Crash course: Ohm's Law for electricians common mistakes edition (part 4)

The mistake that burns up motors

Ohm's Law is V = IR. Every apprentice memorizes it. Then they get on a job site and apply it wrong because real circuits are not textbook circuits. This part of the crash course hits the errors that show up in panels, junction boxes, and motor terminals, the ones that cost you a callback or a warranty claim.

First up: applying V = IR to motor loads at startup. Motors are not resistors. A 5 HP motor with 7 ohms of stator resistance does not pull 34 amps on startup at 240V. It pulls locked-rotor current, often six to eight times full-load amps, because counter-EMF has not built yet. Use NEC Table 430.251(B) for locked-rotor values, not Ohm's Law, when sizing disconnects and short-circuit protection per NEC 430.52.

Same trap shows up with transformers, solenoids, and any inductive load. DC resistance tells you nothing useful about AC behavior until you factor in reactance and power factor.

Voltage drop: the math everyone fudges

NEC 210.19(A) Informational Note 4 recommends branch circuit voltage drop stay under 3%, and combined feeder plus branch under 5%. Most guys eyeball it or trust a chart. The chart assumes a power factor of 1.0 and ignores conductor temperature.

Real voltage drop on a long run uses Vd = 2 x K x I x L / CM for single phase, where K is 12.9 for copper at 75C. Bump K to roughly 14 if the conductor is running hot in conduit on a south wall in July. A 200 foot run of #10 copper feeding a 20A load at 120V looks fine on paper at 2.6% drop, but at 90C conductor temp you are closer to 3%, and if the load is 24A continuous on a 30A breaker you are over.

If the run is over 100 feet, upsize one gauge and stop guessing. The wire costs less than the truck roll.

Parallel resistance traps in grounding

Ground electrode systems per NEC 250.53 require supplemental electrodes when a single rod exceeds 25 ohms to earth. Electricians assume two rods in parallel cut resistance in half. They do not. Parallel rod resistance depends on spacing. Rods less than 6 feet apart share a resistance shell and give you maybe a 20% reduction, not 50%.

NEC 250.53(A)(3) requires a minimum 6 foot separation for this reason. Push it to 10 or 12 feet if the soil is dry or rocky. Measure with a clamp-on ground tester or fall-of-potential method before you sign off.

• Single 8 foot rod in average soil: 25 to 100 ohms
• Two rods at 6 foot spacing: roughly 60 to 65% of single rod value
• Two rods at 20 foot spacing: closer to 50% of single rod value
• Concrete encased electrode per 250.52(A)(3): often under 10 ohms, no supplement needed

Series circuits and the neutral current myth

A common error on multiwire branch circuits: assuming the neutral carries the sum of both hot conductors. On a shared neutral MWBC per NEC 210.4, the neutral carries the difference between the two hots when they are on opposite phases of a single phase system, not the sum. If both hots land on the same phase because someone moved a breaker, the neutral carries the sum and overheats fast.

Ohm's Law applied to the neutral tells the story. If phase A pulls 15A and phase B pulls 12A on opposite legs, neutral carries 3A. Wire them on the same leg and neutral carries 27A on a #12 conductor rated 20A. This is why NEC 210.4(B) requires a common disconnect for MWBCs feeding the same yoke or device.

Before energizing any MWBC, tick the two hots with a non-contact tester against each other. If the tester stays quiet, they are on the same phase. Move one breaker.

Power dissipation and conductor heating

P = I squared R is where undersized conductors go to die. Watts dissipated in a wire rise with the square of current. Doubling the load quadruples the heat. A #14 conductor at 15A dissipates roughly 0.7 watts per foot. Push it to 20A and you are at 1.24 watts per foot, which is why NEC 240.4(D)(3) caps #14 copper at 15A regardless of what the ampacity table says.

This matters in bundled conduit runs. NEC 310.15(C)(1) requires ampacity adjustment when more than three current-carrying conductors share a raceway. Four to six conductors derate to 80%. Ten to twenty drop to 50%. Ignore this and insulation breaks down over years, not hours.

• Check conductor count before pulling, not after
• Neutrals on MWBCs count as current-carrying per 310.15(E)
• Ambient temperature correction stacks on top of bundling derate
• Terminals are rated 60C or 75C, which caps your usable ampacity regardless of conductor temp rating

Quick field checks before you leave

Before closing the panel, run these four Ohm's Law sanity checks. They take 90 seconds and catch most of the mistakes above.

1. Measure voltage at the furthest outlet under load. If it sags more than 5% from the panel reading, your drop math was wrong.
2. Clamp the neutral on any MWBC. Reading should be the difference of the two hots, not the sum.
3. Verify ground resistance on new electrode systems per NEC 250.53(A)(2). Under 25 ohms or install supplemental.
4. Check terminal temp with an IR gun after 15 minutes at full load. Anything over 60C on a 75C rated lug means you have a torque or sizing problem.

Ohm's Law is simple. Applying it to real loads, real conductors, and real code requirements is where the money is. Next part of the crash course covers three phase math and the mistakes that show up on wye versus delta services.