.3 kg) weight

was dropped in a guided free-fall from a15 ft.

(4.57 m) height onto samples of IMC and Rigid

conduit in all trade sizes (a total impact load of

375 ft.- lbs. (508 N.m). Six samples of each

type and size of conduit were tested. Each

conduit sample had been filled with the

number and size of insulated conductors

to the 40% “fill” permitted by the NEC. Two

conductors were connected to a lamp-load

and the circuit protected by a15A fuse. The

degree of conduit deformation was measured

after each test, and the conduit also was

sectioned to allow for visual inspection of

the conductors for insulation damage.

The percentage of the original internal area

remaining after the impact tests is shown in

Table A, based on an average of the six

samples tested in each trade size.

The primary consideration regarding impact

resistance is the total internal area remaining

after impact, and its effect upon conductor

protection. Table B uses Table A data

converted to actual available internal area.

In most of the test sizes, IMC deformation

left more available internal area than GRC. The

exceptions were insignificant in the differences

between GRC and IMC. All dimensional data

used for calculation purposes is within Allied’s

normal manufacturing specification tolerances.

Test Result Summary: The after-impact

internal area exceeded the 40% allowable

fill in all tests, for both IMC and Rigid

conduit. Deformation did not cause

rupture of the insulation or severing of the

conductors in either IMC or Rigid conduit,

and in all cases the conductors

were easily withdrawn from the tested

conduits. It can be concluded that Allied’s

“Cold-working” IMC forming process produces

greater hardness and stiffness. IMC protects

insulated conductors well in excess of NEC

requirements, and is virtually equal to

Rigid in impact strength.

Beam Strength TestTest Objective: To determine the relative

capabilities of IMC and GRC to resist bending

or flexure, and to show how IMC meets beam

strength requirements specified in U.L.1242.

Test Description: Tests were made on

IMC and GRC test samples in all trade sizes.

A 24-inch (610 mm) length supported on two

1/2- inch (12.7 mm) thick stanchions spaced

22 inches (559 mm) apart was subjected to a

force applied through a 2 inch - wide (50 mm)

load block at the conduit’s center. The force

was applied to produce deflection at the rate

6


Test Result Summary: The 3/4 (21) trade size IMC cross-section and interior finish afford

about 50% less pulling force to pull-in conductors than through GRC. Similar results may be

expected in other trade sizes of IMC.

Pulling-Force MeasurementsTestSample Force to Pull – In Force to Pull – OutNumber IMC GRC IMC GRC

Lbs. N Lbs. N Lbs. N Lbs. N

1 130 578 165 734 84 374 178 792

2 85 378 120 534 100 445 184 818

3 86 383 180 801 88 391 185 823

Average 100 445 155 689 91 405 182 810

Cross-section of 1 in. (27 mm) IMC after impact test. Cross-section of 1 in. (27 mm) GRC after impact test.

Table ATrade Size U.S. 1/2 3/4 1 11⁄4 11⁄2 2 21⁄2 3 31⁄2 4Designator Metric 16 21 27 35 41 53 63 78 91 103

IMC 63 69 96 86 90 91 94 96 95 97

GRC 84 65 96 93 93 95 97 97 98 80

PercentOriginalInternalArea

Wire Pull Tests

Table BTrade Size U.S. 1/2 3/4 1 11⁄4 11⁄2Designator Metric 16 21 27 35 41

IMC .226 .419 .945 1.456 2.050

GRC .256 .347 .830 1.391 1.893

∆ sq. in. .030 .072 .115 .065 .157

∆ sq. mm 19 46 74 42 101

Trade Size U.S. 2 21⁄2 3 31⁄2 4Designator Metric 53 63 78 91 103

IMC 3.336 4.980 7.798 10.275 13.480

GRC 3.188 4.644 7.171 9.689 10.184

∆ sq. in. .178 .336 .627 .586 3.30

∆ sq. mm 115 217 404 378 2129

InternalArea (sq. in.)remainingafter impact

InternalArea (sq. in.)remainingafter impact


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of 0.1 inches per minute (2.54 mm/min), and

measurements were made of the deflection in

inches and the load in pounds. The test was

stopped when the deflection equalled 2.250

inches (57.15 mm) – the limitation of the

instrumentation. A chart or trace of the output

from the instrumentation produced a typical

load-deflection curve as shown. Also shown is

a table which includes average force to yield in

pounds for each size of IMC and GRC, plus the

U.L.1242 requirement in pounds for IMC.

