Fire Alarms

Voltage Drop Calculations

Presented September 18, 2007

By Larry Tate

© 2007

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Fire Alarms - Voltage Drop CalculationsTable of Contents

Page - Subject

3 The Code Requirement & Qualifier

4 The Purpose of Voltage Drop Calculations

5 Calculation Considerations

6 Fire Alarm & NAC Panels

8 The End-of-Line (EOL) Voltage Drop Calculation

9 NAC Voltage Drop Calculation Example

10 The EOL Voltage Drop Calculation Formula

11 The Point-to-Point Voltage Drop Calculation Formula

14 Regulated of FWR?

15 Info Needed with Any Voltage Drop Calculation Submittal

16 Estimating Wire Lengths for Voltage Drop Calculations

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Fire Alarms - Voltage Drop Calculations

• The Code Requirement:

NFPA72, 2002 Edition, excerpt from 4.5.1.1, “…At the authority having jurisdiction’s request, complete information regarding the system or system alterations, including specifications, shop drawings, battery calculations, and notification appliance circuit voltage drop calculations shall be submitted for approval.”

Qualifier: Most listed fire alarm systems sold in the USA are 24 volt systems. A few (mostly combination burg/fire panels) operate on 12 volts. For the purpose of this course, all discussion and examples presented assume that the control panels are 24 volt systems. In any event, the same theories and math apply to 12 volt systems too, though the panel cut-off voltage will be half that of 24 volt systems.

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The Purpose of Voltage Drop Calculations• To confirm system will function in a worst case situation.

At end of secondary power standby period.

With all notification appliance circuit (NAC) zones in alarm.

• Submitting voltage drop calcs meets NFPA 72 minimum installation requirements for AHJ submittals.

– It’s important that those calcs are based on reality to ensure the system will work.

• Reasons why accuracy is important:

– Confirms notification circuits will work at all times (if installed right).

Confirms quantity of power supplies needed

Supports equipment wall space planning

Supports 120 VAC primary supply circuit coordination planning

Supports development of future circuit design changes (like tenant improvements) where practical

• Additional voltage drop calculations for auxiliary power supply output circuits (like power for beam or flame detectors) makes good sense to ensure the design will work as planned.

– Not required, but should be someday. That’s another story for another day.

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Calculation Considerations• There are several ways (methods) to perform voltage drop calculations:

– End-of-line (EOL), point-to-point (PTP), load-centering.

• Load centering isn’t often used, though conservative.

• EOL method is most simple, most conservative, and thus least accurate.

– Fewer math steps makes it easy to do by hand with a calculator.

– Results can provide lots of “head room” for future.

• PTP method is also always somewhat conservative; has much more math involved; and, provides more accurate results.

– Generally used with a spreadsheet program because of the many math steps.

– Is often used in calc programs provided by panel manufacturers.

– Is less conservative than EOL method, allowing more devices on a circuit.

– The difference in results from the EOL method can be as much as 30% or more.

• Either method is valid and “safe” to use.

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Fire Alarm & NAC Panels

• Per the new U.L.-864, 9TH Edition Standards for fire alarm control panels:

– All panels must have a demonstrated 20.4 VDC “panel cut-off”

• That is 85% of the 24 volt battery capacity, i.e. the end of “standby”.

• The panel must stop working then to not destroy the batteries.

• All fire alarm control panels (and power supplies, which are included) have an internal voltage drop.

– The voltage at the NAC output terminals is always less than 20.4 volts at cut-off.

– The amount of that drop varies with every panel (from about .5 volts to 2.5 volts).

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Fire Alarm and NAC Panels (continued)

Therefore:

It is the terminal voltage at cut-off that must be used for a meaningful voltage drop calculation.

• Note: You won’t typically find this voltage figure on engineering data sheets.

– Usually, one has to get it from the panel manufacturers’ engineering department.

– Some manufacturer’s calc programs use that terminal voltage (if current).

• This aspect needs to be confirmed with the manufacturer and can’t be assumed.

– Get it in writing…or at least by e-mail from the manufacturer’s rep.

• Important: System designers, using their own spreadsheet programs, should supply the values they used (with factory confirmation of validity) to the design reviewer.

– It is easy to set up the spreadsheet program to list all values used.

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The End-of-Line (EOL) Voltage Drop Calculation

• To create a basic EOL voltage drop calculation:

– Add up current draw for all devices on the circuit using U.L. Max. figures.

• Those figures are typically on all CURRENT data sheets for notification devices listed for use in public mode systems (see handout).

– Add up total wire length for entire circuit, i.e. run lengths times two (if class B)

– Multiply total wire length times the wire resistance value per foot for total circuit wire resistance.

– Multiply the total circuit wire resistance times the total current draw for all devices to get the voltage drop.

– Subtract the voltage drop from the panel cut-off terminal voltage to get the voltage delivered to the last device on the circuit, which must exceed 16 volts.

• This method assumes voltage drop at each device will be the same – it’s not really.

