Crash course: Ohm's Law for electricians what apprentices get wrong (part 3)

Ohm's Law is the first formula every apprentice memorizes and the first one they misapply on the job. V=IR looks simple on a whiteboard. In a hot panel with voltage drop, temperature derating, and a motor pulling locked rotor amps, simple gets you burned. This part 3 covers the field mistakes that show up on real calls, and how to sanity-check your numbers before you pull the trigger on conductor size or breaker selection.

The wheel is a crutch, not a calculator

Apprentices love the Ohm's Law wheel because it removes algebra. The problem is the wheel assumes pure resistive DC or unity power factor AC. The moment you add a motor, a ballast, a VFD, or any inductive load, P=VI stops giving you the real watts. You get apparent power (VA), not true power (W).

On a single-phase 240V circuit pulling 10A through a motor with 0.8 PF, the wheel says 2400W. Reality is 1920W true power and 2400 VA apparent. Your conductor sizing follows the amps, but your energy calcs and generator sizing follow the VA. Confusing the two is how undersized gensets get spec'd.

• Resistive loads (heaters, incandescent, resistance welders): wheel works fine.
• Inductive loads (motors, transformers, ballasts): use VA for sizing, W for billing.
• Three-phase: multiply by 1.732 (square root of 3) for line-to-line calcs.

Voltage drop kills the formula on long runs

Ohm's Law assumes the voltage at the load equals the voltage at the source. On a 200 foot run of #12 feeding a 20A load at 120V, you've already lost roughly 6.4V to conductor resistance. The load sees 113.6V, not 120V. Plug 120V into your calc and your current prediction is wrong.

NEC 210.19(A) Informational Note 4 recommends branch circuit voltage drop not exceed 3%, with combined feeder and branch under 5%. This is not enforceable code, but it is what the inspector and the equipment manufacturer expect. Undervoltage on motor circuits causes overcurrent at startup, which trips breakers and burns windings.

If a circuit nuisance trips only at the far end of a long run, measure voltage under load before you swap the breaker. Nine times out of ten the conductor is undersized for the distance, not the breaker bad.

Temperature changes resistance, and resistance changes everything

Copper resistance increases roughly 0.4% per degree C above 20C. A conductor sitting in a 40C attic has about 8% more resistance than the same conductor on the bench at room temp. Your voltage drop calc done at 75F understates real-world drop in summer.

This is why NEC Table 310.16 has separate columns for 60C, 75C, and 90C insulation, and why ambient temperature correction factors in 310.15(B) exist. The conductor ampacity drops because the conductor cannot shed heat as fast when ambient is high. Ohm's Law still works, but the R in V=IR is not a fixed number stamped on a chart.

• Attic and rooftop runs: apply 310.15(B)(1) ambient correction.
• Conduits with more than 3 current carrying conductors: apply 310.15(C)(1) adjustment.
• Both corrections stack. Multiply, do not pick the worse one.

Series vs parallel: where apprentices invert the math

In series, resistances add. In parallel, they get smaller than the smallest branch. Apprentices routinely invert this when troubleshooting because they think of the loads, not the resistors. Two 100W bulbs in parallel draw more current than one. Two in series draw less, and each sees half the voltage.

This matters when you find a junction box with three loads tapped off one circuit and you're trying to predict total draw. Parallel branches each see full voltage. Total current is the sum of branch currents. Total resistance seen by the source is lower than any single branch.

1. Series: I is the same everywhere, V divides across loads, R adds.
2. Parallel: V is the same everywhere, I divides across branches, R combines as 1/Rt = 1/R1 + 1/R2 + ...
3. If you cannot tell which you're looking at, trace the neutral. Shared neutral with multiple hots usually means parallel branches off a common bus.

Inrush, locked rotor, and why the steady-state number lies

A 5HP motor at 230V single-phase pulls about 28A running per NEC Table 430.248. Locked rotor can hit 6 to 8 times that, briefly. Ohm's Law at the nameplate gives you the steady-state resistance, but the cold motor has lower winding impedance and the rotor is not turning, so back-EMF is zero. Current spikes accordingly.

This is why NEC 430.52 lets you size the branch overcurrent device well above the motor FLA, up to 250% for inverse time breakers on most motors. The conductor still gets sized at 125% of FLA per 430.22, because the conductor needs to handle continuous running current, not the half-second inrush.

If a motor breaker holds for years then suddenly trips on startup, check the contactor and the run capacitor before you upsize the breaker. Worn contacts and weak caps extend inrush duration past the breaker's tolerance curve.

Sanity checks before you trust the number

Every Ohm's Law calc on the job should pass three checks before you act on it. Does the answer match the order of magnitude you'd expect from experience. Does the voltage at the load match the voltage at the source after drop. Does the load type (resistive, inductive, electronic) match the formula you used.

If any check fails, stop and remeasure. A clamp meter on the conductor under load tells you more in five seconds than ten minutes of math with the wrong assumptions. The formula is a starting point, not the final answer.