Crash course: Ohm's Law for electricians contractor's perspective (part 1)

Ohm's Law: the only formula you actually need

V = I × R. Voltage equals current times resistance. Memorize it, because every load calc, voltage drop check, and troubleshooting call traces back to it. If you can rearrange this one equation, you can solve 80% of the math you'll hit on a job site.

Three forms, same law:

• V = I × R (find voltage)
• I = V / R (find current)
• R = V / I (find resistance)

Power ties in through the wheel: P = V × I. From there you can derive P = I²R and P = V²/R. That's the full toolkit. Anything fancier is just algebra on top of these.

What it actually means in the field

Voltage is pressure. Current is flow. Resistance is anything that pushes back, conductors, connections, loads, corroded lugs, bad splices. When a circuit misbehaves, one of those three changed. Your job is figuring out which one.

Example: 240V circuit feeding a 12 ohm heating element. I = 240 / 12 = 20A. Power draw is 240 × 20 = 4800W. Now you know why it's on a 30A breaker with #10 copper and not #12. The math drove the install, not the other way around.

Tip: if a load is pulling more amps than the nameplate says, resistance dropped somewhere it shouldn't have. Think shorted turns in a motor winding or a partial ground fault, not a "bad breaker."

Voltage drop: where Ohm's Law pays your bills

NEC 210.19(A) Informational Note 4 recommends branch circuit voltage drop not exceed 3%, with combined feeder and branch not exceeding 5%. It's not a hard code rule for most installs, but inspectors and engineers will hold you to it on plan-and-spec work, and motors and electronics will punish you if you ignore it.

The field 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 conductor's circular mils from Chapter 9, Table 8. For three phase, swap the 2 for 1.732.

1. Pull the load in amps from the nameplate or calc.
2. Measure the one-way run.
3. Plug it in, check the drop against your 3% target.
4. If it fails, upsize the conductor and recheck. NEC 250.122(B) says you upsize the EGC proportionally.

Troubleshooting with Ohm's Law

Every weird call on a job is an Ohm's Law problem in disguise. Lights dimming when the AC kicks on? Voltage sagging because inrush current spiked across the resistance of an undersized feeder. Breaker tripping on a known-good circuit? Resistance changed, usually toward zero, somewhere in the load or wiring.

Use your meter to verify, not guess. Read voltage at the panel, then at the device under load. The difference is your drop across the conductor. Divide that drop by the measured current and you've got the actual resistance of the run, splices and all. Compare it to the theoretical resistance from Table 8 and you'll know if you've got a high-resistance connection hiding somewhere.

Tip: a hot lug or warm splice is almost always a resistance problem. Power dissipated as heat is I²R. Double the current, quadruple the heat. That's why loose neutrals burn houses down.

Sizing loads and breakers

NEC 210.20(A) requires overcurrent protection sized for 125% of continuous loads plus 100% of non-continuous. Ohm's Law gets you the load current in the first place. Nameplate watts divided by nameplate volts equals amps. Then apply the multipliers.

For motors, NEC 430.6(A)(1) says use the FLC tables in 430.247 through 430.250, not the nameplate, for conductor and OCPD sizing. Ohm's Law still drives the underlying behavior, but the code intentionally uses table values to standardize the math across manufacturers.

• Resistive loads (heat, incandescent): use V = I × R directly.
• Motor loads: use NEC tables for sizing, Ohm's Law for diagnosis.
• Electronic loads: nameplate watts and power factor matter more than raw resistance.

Common mistakes that cost you money

Guys mix up series and parallel resistance and miscalculate total load. In series, resistances add. In parallel, the reciprocal of the total equals the sum of the reciprocals, so total resistance is always less than the smallest branch. Two 10 ohm loads in parallel pull double the current of one, not half.

The other big one is ignoring temperature. Conductor resistance climbs with heat, roughly 0.4% per degree C for copper. A run that calcs fine at 25°C ambient can fail voltage drop in a 40°C attic. NEC 310.15(B) ambient correction factors handle the ampacity side, but voltage drop is on you to verify in real conditions.

Part 2 will cover three phase math, power factor, and how Ohm's Law behaves on the secondary side of a transformer. Keep your calculator and your Chapter 9 tables handy.