Test Result Summary: IMC far surpasses

the requirements of U.L.1242. The data

shows that IMC is stiffer than GRC. It can be

concluded that IMC will provide more

consistent resistance to mechanical abuse

because its dimensional tolerances are more

closely held than are those required for GRC.

Threaded Coupling Pull-Out TestTest Objective: To compare the relative

strength of thread configuration of IMC and

GRC; also the compliance of IMC with

U.L.1242.

Test Description: Standard stock, straight-

thread tapped couplings were threaded onto

each end of12- inch lengths (305 mm) of IMC

and GRC in all trade sizes, with adapters

screwed in, and pipe wrench tightened. Each

assembly was then subjected to an axial pull

force until failure occurred. An increasing load

was applied until the threads failed or the

conduit fractured. The average load pounds

required to fracture both types of conduit,

together with U.L.1242 specification for each

size IMC, is tabulated here.

Test Result Summary: IMC, when

threaded with 3/4 in. per foot (21mm)

tapered thread-form and coupled, averaged

from 80% to 98% of the strength of GRC, and

substantially exceeds U.L.1242 requirements

for IMC. Close dimensional control of the

wall thickness and outside diameter of IMC

assures consistent performance in excess of

minimums. GRC is permitted much larger

dimensional tolerance, making possible many

variations of wall, I.D. and O.D. dimensions.

Consequently, consistent performance of

GRC is less certain. It should be noted that

forces of a magnitude to cause joint failure in

either IMC or GRC would cause the electrical

system to fail for other reasons before the

conduit would fail.

Deflection

Lo

ad in

Po

un

ds

1000

875

750

625

500

375

250

125Force toPermanent Deflection

Force toYield (IMC)

Force toYield (Rigid)

Trade Size 1⁄2 IMC

Trade Size 1⁄2 Rigid Conduit

Conduit Beam Strength

Trade Size Average OD Average OD Avg Force To Yield Avg Force to Yield U.L. RequirementDesignator IMC GRC IMC GRC for IMC

U.S. Metric in in lb N lb N lb N

1/2 16 0.815 0.828 552 2455 446 1984 375 1700

3/4 21 1.030 1.040 974 4333 868 3861 950 4200

1 27 1.289 1.323 1750 7784 1686 7500 1400 6300

11⁄4 35 1.640 1.668 2627 11,685 3060 13,612 2000 9000

11⁄2 41 1.882 1.907 3893 17,317 3758 16,716 3800 13,400

2 53 2.363 2.391 5200 23,131 6083 27,059 4100 18,300

21⁄2 63 2.851 2.876 12,158 54,081 11,192 49,785 7000 32,000

3 78 3.476 3.502 13,958 62,088 19,850 88,297 7000 32,000

31⁄2 91 3.969 4.013 16,183 71,986 22,850 101,642 10,000 45,000

4 103 4.460 4.519 17,017 75,695 23,075 102,643 10,000 45,000

Threaded Coupling Fracture ForcesAverage Load in lb to

Trade Fracture 3/4 in. TaperSize

Designator IMC GRC

U.S. • Metric lb N lb N1/2 • 16 10,658 47,409 10,920 48,575U.L. Requirement* 5,000 22,200

3/4 • 21 12,840 57,115 13,978 62,177U.L. Requirement* 8,000 35,600

1 • 27 19,579 87,092 18,120 80,602U.L. Requirement* 13,000 57,800

11⁄4 • 35 23,604 104,996 24,112 107,256U.L. Requirement* 16,000 71,200

11⁄2 • 41 27,850 123,883 30,892 137,414U.L. Requirement* 24,000 106,800

2 • 53 34,300 152,574 40,708 181,078U.L. Requirement* 28,000 124,600

21⁄2 • 63 59,233 263,482 61,291 272,636U.L. Requirement* 30,000 133,400

3 • 78 57,150 254,216 66,208 294,508U.L. Requirement* 35,000 155,700

31⁄2 • 91 69,666 309,890 81,367 361,938U.L. Requirement* 10,000 178,000

4 • 103 74,233 330,205 93,250 414,797U.L. Requirement* 40,000 178,000

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8

Threaded Coupling Deflection Test

Test Objective: To determine the relative

capabilities of coupled lengths of IMC and GRC

to resist bending forces.

Test Description: Two12- inch lengths

(305 mm) of conduit were assembled into

a coupling and supported on two 1/2 in.