– Can have a 20% to 40% margin of error (in extra “headroom”).

• Note comparison in sample calculation results for margin of error.

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NAC Voltage Drop Calculation Example

• Notes for Example:

– Run lengths are shown above.

– Wire length = two times the run length.

– Current draw shown is RMS @16 VDC (i.e. U.L. Max.)

– In this example, panel cut-off terminal voltage assumed to be 19.1 VDC

BPS1 DANAC

POWERBOOSTER

#12 conductors 1..98Ω per 1000 feet200’ run 50’ run 150’ run 100’ run

NAC 1-1(75 mA)

NAC 1-2(125 mA)

NAC 1-3(200 mA)

NAC 1-4(100 mA)

EOL

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The EOL Voltage Drop Calculation Formula• The EOL Voltage Drop Calculation Formula:

1. STEP 1: D (wire run distance) X 2 (two conductors) X R/ft. (wire resistance per ft.) = Rt (total circuit resistance)

Note: Wire resistance figures used should either be from Chapter 9, Table 8 in the NEC, or, from the wire manufacturer IF the wire part number appears on the plans. Reviewer should have this info confirmed.

2. STEP 2: Rt X I (total circuit load using U.L. Max.) = Total circuit voltage drop at EOL.

3. STEP 3: Subtract voltage drop from terminal voltage to confirm voltage at last device, which must exceed 16 volts.

That’s all there is to it!

• A sample calc using the example on previous page:

– 500’ x 2 x .00198 ohms (NEC listed value for 12 AWG stranded) = 1.98 ohms

– 1.98 ohms x .5 amps = .99 volts total voltage drop

– 19.1 VDC (cut-off terminal voltage) – .99 VDC = 18.11 VDC @ last device

I know…12 AWG isn’t a good choice for use with notification devices!

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The Point-to-Point Voltage Drop Calculation Formula

• The point-to-point calculation:• First calculation:• R1 (segment resistance) X added U.L. Max current for NAC1-1, NAC1-2, NAC1-3 & NAC1-4

Subtract total from the cut-off terminal voltage to get voltage at NAC1-1.• Second calculation:

– R2 (segment resistance) X added current for NAC1-2, NAC1-3 & NAC1-4– Subtract total from the NAC1-1 voltage to get voltage at NAC1-2.

• Third calculation:– R3 (segment resistance) X added current for NAC1-3 & NAC1-4– Subtract total from NAC1-2 voltage to get voltage at NAC1-3

• Fourth calculation:– R4 (segment resistance) X current for NAC1-4– Subtract total from NAC1-3 voltage to get voltage at NAC1-4 EOLR.If this last value is greater than 16 volts then the circuit should work.

BPS1 DANAC

POWERBOOSTER

#12 conductors 1..98Ω per 1000 feet200’ run R1 50’ run R2 150’ run R3 100’ run R4

NAC 1-1(75 mA)

NAC 1-2(125 mA)

NAC 1-3(200 mA)

NAC 1-4(100 mA)

EOL

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The Point-to-Point Voltage Drop Calculation (continued)• A sample calc using the example on the previous page:

NAC1-1 : 400’ (total) x .00198 ohms = .792 ohms (from BPS10 to NAC1-1)

.792 ohms x .5 amps (total of all NAC devices) = .396 volts drop @ NAC1-1,

19.1 - .396 = 18.704 VDC @ NAC1-1

NAC1- 2: 100’ (total) x .00198 ohms = .0198 ohms (from NAC1-1 to NAC1-2)

.0198 ohms x .425 amps (NAC1-2, -3 & -4) = .008415 volts drop @ NAC1-2

18.704 - .008415 = 18.691585 VDC @ NAC1-2

NAC1- 3: 300’ (total) x .00198 ohms = .594 ohms (NAC1-2 to NAC1-3)

.594 ohms x .3 amps (NAC1-3 & -4) = .1782 volts drop @ NAC1-3

18.691585 - .1782 = 18.513385 VDC @ NAC1-3

NAC1- 4: 200’ (total) x .00198 ohms = .396 ohms (NAC1-3 to NAC1-4)

.396 ohms x .1 amps (NAC1-4 only) = .0396 volts drop @ NAC1-4

18.513385 - ..0396 = 18.473785 VDC @ NAC1-4 (last device on circuit)

TOTAL CIRCUIT VOLTAGE DROP: 19.1 – 18.473785 = .626215 VDC

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The Point-to-Point Voltage Drop Calculation (continued)• In the PTP calculation method there is a computation for each wiring segment.

– Uses revised (updated) voltage at each device due to drops for each segment.

– Relies on a fixed current draw value (hopefully using U.L. Max values), making it conservative.

• In reality, the higher the voltage (closer to the panel), the lower the current draw of that device, making the calc somewhat conservative too.

– Can be very time consuming to do by hand due to the many steps. Thus, most designers need to use a spreadsheet program to employ this method.

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Regulated or FWR?

