Crash course: Ohm's Law for electricians code-compliant version (part 5)

Ohm's Law, the three forms you actually use

Voltage equals current times resistance. That is the whole law. On the truck you need three rearrangements and nothing else:

• E = I × R (find voltage across a load or a length of conductor)
• I = E ÷ R (find current when you know the applied voltage and the load resistance)
• R = E ÷ I (find resistance from a measured voltage drop and a known current)

Add the power wheel and you get P = I × E, P = I² × R, and P = E² ÷ R. That last one is the fastest way to sanity check a resistive heater nameplate against a branch circuit before you pull wire.

Sizing conductors against voltage drop

The Code does not mandate a voltage drop limit in 210.19 or 215.2, but Informational Notes No. 2 under both articles recommend 3% on branch circuits and 5% total from service to outlet. Inspectors in performance jurisdictions will hold you to it, and engineers will spec it on the prints. Treat it as a hard number on anything over 100 feet.

For a two wire run, voltage drop equals 2 × I × R, where R is the conductor resistance for the one way length pulled from Chapter 9, Table 8. For three phase, the multiplier is 1.732 instead of 2. Keep the copper uncoated column handy, it is the one you want for THHN and XHHW.

Rule of thumb on 120V branch circuits: if the one way run is past 75 feet and you are loading more than 12 amps continuous, bump from 12 AWG to 10 AWG before the inspector asks.

Using Ohm's Law to verify ampacity and overcurrent

Resistive loads are the easy case. A 4500W water heater at 240V draws 4500 ÷ 240 = 18.75A. Continuous load, so 125% per 210.19(A)(1) and 215.3 gives you 23.4A minimum conductor ampacity and OCPD. 10 AWG copper at 75°C from Table 310.16 handles it at 35A, and a 30A breaker per 422.13 covers the fixed storage heater rule.

Motors are not an Ohm's Law problem, they are a Table 430.250 problem. Do not back into motor FLA with E ÷ R, you will undersize every time because locked rotor and power factor are not in the equation. Use the tables, apply 430.22 for conductors and 430.52 for short circuit protection.

• Resistive loads (heat, incandescent, straight resistance): Ohm's Law is exact
• Motors, transformers, electronic ballasts: use NEC tables, not Ohm's Law
• LED drivers and switch mode supplies: use nameplate input amps, power factor lies to you

Fault current, the one calculation that saves lives

110.9 requires equipment interrupting ratings to be adequate for the available fault current at the line terminals. 110.24 requires that available fault current be field marked on service equipment. Both come back to Ohm's Law with the transformer and conductor impedance as R.

The point to point method is Ohm's Law dressed up. You take the transformer secondary short circuit current, then divide it down by the impedance of the feeder run to the panel. If the result at your panel exceeds the breaker AIC rating stamped on the label, you have a 110.9 violation and a pending explosion. Series rated combinations under 240.86 are the only legal workaround, and they have to be tested and labeled.

When in doubt on a commercial service change, pull the POCO infinite bus letter, run the calc to the first panel, and mark it on the dead front. It takes ten minutes and it is what the AHJ wants to see.

Grounding and bonding, why low resistance matters

250.4(A)(5) says the earth shall not be used as the sole equipment grounding path. The reason is Ohm's Law. A ground rod at 25 ohms per 250.53(A)(2) carrying 120V to a fault gives you 120 ÷ 25 = 4.8A, nowhere near enough to trip a 20A breaker. The fault stays energized, the enclosure stays hot, somebody dies.

That is why the equipment grounding conductor sized per 250.122 has to be a low impedance metallic path back to the source. On a 20A circuit, a 12 AWG EGC gives you fractions of an ohm, so a bolted fault pushes hundreds of amps and clears the breaker in one cycle. Run the numbers once and you stop arguing with apprentices about why the green wire matters.

Quick field checks you can do with a meter

Ohm's Law is a troubleshooting tool, not just a design tool. Measure, calculate, compare. If the numbers disagree with the nameplate, the problem is in front of you.

1. Voltage drop test: measure voltage at the panel and at the load under full load. Difference divided by panel voltage is your percent drop.
2. Neutral integrity: on a MWBC, measure line to neutral on both legs. If they swing apart under load, the neutral is loose, per 300.13(B) you cannot depend on a device for continuity.
3. EGC continuity: low range ohms from the farthest device ground screw back to the panel ground bar. Over 1 ohm on a branch circuit means a loose connection or a broken splice.
4. Load verification: clamp amps, multiply by measured voltage, compare to nameplate watts. Off by more than 10%, something is wrong.

Keep the three equations on the inside of your panel schedule cover. The math has not changed since 1827, and the Code is still written around it.