(13 mm) thick stanchions spaced 22 inches

(559 mm) apart. With the coupling centered

between the supports, force was applied to the

coupling through a 2 in.- wide (51 mm) load

block to produce a deflection at the rate of

0.1 inches (2.54 mm) per minute. Tests were

conducted on IMC and GRC in all trade sizes.

The average forces at the limit of deflection

are shown.

Test Result Summary: Threaded

couplings on both IMC and GRC uniformly

resisted the lateral forces applied,

without rupturing.

Cantilevered Beam Strength Test

Test Objective: To determine the relative

capabilities of IMC and GRC for use as service

masts supporting horizontal service

conductors and service drops.

Test Description: Different lengths (46 in.,

64 in. and 70 in. –1245 mm, 1626 mm and

1778 mm) of each of three trade sizes (2,

21⁄2 and 3 – 53, 63 and 78) of IMC and GRC

were mounted and restrained in the Tinius

Olsen machine. Force was applied at the

rate of 1 inch (25.4 mm) per minute and

measurements were made of the total

deflection and permanent deflection at each of

several loads. Three tests were made on each

size sample. Results were averaged and are

tabulated below.

Test result summary: The force required

to permanently deflect IMC was greater than

for GRC in almost every case. IMC is generally

equal to or superior to GRC performance for

service mast use.


Coupling Deflection Yield Forces

GRCIMCTrade Size

20,000

15,000

10,000

5,000

1,000 1⁄2 3/4 1 11⁄4 11⁄2 2 21⁄2 3 31⁄2 4

Service Mast Cantilever Beam Test

Trade Size Force to Obtain Permanent Deflection Force to Obtain 3”

CantileverDesignator At 1/4” At 1/2” At 1” Total Deflection

Length (L) US Metric IMC IMC GRC GRC IMC IMC GRC GRC IMC IMC GRC GRC IMC IMC GRC GRC

lb N lb N lb N lb N lb N lb N lb N lb N

2 53 461 2051 398 1770 516 2295 426 1895 550 2447 443 1971 561 2495 455 2024

21⁄2 63 1029 4577 905 4026 1163 5173 944 4199 1267 5636 991 4408 1235 5494 1034 4599

3 78 1471 6543 1479 6579 1626 7233 1550 6895 1719 7646 1625 7228 1758 7820 1679 7469

2 53 318 1415 310 1379 342 1521 3326 14,795 363 1615 342 1521 338 1503 341 1517

21⁄2 63 777 3456 630 2802 857 3812 649 2887 921 4097 669 2976 803 3572 672 2989

3 78 967 4301 946 4208 1073 4773 1013 4506 1057 4702 1058 4706 1103 4906 1065 4737

2 53 270 1201 262 1165 300 1334 283 1259 323 1437 294 1308 277 1232 288 1281

21⁄2 63 736 3274 579 2576 7877 35,039 601 2673 816 3630 623 2771 617 2745 613 2727

3 78 904 4021 923 4106 1000 4448 975 4337 1086 4831 1009 4488 930 4137 994 4422

46”

(1425 mm)

64”

(1626 mm)

70”

(1778 mm)


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9

Section 345-4 of the1975 National

Electrical Code (NEC), which restricted IMC

from use in hazardous locations, was removed

with the adoption of the1978 NEC. The1978

NEC recognized IMC for use in all hazardous

locations including Class1, Division1.

Allied commissioned Underwriters’

Laboratories, Inc., (U.L.) to conduct a fact-

finding investigation of IMC for use in

hazardous locations. Such an extensive

investigation had not previously been made

of any other type of conduit, including Rigid.

U.L. File E58688, dated July 22,1976,

contains a report of this investigation, and

is available from Allied.

The data set forth in the U.L report was

submitted to National Electrical Code Panels

8 and14. This was in support of Allied’s

proposals to (1) eliminate the hazardous

locations restriction and (2) include IMC in

those Articles and Sections of the Code

pertaining to use in hazardous locations. Those

proposals were accepted by the Code panels,

and included in the1978 NEC.

Summary of U.L. File E58688,Fact-Finding Report onIntermediate Metal ConduitFor Use In HazardousLocations

This summary of important areas of the Fact-

Finding Report was used as support of the

proposal to the Code panels to recognize IMC

for use in hazardous locations. This recognition

was secured with passage of the1978 NEC.

In all areas, parallel tests were conducted on

GRC and IMC. At the time these U.L. tests were

performed, only GRC was recognized by the

NEC as acceptable for use in all classes, groups

and divisions of hazardous locations.