• Fire alarm system power supplies are typically either “regulated” or “full wave rectified” (FWR).

FWR power outputs are much less “linear” than regulated outputs, and therefore less efficient.

Thus, devices powered by FWR power draw more current than with regulated outputs.

HOWEVER, Power supply type has nothing to do with battery or voltage drop calculations (sigh of relief by plan reviewers since they can ignore that).

THEREFORE, the only reason to use FWR current draw figures is to confirm max. power supply loading on primary power when the power supply is FWR.

• Batteries are one of the most pure forms of regulated power supplies.

Output is very linear and efficient for connected fire alarm devices.

Battery and voltage drop calculations should only be done with the assumption that system is on secondary (battery) power.

Once on secondary battery power, all power supplies become regulated.

NOTE: We don’t need to get any more technical than that!

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Info Needed with Any Voltage Drop Calculation Submittal1. The NAC terminal voltage for each panel type used in the calculations, supported by

manufacturer’s confirmation.

2. The U.L. Max current draw figures used in calculations, supported by data sheets for each model used with system, which should match the parts list on drawings.

3. The resistance per foot for each wire gauge (AWG) used in NAC circuits, either using the NEC tables or manufacturer’s figures IF supported by a data sheet and specified by part number(s) on the drawings.

4. The basis of the math used within the calculation program, i.e. point-to-point, end-of-line, or “something else”, spelling out all math steps used.

– Note: “Something else” is probably a baseless figment of someone’s imagination or wishful thinking. There’s a lot of that out there.

– Also note: If the calc program came from the manufacturer of the panels being used; and, is current enough to take the panel cut-off voltage into account; then, it is probably valid since their name is on it (i.e. manufacturer’s instructions). The manufacturers typically don’t volunteer to provide the details on the math in programs they supply to their distributors for several reasons. A manufacturers’ (or anyone’s) calc program should be capable of printing out quantities and current draw figures used for all devices in the calculations, which should be included in any submittals to the local AHJ.

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Estimating Wire Lengths for Voltage Drop Calculations• Estimating wire lengths for each circuit is not difficult.

– It must be conservative to ensure it will work..

• Being short on wire length estimates can result in system failures.

• When estimating wire run lengths:

– Allow worst case for power supply locations in fire alarm panel rooms.

– Allow worst case for wire run elevation (i.e. run at beam height) and length of vert. drop into devices.

– Wire runs are always (code) required to follow building lines.

• The wire run length for all NAC circuits should be estimated and provided, regardless of length, both for current review and future reference.

– These values, along with calculated resistances, are what the installers should use to compare their required circuit measurements when confirming there are no shorts or opens. Hopefully, all jurisdictions are enforcing that part.

– This info can also be useful when doing tenant improvements.

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Those Non-UL 1971 (Higher Voltage Demand) Devices

Non-UL 1971 NAC devices can only be used in “private mode” fire alarm systems.

How to provide meaningful calculations can be the $64,000 question.

No requirement to operate from 16 VDC to 33 VDC, making voltage drop calculations “difficult” and sometimes baseless.

Operating voltage can be higher than the power supply cut-off voltage.

Sometimes, pressing the manufacturer to provide the listed operating range is difficult though called for. It’s often not on their data sheets.

Frequently, the only way to allow for voltage drops and the panel cut-off is to “throw in” a 50% to 200% additional battery capacity as a “best guess” allowance.

When non-UL 1971 devices are used, it’s best to call for a functional battery standby test to confirm it will work.

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Calcs for “Worst Case” Circuits Only?

• What is a “worst case” NAC circuit?

– Only the one with the longest wire run?

– Only the one with the most devices connected?

• Answer: You probably won’t know until you run (or see) the calcs.

– The “worst case” could be (for instance) the third longest circuit with the third greatest device load connected.

– There frequently won’t be a simple “worst case” of having both the longest wire run plus the largest load connected.

– So, providing multiple circuit calculations, for all but the obviously smallest combined loads and wire lengths, are justified.

– The people who try to cut corners on this are typically the ones who also leave important stuff out that can result in catastrophic system failures.

A Note to Plan Reviewers and Final Inspecting Local AHJ’s:

If you consistently review the integrity of the calculations (i.e. the validity of the values being used); and, confirm that the wiring integrity testing is done in conjunction with those calculation values; you will probably end up with far fewer systems with design problems in your jurisdiction.

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Presentation Credit & Reserved Copyright Restrictions

This presentation on voltage drop calculations was created by Larry Tate, primarily for internal use by our fire life safety systems designers and engineers at CH2M Hill, who has graciously provided permission to share this information with those interested through the Northwest Automatic Fire Alarm Association. The copying of this presentation for internal company or jurisdiction distribution and use is hereby permitted. However, the use of this copyrighted material in any commercial or (for fee) applications; or, the editing or altering of this material in any way, without the permission of the author and CH2M Hill, is strictly forbidden.

Larry Tate

Senior Designer, TS4

Electrical/Life Safety Systems

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