Crash course: Ohm's Law for electricians contractor's perspective (part 3)

Picking up where Part 2 left off

Part 1 covered the basic triangle. Part 2 hit power and the wheel. Part 3 is where Ohm's Law stops being a classroom exercise and starts paying for itself on the job. Voltage drop, conductor sizing sanity checks, troubleshooting hot conductors, and reading what the meter is actually telling you.

If you can do these four things in your head or on a phone calculator, you stop guessing and start diagnosing.

Voltage drop the way the field actually uses it

NEC 210.19(A) Informational Note No. 4 and 215.2(A)(1) Informational Note No. 2 recommend keeping branch circuit drop at 3% and combined feeder plus branch at 5%. It is a recommendation, not a hard rule, but inspectors and engineers treat it like one on commercial jobs.

The single phase formula every electrician should have memorized: VD = (2 x K x I x D) / CM. K is 12.9 for copper, 21.2 for aluminum. D is one way distance in feet. CM is circular mils from Chapter 9 Table 8. For three phase, swap the 2 for 1.732.

• 20A continuous load, 120V, #12 copper (6530 CM), 100 ft one way: VD = (2 x 12.9 x 20 x 100) / 6530 = 7.9V, or 6.6%. Fails.
• Same load on #10 (10380 CM): 4.97V, 4.1%. Still over.
• Bump to #8 (16510 CM): 3.13V, 2.6%. Passes.

This is why long home runs in warehouses and parking structures get upsized two trade sizes before anyone touches a calculator. Ohm's Law tells you why before the engineer's stamp tells you to.

Using Ohm's Law to verify conductor heating

Ampacity tables in 310.16 give you the legal answer. Ohm's Law gives you the physical one. Power dissipated in a conductor is I squared R. Double the current, quadruple the heat. That is why a loose lug or a corroded splice goes from warm to glowing in a hurry once load climbs.

Field check: pull the resistance per foot from Chapter 9 Table 8, multiply by your run length, then by I squared. That is the watts being dumped into the conductor itself. On a 200 ft run of #6 copper (0.491 ohms per 1000 ft) carrying 55A, you are burning roughly 594 watts in the wire. That heat has to go somewhere, and 310.15(B) adjustment factors exist because of it.

If a breaker keeps tripping and the conductor feels warmer than ambient by more than about 30 degrees F at steady load, stop resetting it. Measure voltage drop end to end under load and back-calculate resistance. A connection has gone high resistance somewhere.

Troubleshooting with V = IR in your head

Most service calls come down to one of three things: open, short, or high resistance. Ohm's Law sorts them fast.

1. Voltage present, no current, load not running: open somewhere downstream of your meter point.
2. Voltage collapses to near zero when load engages, breaker trips: short or grossly undersized conductor, or a load drawing far more than nameplate.
3. Voltage sags 10 to 20% under load but recovers when load drops: high resistance connection, almost always a lug, splice, or backstabbed device.

The third one is the killer because it does not trip anything. It just cooks devices, dims lights, and eventually starts a fire. NEC 110.14 exists specifically because of bad terminations, and torque specs in 110.14(D) became mandatory in the 2017 cycle for a reason.

On a nuisance trip call, measure voltage at the breaker line side, load side, at the device, and across the neutral to ground at the panel. Four readings, two minutes, and Ohm's Law tells you which segment is dirty.

Motor and transformer quick math

Single phase motor full load amps: FLA = (HP x 746) / (V x efficiency x power factor). For rough field work, assume 0.85 for both efficiency and PF on standard motors and you will be within a few percent of the 430.248 table. Three phase: divide by an additional 1.732.

Transformer secondary current: I = kVA x 1000 / V for single phase, or kVA x 1000 / (V x 1.732) for three phase. A 75 kVA 480 to 208Y/120 transformer pushes 208A on the secondary. That sets your secondary OCPD per 450.3(B) and your conductor sizing per 240.21(C).

• 10 HP, 240V, single phase, standard motor: roughly 50A FLA. Table 430.248 says 50A. Close enough for a sanity check.
• 25 HP, 480V, three phase: FLA math gives about 32A. Table 430.250 says 34A. Use the table for sizing, the math for verification.

What to keep on your phone

You do not need to memorize everything. You need to recognize when a number looks wrong. If a 20A circuit is reading 14 amps and the lights are dim, something is off. If a 480V feeder reads 460V at the panel and 430V at the disconnect under load, you have a 6.5% drop in one segment and a problem.

Build a short reference: K values, the four Ohm's Law and four power formulas, single and three phase voltage drop, transformer secondary amps, and motor FLA. That is the working electrician's cheat sheet. Everything else is in the book.