The investigation covered four areas:

1.) Explosion Pressure Tests

2.) Flame Propagation Tests

3.) Hydrostatic Strength Tests

4.) Tensile Strength Tests

The test configurations were designed to

simulate actual installations which may be

found in the field.

Explosion Pressure TestsThe explosion tests were run in order to

determine as closely as possible the maximum

explosion pressures which would result from

different mixtures of explosive gasses and

piping/devices or test configurations. Four

hundred and ninety nine tests were run and at

no time was the pressure great enough to

rupture the conduit.

Precise calculation of maximum explosion

pressures is not possible even when the

characteristics of the flammable gas, air-gas

ratio, contained volume, volume configuration

and ignition energy are known. As a result,

it is necessary to conduct a large number of

explosion tests in which the above factors are

varied.

The maximum explosion pressures

obtained are then used as a comparison basis

for hydrostatic test results. The combination of

factors which produces the greatest explosion

pressure is also used for the flame propagation

test. The maximum pressure for any series

of tests is selected for this comparison,

since no significance can be attached to an

average value.

Typical Explosion Test configuration. All threaded conduit and pipe connections in each test configuration were wrench-tightened. The test configuration was filledwith explosive gas-air or vapor- air mixture until the original air was displaced. Samples ofthe explosive mixtures were withdrawn foranalysis from the pipe carrying the explosivemixture, and the exhaust line. Inlet and outletvalves were then closed and the mixtureignited by a spark plug. Pressure developedduring the test was recorded. At theconclusion of each test, the products ofcombustion were removed from the system.

Hazardous Location Test Data

MixtureSupply

Line

PneumaticallyOperated

Valve

PneumaticallyOperated

Valve

MixtureExhaust

Line

PneumaticControl

Lines

Transducer

Spark PlugTransformer

TestConfiguration Spark

Plug

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10

Flame Propagation TestsThe propagation tests were conducted in

order to determine the possibility of flame

propagation through a joint consisting of two

sections of taper-threaded conduit engaged

into a standard straight thread electrical

conduit coupling. In each trade size the same

straight thread standard electrical coupling

was used in both the GRC test and the IMC

test. (No tests were conducted using integral,

set screw or compression couplings, since

these types are not recognized as acceptable

for use in all classes, groups and divisions of

hazardous locations.)

In the test the coupled joint was enclosed

in a transparent container filled with an

explosive mixture. An explosive mixture was

detonated within the coupled conduit and

instrumentation determined whether the flame

within the conduit was propagated to the

mixture surrounding the joint. Visual inspection

also indicated propagation, if it occurred, since

the plastic panels of the enclosure would blow.

Two hundred tests were run. Listed below is

a summary of these tests as shown on page

T1-9 of the U.L. Fact-Finding Report. This

summary shows that in the eight test series

run, IMC and GRC were equal in four of the

tests, IMC outperformed GRC in two of the

tests, and GRC outperformed IMC in two

of the tests.

A properly made-up joint in an electrical

conduit system intended for a hazardous

location is one where all threads are engaged

so that the external chamfer on the end of the

conduit mates with the internal chamfer of the

coupling. The results of these tests show that

flame will not propagate to the exterior

of a conduit system with joints made in

this manner.

The last category, “Flame Did Not

Propagate,” of the following tabulated results

shows the minimum number of turns of

thread engagements needed to prevent flame

propagation in the same sample. The number

of threads engaged in a proper engagement

always exceeds these values.

The detailed results of the propagation tests

will be found in Appendix D of the U.L. report.

Flame Propagation Test Configuration. Test method and apparatus used were thesame as in the explosion tests except for the addition of a sealed observation chamberaround each threaded joint being tested.Threads on both the IMC and GRC samplesused were checked to assure that NPTthreads were provided. Each series covered a range of thread engagement to determinethe number of threads which did not result inflame propagation through the threaded joint.

Flame Did Not Propagate

Conduit 1 (27) Trade Size

Total FlameTest Explosive Number PropagatedSeries Mixture of of MaximumNo. of No. ofNumber

Type Sample No.Gas in Air Tests No. of Turns

Turns Tests

1 IMC 3 Hydrogen 26 8 9 16

2 GRC 7 Hydrogen 36 8 9 16



5 IMC 3 Acetylene 26 5 6 11

6 GRC 3 Acetylene 29 6 7 10



Allied Intermediate Metal Conduit (IMC)

Coupling

ConduitUnder Test

TestChamber


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Hydrostatic Strength TestsHydrostatic strength tests were performed

on samples of GRC and IMC produced by

different manufacturers.

