Crash course: Ohm's Law for electricians with photos (part 4)

Part 4: Putting Ohm's Law to Work on the Job

Parts 1 through 3 covered the triangle, the math, and power. This one is about the calls you actually run into: sizing conductors, checking voltage drop, diagnosing nuisance trips, and reading a meter before you chase your tail. The numbers don't lie. The panel schedule sometimes does.

Keep V = IR and P = VI taped to the inside of your brain. Everything below is just those two equations dressed up for fieldwork.

Voltage Drop on Long Runs

NEC 210.19(A) Informational Note 4 recommends branch circuits not exceed 3% drop, with combined feeder and branch not exceeding 5%. It is a recommendation, not a hard rule, but inspectors and engineers treat it like one on commercial jobs.

The working formula on a single-phase run is Vd = 2 x K x I x D / CM, where K is 12.9 for copper, I is load amps, D is one-way distance in feet, and CM is the circular mils of the conductor. Rearrange with Ohm's Law and you can solve for the conductor size you actually need, not the one the tap chart suggests.

• 120V, 16A load, 150 feet one-way, #12 copper (6530 CM): Vd = (2 x 12.9 x 16 x 150) / 6530 = 9.48V, or 7.9%. Too much.
• Bump to #10 (10380 CM): Vd = 5.96V, or 4.97%. Still over for a branch circuit.
• Go to #8 (16510 CM): Vd = 3.75V, or 3.1%. That is your conductor.

The ampacity table got you to #12. Ohm's Law got you to #8. Both matter.

Diagnosing Nuisance Breaker Trips

A 20A breaker trips at what looks like a light load. Before you swap the breaker, do the math. Measure voltage at the panel with the circuit loaded, then measure at the device. If the drop is significant, you are losing voltage to resistance somewhere that shouldn't have it.

A loose lug, a backstabbed receptacle, or a corroded splice adds resistance. That resistance dissipates power as heat (P = I squared x R), and on a motor load the motor compensates by pulling more current to hold torque. More current, more heat, tripped breaker. The breaker is doing its job.

If a 15A motor branch is tripping and the motor is cold, put the clamp meter on the conductor at the panel and at the motor. Same current, different voltage = bad connection in between. Same current, same voltage, still tripping = motor or breaker.

Sizing for Continuous Loads

NEC 210.19(A)(1)(a) and 215.2(A)(1)(a) require conductors supplying continuous loads to be sized at 125% of the continuous load. A continuous load per Article 100 is one expected to run for 3 hours or more. Commercial lighting, EV chargers, and a lot of HVAC fall in this bucket.

Ohm's Law tells you the steady-state current. The 125% factor is the code's margin for heat buildup in the conductor insulation over time. Skip it and you get derated insulation, nuisance trips, and a failed inspection.

1. Calculate continuous load current: I = P / V.
2. Multiply by 1.25.
3. Size breaker and conductor to that number, per NEC 310.16 ampacity tables and termination temperature ratings per 110.14(C).
4. Verify voltage drop does not push you to a larger conductor anyway.

Three-Phase: The Square Root of Three

Single-phase: P = V x I x PF. Three-phase: P = V x I x PF x 1.732. That 1.732 is the square root of 3, and it is the most forgotten number on the truck.

A 480V three-phase motor pulling 25 FLA at 0.85 power factor: P = 480 x 25 x 0.85 x 1.732 = 17,670 watts, or about 17.7 kW. If you forget the 1.732, you will undersize your feeder by 42%. Motor nameplate horsepower works out too: 17.7 kW / 0.746 = 23.7 HP, which matches a 25 HP motor with typical efficiency losses.

NEC 430.22 sizes motor branch conductors at 125% of the motor FLA from Tables 430.248 through 430.250. Use the table FLA, not the nameplate, for conductor sizing. Nameplate is for overload protection per 430.32.

Reading the Meter Before You Guess

Ohm's Law is only useful if your inputs are real. A lot of troubleshooting goes sideways because someone assumed 120V at the device when there was actually 108V, or assumed a 15A load when the clamp showed 22A.

Verify under load. Voltage measured with nothing running tells you the source is alive. Voltage measured with the load engaged tells you what the load actually sees. The difference is your drop, and the drop is your diagnosis.

Clamp meter on the hot, voltmeter from hot to neutral at the device, everything energized and running. Three numbers: source voltage, load voltage, load current. From those three you can back into every resistance, every fault, and every undersized conductor on the run.

Quick Reference

The equations do not change. The values you plug in are the whole game. Measure twice, calculate once, and trust the numbers over the nameplate when they disagree.

• V = I x R, P = V x I, P = I squared x R
• Three-phase power: multiply by 1.732
• Voltage drop target: 3% branch, 5% total (NEC 210.19 IN 4, 215.2 IN 2)
• Continuous load: size at 125% (NEC 210.19, 215.2)
• Motor conductors: 125% of table FLA (NEC 430.22)