Crash course: Ohm's Law for electricians common mistakes edition (part 1)

Ohm's Law, the version that actually matters on the job

V = I × R. Volts equal amps times ohms. Rearrange it as needed: I = V / R, R = V / I. That is the whole law. The mistakes are not in the algebra. They are in what techs plug into the variables and what they assume stays constant.

On paper, a 120V circuit through a 12 ohm load draws 10 amps. In the field, that number drifts because conductors heat up, terminations loosen, and nobody actually measured the source voltage at the load. Treat Ohm's Law as the start of the analysis, not the end.

Mistake 1: using nominal voltage instead of measured voltage

"120V" is a label, not a measurement. Utility voltage at the service can sit anywhere from about 114V to 126V per ANSI C84.1 Range A. After voltage drop through the feeder and branch circuit, the load might see 110V or less. If you calculate current from 120V when the load actually sees 112V, your amp prediction is off by roughly 7%.

This matters when you are sizing overcurrent protection, troubleshooting nuisance trips, or chasing a motor that runs hot. NEC 210.19(A) Informational Note 4 and 215.2(A)(1) IN 2 recommend keeping combined feeder and branch circuit voltage drop under 5% for reasonable efficiency. That is a recommendation, not a hard rule, but it is the number inspectors and engineers expect you to know.

Field tip: before you calculate anything, put your meter on the load terminals under load. Open circuit voltage lies. Loaded voltage tells the truth.

Mistake 2: forgetting that resistance is not constant

Copper resistance climbs about 0.4% per degree C. A conductor at 75C carries noticeably more resistance than the same conductor at 20C in the truck. Motor windings, incandescent filaments, and heating elements shift resistance even more dramatically as they warm up. That is why an incandescent bulb draws a big inrush current when cold, then settles.

NEC Chapter 9 Table 8 gives DC resistance at 75C for a reason. If you are pulling numbers off that table for voltage drop on a run that will actually operate at 40C ambient with partial load, your real resistance is lower and your drop is smaller than the book says. The book errs on the conservative side. Use it, but know what it assumes.

• Chapter 9 Table 8: DC resistance and reactance of conductors
• Chapter 9 Table 9: AC resistance and reactance, use this for most AC calcs
• 310.15(B): ambient temperature correction factors

Mistake 3: applying DC Ohm's Law to AC circuits without impedance

On AC, the opposition to current is impedance (Z), not just resistance. Z combines resistance with inductive and capacitive reactance. For a purely resistive load like a baseboard heater, R and Z are close enough to ignore the difference. For motors, ballasts, transformers, and long conductor runs, reactance matters and the power factor is not 1.0.

If you plug R into V = I × R on a motor circuit and expect it to predict current, you will be wrong. Use nameplate FLA or NEC Table 430.250 for three-phase motor current, and use 430.22 for branch circuit conductor sizing at 125% of FLA. Ohm's Law still applies, just with Z instead of R and with the phase angle accounted for.

Mistake 4: confusing series and parallel behavior

In a series circuit, current is the same everywhere and voltage divides across the resistances. In a parallel circuit, voltage is the same across each branch and current divides. Techs get into trouble when they treat a multiwire branch circuit or a parallel feeder run like a simple series loop.

Parallel conductors per 310.10(G) must be the same length, same conductor material, same size (1/0 AWG and larger), same insulation type, and terminated the same way. If one conductor is even slightly shorter or has a tighter lug, it sees lower resistance and hogs current. Ohm's Law predicts exactly this: lower R on one path, higher I on that path. You get a hot conductor that the breaker never sees, because the breaker only sees total current.

Field tip: when you megger or test parallel sets, check them against each other, not just against ground. A 10% resistance mismatch between paralleled conductors is enough to chase down.

Mistake 5: ignoring the neutral on shared circuits

On a multiwire branch circuit, the neutral carries the imbalance between the two or three hots. Techs sometimes assume the neutral current is always small. On circuits feeding nonlinear loads like LED drivers, VFDs, and switch-mode power supplies, the neutral can carry triplen harmonic currents that add rather than cancel. The neutral can run hotter than the phase conductors.

NEC 310.15(E) addresses this. When a major portion of the load is nonlinear, the neutral is considered a current-carrying conductor and counts toward the conductor fill adjustment in 310.15(C)(1). Running your Ohm's Law math only on the phase side and ignoring the neutral is how you end up with an overheated shared neutral in a panel full of computer circuits.

1. Identify the load type. Linear or nonlinear?
2. If nonlinear and dominant, count the neutral as current carrying.
3. Apply the conductor adjustment factors in 310.15(C)(1).
4. Recheck ampacity against your calculated or measured current.

Where part 2 goes

Part 2 covers power calculations (P = V × I and its siblings), voltage drop worked examples with real wire sizes, and the fault current side of Ohm's Law where the source impedance and transformer %Z set the available fault current your gear has to interrupt. Same rules apply: measure before you calculate, and know what your constants actually represent.