Crash course: Ohm's Law for electricians top tips edition (part 5)

Ohm's Law in the field

V = I × R. Three variables, one triangle, every troubleshooting call. If you know two, you solve the third. Power folds in with P = V × I, and the pie chart expands from there. Most field problems reduce to one of these equations once you stop guessing.

The trick is not memorizing formulas. It is knowing which value you actually have in front of you, which one the meter is lying about, and which one the load is doing under stress. Cold resistance is not hot resistance. Nameplate amps are not running amps. Nominal voltage is not terminal voltage.

Voltage drop: the calculation you will actually run

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

Single phase: VD = (2 × K × I × L) / CM. Three phase: swap the 2 for 1.732. K is 12.9 for copper, 21.2 for aluminum at 75°C. CM is the circular mil area from Chapter 9 Table 8. Run it before you pull wire, not after the customer calls about dim lights.

• 12 AWG copper, 20A load, 100 ft, 120V: VD = (2 × 12.9 × 20 × 100) / 6530 = 7.9V, or 6.6%. Too much.
• Bump to 10 AWG: VD drops to 4.9V, or 4.1%. Still marginal.
• 8 AWG: 3.1V, or 2.6%. Good.

Using Ohm's Law on a service call

A motor trips on startup. Nameplate says 10 FLA at 240V. You megger the windings and read 2.4 ohms line to line. Apply I = V/R at locked rotor: 240 / 2.4 = 100A inrush. That is 10x FLA, normal for a Design B motor per NEC 430.52 and Table 430.52. Breaker sizing at 250% instantaneous trip is 25A minimum. If the installed breaker is 20A, you found your problem without touching the motor.

Same logic on heaters. A 4500W element at 240V should draw 18.75A and measure 12.8 ohms cold. Read 25 ohms, it is failing open. Read 2 ohms, it is shorted to the sheath. The meter tells you before you pull the element.

Always measure resistance with the circuit de-energized and the load isolated. A parallel path through a contactor coil or indicator lamp will skew the reading and send you chasing ghosts.

Power, heat, and why terminals fail

P = I² × R is the equation that burns down panels. Resistance at a loose lug doubles from 0.001 to 0.002 ohms. At 40A, heat dissipation jumps from 1.6W to 3.2W in a space the size of a dime. That is the difference between warm and glowing.

This is why NEC 110.14(D) requires torque specs per listing, and why 2017 and later codes mandate calibrated torque tools. Feel and experience are not enough once you are above 30A. Torque it, mark it, move on.

1. Check terminal temperature rating: 60°C, 75°C, or 90°C per NEC 110.14(C).
2. Size conductors to the lowest rated component in the path.
3. Torque to manufacturer spec, not to gut feel.
4. Retorque per NEC 110.14(D) if the listing calls for it.

Three phase math without the headache

Line to line voltage is 1.732 times line to neutral on a wye. A 208Y/120 panel gives you 120V phase to neutral and 208V phase to phase. Power in a balanced three phase load: P = 1.732 × V(LL) × I × PF.

Quick field check: a 5 HP three phase motor at 480V pulls roughly 7.6A per NEC Table 430.250. If your clamp reads 11A on one leg and 7A on the others, you have a voltage imbalance, a bad connection, or single phasing coming. NEMA says derate above 1% imbalance, shut down above 5%.

Measure current on all three legs on every service call to a three phase load. Imbalance is the first symptom of a problem that has not failed yet.

The numbers worth memorizing

You do not need to carry a calculator for the common cases. Burn these into muscle memory and you will solve 80% of field problems in your head.

• Watts to amps at 120V: divide by 120. At 240V, divide by 240. At 208V three phase, divide by 360.
• HP to amps single phase: HP × 8 at 120V, HP × 4 at 240V. Close enough for sanity checks.
• 12 AWG copper is roughly 1.6 ohms per 1000 ft. 10 AWG is 1.0. 8 AWG is 0.63.
• 3% voltage drop on 120V is 3.6V. On 240V, 7.2V. On 480V, 14.4V.

Everything else is a variation of V = IR with different units or a geometry factor out front. Know the triangle, know the pie chart, and the rest is arithmetic.