Crash course: Ohm's Law for electricians code-compliant version (part 1)

Ohm's Law: the three variables you actually use

Voltage, current, resistance. V = I × R. Rearrange for whichever one you're missing. On a service call you're almost never solving for resistance directly, you're using the relationship to sanity check what a meter is telling you or what a circuit should pull.

Memorize the wheel or don't, but keep these three forms in your head: V = IR, I = V/R, R = V/I. Add P = VI for power and you've covered 90% of the math a working electrician runs in a day.

The NEC doesn't cite Ohm's Law by name, but almost every ampacity, voltage drop, and overcurrent rule in the book assumes you understand it. Article 90.1(B) tells you the Code is a minimum for safe installations, not a design manual. Ohm's Law is how you bridge that gap.

Voltage drop: where the math meets 210.19(A)

NEC 210.19(A) Informational Note No. 4 recommends branch circuit conductors sized so voltage drop does not exceed 3%, with a combined feeder and branch total not exceeding 5%. It's not mandatory in most installations, but inspectors and engineers expect you to respect it, and some jurisdictions make it enforceable through amendments.

The working formula for single phase: VD = (2 × K × I × D) / CM. K is 12.9 for copper, 21.2 for aluminum. D is one way distance in feet. CM is the circular mils of the conductor. For three phase, swap the 2 for 1.732.

Run the numbers before you pull wire on a long run. A 20 amp circuit at 120 volts pushing 150 feet on #12 copper lands around 4.8% drop. That's over the recommendation and your customer's motor loads will feel it.

Field tip: if a homeowner complains that lights dim when the fridge kicks on, measure voltage at the panel and at the receptacle under load. If you see more than a 3 to 5 volt swing, you've got a voltage drop or a loose connection, not a "bad breaker."

Ampacity and the 80% rule

NEC 210.19(A)(1) and 215.2(A)(1) require branch circuit and feeder conductors to have an ampacity not less than the noncontinuous load plus 125% of the continuous load. That 125% factor is the flip side of the 80% rule you hear on the job.

Ohm's Law tells you what the load draws. The Code tells you how to size the wire and breaker around it. A 16 amp continuous load needs a conductor and OCPD rated for 16 × 1.25 = 20 amps minimum. Skip that step and you're looking at nuisance trips or, worse, insulation cooking inside a wall.

• Continuous load: expected to run 3 hours or more (NEC Article 100)
• Noncontinuous: anything less than 3 hours of steady operation
• Mixed loads: 100% noncontinuous + 125% continuous, summed
• Breaker and conductor both sized to the calculated value

Check Table 310.16 for the ampacity of the conductor you're proposing, apply any correction and adjustment factors from 310.15(B) and (C), and confirm the result still covers your calculated load.

Power calculations for load sheets

P = VI is the starting point. For resistive loads like heat strips and incandescent lamps, that's all you need. For motors and anything reactive, you're working with apparent power (VA) and real power (watts), and power factor lives between them.

NEC 220.14 gives you the specific loads for branch circuit calculations, and 220.12 sets the general lighting load in VA per square foot by occupancy type. These are VA values, not watts, because the Code is sizing for current, and current is what heats conductors.

1. Total the connected load in VA from 220.14 and 220.12
2. Apply demand factors from 220.42 through 220.61 where permitted
3. Divide by system voltage to get amps
4. Size conductors and OCPD per 210.19 and 215.2

A 240 volt, 4500 watt water heater draws 4500 / 240 = 18.75 amps. Continuous load, so 18.75 × 1.25 = 23.4 amps minimum. You're on a 25 or 30 amp breaker with #10 copper, depending on terminal temperature rating and Table 310.16.

Troubleshooting with Ohm's Law

When a circuit is misbehaving, Ohm's Law turns your meter into a diagnostic tool. Measure voltage across a suspected bad connection under load. Any meaningful voltage across what should be a solid conductor means resistance where it shouldn't be, and that resistance is dissipating power as heat.

A loose neutral on a 20 amp circuit pulling 15 amps with even a half ohm of bad connection is dropping 7.5 volts and burning 112 watts right at the splice. That's a fire starter. The math matches what your thermal camera shows.

Field tip: on nuisance GFCI trips under NEC 210.8(A), measure neutral to ground voltage with the load running. More than a volt or two usually points to a shared neutral, a wet box, or an appliance with internal leakage, not a bad device.

Part 2 will cover three phase math, motor calculations under Article 430, and how to use Ohm's Law to verify conductor sizing on feeder upgrades. Keep the formulas close and the Code book closer.