Line Loss: Easy as 1, 2, 3

Today, we had a power lab (hands-on workshop) at Dadco Power and Light in Sun Valley, California. This is an annual event hosted by IATSE Local 728 for their members, and the owner of the company, Ron Dahlquist, generously provides space and gear. As part of the workshop, I wanted to demonstrate voltage drop (a.k.a. line loss). What is voltage drop? I'm glad you asked.

Every time you connect a load and turn it on, the supply voltage is applied to the circuit, but some of the voltage is dropped across the wiring, and what's left is applied to the load. The amount of voltage that is dropped across the conductors depends on the amount of current flowing through them and the resistance of the conductors. If the current is too high, or if the conductors are too small or too long, then too much voltage will be dropped and not enough applied to the load. How do you figure out ahead of time if that's the case? You're awfully curious, and I like that.

According to Ohm's law, the voltage that is dropped across the conductors is the current times the resistance of the wire. In the back of the National Electrical Code (NFPA 70), there is a table that gives you the resistance of copper wire based on the length and the gauge. I like to use that to predict voltage drop (sometimes called line loss).

At the workshop we had a setup with a 12kW tungsten lamp connected with 200' of banded feeder cable (#2 AWG). According to Chapter 9, Table 9, the resistance of 1000' of #2 AWG copper wire is 0.19 ohms. Since we had a 200' run, that means we had 400' of copper (200' to the load and 200' back to the supply). And since 400' divided by 1000' is 0.4, we calculated a total resistance of 0.076 ohms (0.19 × 0.4 = 0.076). Since the light operates at 120V, we also calculated a current of about 100 amps (12,000 ÷ 120 = 100). Therefore, we expected to measure a voltage drop of 7.6 volts.

After firing up the genny, we measured the current at the generator and it was 125 volts. Then we measured the current at the load, and it was 118.6. That means we had a voltage drop of 6.4 volts.

Our calculation was pretty close, but we weren't as close as Ron's estimate. Just before we measured the voltage drop, Ron whispered in my ear. "I'll bet the voltage drop is 6 volts," he said. He was closer than we were. How did he know?

Ron later told me that he has a rule of thumb that comes from years of experience. He calls it the 1-2-3 rule. It says that you will get about 1 volt of line loss per 100' of 4/0 AWG cable with 100 amps, 2 volts of line loss per 100' of 2/0 AWG cable with 100 amps, or 3 volts of line loss per 100' of #2 AWG with 100 amps.

The Roman historian Tacitus said, "Experience teaches." Yes, it does, but there is no better combination than experience and training.