Crash course: Ohm's Law for electricians vs the old way (part 2)

Quick Recap From Part 1

Part 1 covered the basics: V = I × R, P = V × I, and why memorizing the wheel chart matters less than knowing which two values you actually have in your hand. Voltage from the meter, current from the clamp, resistance from continuity. Pick two, solve for the third.

Part 2 is about applying it on the job. Voltage drop, conductor sizing, motor calcs, troubleshooting a circuit that reads weird. The old way was guess and check, then call the journeyman. The new way is plug numbers into a formula on your phone and move on.

Voltage Drop The Field Way

NEC 210.19(A) Informational Note No. 4 recommends branch circuits stay under 3% drop, with total drop (feeder + branch) under 5%. Not a code rule, but inspectors and engineers treat it like one. The old way was a slide rule and a coffee stained chart in the back of the truck.

Single phase voltage drop: VD = (2 × K × I × L) / CM. K is 12.9 for copper, 21.2 for aluminum. L is one way length in feet. CM is circular mils from NEC Chapter 9, Table 8. Three phase, swap the 2 for 1.732.

• 120V circuit, 3% = 3.6V max drop
• 240V circuit, 3% = 7.2V max drop
• 480V circuit, 3% = 14.4V max drop
• Run length over 100 ft on a 20A circuit? Bump to #10 and stop guessing

If the homeowner's well pump is tripping on start and the run is 300 ft of #12, you found the problem before you opened the panel. Locked rotor current on a small pump can be 5x to 7x FLA. Do the math at startup amps, not running amps.

Sizing Conductors With Ohm's Law In Mind

NEC 310.16 gives you ampacity. That's the ceiling for heat. Ohm's Law tells you whether the load at the far end will actually see usable voltage. Two different problems, both have to be solved.

A #12 THHN is good for 20A at 75°C per Table 310.16. But push that 20A load 150 feet on 120V and you're at roughly 6.2V drop, well over 5%. Code compliant on ampacity, garbage on performance. The motor stalls, the LED driver flickers, the customer calls back.

1. Calculate load in amps (VA divided by voltage)
2. Pick conductor size from 310.16 for ampacity
3. Run voltage drop at that size and length
4. If over 3% on branch, upsize one AWG and recheck
5. Verify EGC sizing per 250.122 if you upsized for VD

Motor And Resistive Load Math

Motor nameplate gives FLA, but NEC 430.6(A)(1) says use Tables 430.247 through 430.250 for sizing conductors and overcurrent, not the nameplate. Conductors at 125% of table FLA per 430.22. Overload protection sized off the nameplate per 430.32. Two different numbers, two different purposes.

Resistive loads are easier. Water heater nameplate says 4500W at 240V. I = P/V = 18.75A. Continuous load per 422.13 means 125%, so 23.4A minimum circuit. Round up to a 25A or 30A breaker on #10. Done.

Nameplate watts on a heater assume nameplate volts. If the panel is sitting at 228V instead of 240V, that 4500W element is actually putting out about 4060W. Customer complains the shower is lukewarm, and you find a loose lug at the meter. Ohm's Law caught it before the IR camera did.

Troubleshooting With Numbers Instead Of Guesses

A circuit reads 118V no load, 96V under load. That's a 22V drop on a circuit that should drop maybe 3V. Something is adding resistance. Loose neutral, corroded splice, undersized wire from a previous hack job, or a backstabbed receptacle giving up.

R = V / I tells you how much extra resistance is in there. Drop of 22V at 12A load means 1.83 ohms of unwanted resistance somewhere in the path. A clean #12 run of 100 ft is about 0.19 ohms. You're looking for a connection problem, not a wire problem.

• Voltage drop across a closed switch should be near zero. Anything over 0.5V means burned contacts
• Neutral to ground voltage under load tells you neutral health. Over 2V on a 120V circuit, start hunting
• Resistance to ground on a de-energized motor winding should match phase to phase within 10%

The Phone In Your Pocket Is The New Wheel Chart

Old way: paper wheel, mental math, ask the foreman. New way: punch in two values, get the third, get the conductor size, get the voltage drop, get the breaker size, all referenced to the article that backs it up. NEC 110.3(B) still applies. Listed equipment, used per instructions. The math just gets you to the right gear faster.

Ohm's Law isn't theory. It's the reason your service call ends in a working circuit instead of a callback. Know it cold, run the numbers before you cut wire, and let the code book back up what the math already told you.