Crash course: Ohm's Law for electricians what apprentices get wrong (part 2)

Recap: Where Part 1 Left Off

Part 1 covered the basics: V = IR, P = IV, and the triangle tricks. This one digs into the mistakes that show up on real jobs, the ones that cost apprentices hours of troubleshooting or get them a correction tag from the inspector. If you have not read Part 1, the short version is this: voltage pushes, current flows, resistance resists, and power is the product of the first two.

Ohm's Law itself is simple arithmetic. The trouble starts when apprentices apply it to the wrong part of the circuit, forget about voltage drop, or confuse AC behavior with DC. Let's go through the common traps.

Mistake 1: Ignoring Voltage Drop on Long Runs

Ohm's Law tells you that any conductor with resistance will drop voltage under load. Apprentices size wire from the ampacity table in NEC 310.16 and call it done. That works for heat, not for performance. A 120V circuit feeding a motor 180 feet away on #12 copper can easily drop 5% or more, and the motor will run hot, pull more current, and fail early.

NEC 210.19(A) Informational Note 4 and 215.2(A)(1) Informational Note 2 recommend a maximum 3% drop on branch circuits and 5% combined on feeders plus branch circuits. These are not code mandates in most cases, but they are the industry baseline.

• Use VD = 2 x K x I x L / CM for single-phase, where K is 12.9 for copper, I is load amps, L is one-way length in feet, and CM is circular mils of the conductor.
• For three-phase, use 1.732 (root 3) instead of 2.
• Run the math before you pull the wire. It is cheaper to upsize once than to repull later.

Field tip: if a run is over 100 feet and loaded over 80% of breaker rating, do the voltage drop math. Do not guess. A phone calculator takes 30 seconds.

Mistake 2: Treating Impedance Like Resistance

On AC circuits with motors, ballasts, or transformers, the opposition to current is impedance (Z), not just resistance (R). Impedance includes inductive and capacitive reactance. Plug resistance into Ohm's Law on a motor circuit and your current prediction will be wrong, sometimes badly.

This is why NEC 430.6(A)(1) tells you to size motor branch circuit conductors from the FLC tables in 430.250, not from the motor nameplate and not from a resistance measurement with a meter. The nameplate tells you actual running current; the tables give you the value the code uses for sizing.

For heating elements, incandescent lamps, and resistive loads, R and Z are effectively the same and Ohm's Law applies cleanly. Know which kind of load you have before you start calculating.

Mistake 3: Confusing Series and Parallel Behavior

In a series circuit, current is the same everywhere and voltages add up. In a parallel circuit, voltage is the same across each branch and currents add up. Apprentices mix these up when troubleshooting, especially on multiwire branch circuits and shared neutrals.

A loose neutral on a shared-neutral circuit is a classic example. With the neutral open, the two hot legs now see each other in series through the loads. A 120V appliance on one leg can suddenly see 200V or more, depending on the load balance on the other leg. Things burn up fast.

1. NEC 210.4(B) requires a simultaneous disconnect for all ungrounded conductors of a multiwire branch circuit at the panel.
2. NEC 300.13(B) prohibits using device terminals as the splice point for grounded (neutral) conductors on multiwire circuits. Pigtail the neutrals.
3. Torque your neutral bar screws to spec. A loose neutral is the fastest way to prove series-circuit behavior to a homeowner, in the worst possible way.

Mistake 4: Power Factor Blindness

P = IV works for DC and for purely resistive AC. On inductive AC loads, real power is P = IV x PF, where PF is power factor. An apprentice who sizes a generator or a conductor using P = IV on a motor load will come up short because current is higher than the resistive math predicts.

For a 10 HP, 240V, three-phase motor with a PF of 0.85 and efficiency of 0.90, the current is roughly: I = (HP x 746) / (1.732 x V x PF x eff). That is about 23.4A, which tracks with NEC Table 430.250. Skipping PF gives you a number that looks fine on paper and trips breakers in the field.

Field tip: when the nameplate amps do not match your Ohm's Law prediction, suspect power factor or efficiency before you suspect the motor. Nine times out of ten the motor is fine and your math was incomplete.

Mistake 5: Forgetting Temperature and Continuous Loads

Conductor resistance rises with temperature. A #12 THHN at 75C has noticeably more resistance than at 25C, and that changes your voltage drop under real operating conditions. The ampacity tables in NEC 310.16 already bake in a temperature assumption. Derating per 310.15(B) and (C) adjusts for ambient and bundling.

Continuous loads, defined in NEC Article 100 as loads expected to run for three hours or more, must be sized at 125% per 210.19(A)(1) and 215.2(A)(1). Ohm's Law gives you the steady-state current, but the code wants the circuit built with headroom so the breaker and conductors stay cool.

• Calculate continuous load current, multiply by 1.25, then size the conductor and OCPD from that number.
• Check ambient temperature where the conductor actually runs, not where the panel is.
• If you are bundling more than three current-carrying conductors in a raceway, apply 310.15(C)(1) adjustment factors.

Quick Reference Card

Keep this on your phone or in your toolbag:

• V = IR, I = V/R, R = V/I (DC or resistive AC)
• P = IV x PF (single-phase AC with reactive load)
• P = 1.732 x V x I x PF (three-phase AC)
• VD single-phase = 2 x K x I x L / CM
• VD three-phase = 1.732 x K x I x L / CM
• Continuous load sizing: multiply by 1.25

Master the formulas, then master when they do not apply. That is the difference between an apprentice who passes the test and a journeyman who passes inspection.