How voltage drops over long distances

Usually in the controls business, you try to locate your controllers as close as possible to the devices they are controlling. Sometimes that’s not doable, like in the case where you have rooftop units in a cold climate. Controllers can’t handle below freezing temperatures, so you have to locate the controllers somewhere in the building, maybe hundreds of feet away. How do you know that the resistance of a long run of wire won’t cause a voltage drop bad enough to cause problems?

As you probably know, a long enough wire run will cause a voltage drop, but it has to be pretty long to cause any problems. A voltage drop on a signal coming back to a Voltage Input from a sensor will cause the reading to be inaccurate. The National Electrical Code contains tables for figuring this out, but here’s a couple of ways of doing it on your own.

You have to know the length and resistance of the wire, the source voltage, and the maximum current of the circuit. Here’s a web site that can give you the resistance of light gauge wires: http://www.cirris.com/testing/resistance/wire.html

Most 24 volt control wire is 18 AWG, and according to the above mentioned website, the resistance of 18 gauge wire is .00639 Ohms per foot. Since the maximum current on a Computrols Professional controller 24VDC Binary Output is 50mAmps, we’ll use that as the current. Let’s look at the formula and circuit diagram:

In this example, let’s go with a wire distance of 1000’, which is probably more distance than you’ll ever have to run. The variables are: V1 = 24VDC; R = 0.00639/Ft.; L = 1000’; I = 0.05 Amps.

24 – (2 x 1000 x 0.00639 x 0.05) = 24 - 0.639 = 23.361 Volts at End

As you can see, the voltage at the end is still over 23 Volts, so 1000’ should not be a problem powering a device. Even at 2000’, the voltage drop is just 1.278 Volts.

In the case of powering a long line of VAV controllers from one transformer, it would be very complicated to calculate the voltage drop. Computrols recommends that you use a 28V transformer to compensate for any voltage drop.

You probably don’t have to worry about voltage drops when powering a device or relay as in the above example, but a Voltage Input could pose an accuracy problem. We recommend the use of Voltage-to-Current converters or 4-20mA transmitter. This also makes it noise immune from nearby 60Hz sources.

In the case of a long run to a thermistor, we also suggest using a 4-20mA device instead of a plain thermistor. No matter what the resistance due to wire length, the current will remain constant.