The test results are shown on Page T1-11 of

the U.L. report.

In a comparison of the data on Page T1-11

with that on Page T1-7, Maximum Pressure,

PSI shows that the hydrostatic strength of

both types of pipe in all cases exceeds the

maximum pressure obtained in explosion by

at least the following factors:

Hydrostatic Strength Test results. Samplesof trade size 1 (27) and trade size 4 (103) IMC and GRC with steel plates welded oneach end were connected to the hydrostatictest apparatus. Pressure was increasedgradually until ultimate results were obtained.Test time duration was recorded for each test.

Tensile Strength TestsTensile tests were performed on trade size

1/2 (16 mm ) GRC and IMC. A record of these

results will be found on Page T1-13 of the

U.L. report.

A review of these results shows that the

calculated tensile strength of IMC in all

instances exceeded that of GRC.

This result is caused by the inherent

differences in both the basic material and

method of manufacture used to produce IMC

as compared to the basic material and method

of manufacture used to produce GRC.

11

The data presented in this Specification Guide is only partial and the complete reports are available upon request.Statements made herein are intended to indicate the performance of IMC when compared to types of conduit underclosely controlled conditions. This brochure is for informational purposes only, and Allied Tube & Conduit makes noexpress or implied warranties or representations other than those with regard to the product in effect at the time ofsale as set forth in Allied’s then current printed terms and conditions.

Trade Multiples ofSize Type Explosion Pressure

U.S. Metric

1 27 GRC 3.3 Times

1 27 IMC 6.3 Times

4 103 GRC 55.5 Times

4 103 IMC 24.9 Times

A

D E

B C

GRC – Trade Size 4 (103)

IMC – Trade Size 4 (103)

Manufacturer/ 1/2 (16) Load at Yield Tensile StrengthSample No. Conduit Type Lb N PSI Mpa

A/1 IMC * 11,660 51,866 66,400 458

A/2 IMC 11,600 51,599 63,700 439

A/3 IMC 11,600 51,599 70,700 487

A/4 IMC 11,650 51,822 67,200 463

A/5 IMC 11,660 51,866 68,600 473

AVG† 11,610 51,644 67,300 464B/6 IMC 11,360 50,532 68,100 470

B/7 IMC 10,960 48,753 69,000 476

B/8 IMC 11,160 49,642 68,500 472

B/9 IMC 11,490 51,110 69,600 480

B/10 IMC 10,520 46,795 62,200 429

AVG† 11,100 49,375 67,500 465C/11 IMC 11,720 52,133 68,400 472

C/12 IMC 12,720 56,581 73,400 506

C/13 IMC 11,750 52,267 69,500 479

C/14 IMC 11,600 51,599 67,900 468

C/15 IMC 11,570 51,466 67,400 465

AVG† 11,870 52,800 69,300 478D/16 GRC ** 13,660 60,763 56,600 390

D/17 GRC 12,620 56,137 51,700 356

D/18 GRC 14,900 66,279 61,700 425

D/19 GRC 13,520 60,140 58,000 400

D/20 GRC 13,800 61,385 58,700 405

AVG† 13,700 60,941 57,300 395E/21 GRC 14,260 63,432 60,000 414

E/22 GRC 14,240 63,343 60,400 416

E/23 GRC 14,240 63,343 61,400 423

E/24 GRC 14,220 63,254 61,200 422

E/25 GRC 14,220 63,254 60,800 419

AVG† 14,240 63,343 60,800 419F/26 GRC 13,760 61,208 61,700 425

F/27 GRC 13,695 60,918 62,700 432

F/28 GRC 13,695 60,918 61,400 423

F/29 GRC 13,750 61,163 61,400 423

F/30 GRC 13,730 61,074 61,200 422

AVG† 13,726 61,056 61,700 425

* Intermediate Metal Conduit ** Galvanized Rigid Conduit † Average of five samples

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© Allied Tube & Conduit Printed in U.S.A. ATC-L-1702-0608

• Allied Tube & Conduit • AFC Cable Systems® • Cope® Cable Tray • Power-Strut® Metal & Fiberglass Framing

Allied Tube & Conduit, AFC Cable Systems, Cope, Power-Strut, and Tyco are trademarks or registered trademarks of Tyco and/or its affiliates inthe United States and in other countries. All other brand names, product names, or trademarks belong to their respective owners.

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