Crash course: Ohm's Law for electricians contractor's perspective (part 2)

Pick up where Part 1 left off

Part 1 covered the basics: V = I × R, the units, and why a loose neutral cooks insulation. Part 2 is the contractor's view. How Ohm's Law shows up on the bid sheet, in the panel, and on the truck when something is wrong.

You do not need to derive equations on a job site. You need to know which version of the formula to grab, when to use the power triangle instead, and where the code forces your hand.

The three forms you actually use

Memorize these. Everything else is algebra you can do on a phone.

• V = I × R ... voltage drop across a conductor or load.
• P = I × V ... wattage on a single-phase circuit.
• P = I² × R ... heat dissipated in a conductor or termination.

That last one is the troubleshooting form. Loose lugs, undersized conductors, and corroded splices all fail because I²R heat scales with the square of the current. Double the load on a marginal connection and you quadruple the heat. That is why a 40 amp draw on a tired 50 amp breaker still smells like burnt phenolic.

For three-phase, swap to P = I × V × 1.732 × PF. Power factor matters once motors enter the picture. Resistive loads (heat strip, incandescent, water heater elements) run at PF 1.0, so you can ignore it. Motors and electronic ballasts do not.

Voltage drop on the bid

NEC 210.19(A) Informational Note 4 and 215.2(A)(1) Informational Note 2 recommend a 3 percent drop on branch circuits and 5 percent total feeder plus branch. It is not mandatory in most cases, but the AHJ on commercial work will hold you to it, and the engineer's spec almost always will.

The field formula for single-phase, copper:

VD = (2 × K × I × D) ÷ CM. K = 12.9 for copper, 21.2 for aluminum. D is one-way distance in feet. CM is circular mils from Chapter 9 Table 8.

Run the math before you pull wire on long runs. A 200 foot 20 amp circuit on #12 copper drops about 6.4 volts at 120 V, which is over 5 percent. Bump to #10 and you are back inside spec. Catching that on the bid is the difference between profit and a callback rewire.

Sizing for heat, not just ampacity

NEC 310.16 gives you the ampacity table. Ohm's Law tells you why the table exists. Conductor resistance times current squared equals watts of heat per foot. The insulation rating (60, 75, 90 degree C) is the ceiling.

1. Pull the load in amps.
2. Apply 125 percent for continuous load per 210.19(A)(1) and 215.2(A)(1).
3. Apply ambient and bundling derates from 310.15(B).
4. Pick the conductor from 310.16 at the termination temperature rating, usually 75 C per 110.14(C).

Skip step 3 on a rooftop conduit run and you will trip breakers in August. The conductor is doing exactly what Ohm's Law predicts: more resistance with temperature, more drop, more heat, thermal runaway at the weak point.

Troubleshooting with a meter

A clamp meter and a DMM are an Ohm's Law calculator. Three readings tell you almost everything.

• Voltage at the panel vs. at the load. Difference is your drop. Drop ÷ current = circuit resistance.
• Current under load. Compare to nameplate. Higher than expected means lower resistance somewhere, often a partial short or a failing motor winding.
• Resistance with the circuit dead and isolated. Compare splice to splice. A hot lug will read measurably higher than its neighbor.

For a 240 V dryer pulling 24 amps, expect roughly 10 ohms of effective load resistance. If you read 30 volts of drop from panel to receptacle, that is 1.25 ohms of wire and connection resistance burning 720 watts of heat somewhere it should not be. Find it before the customer does.

Tip from the truck: when a breaker trips and resets cold but trips again under load, measure voltage drop across the breaker itself with the load running. More than half a volt across the breaker contacts means the breaker is the problem, not the circuit.

Where the code locks it in

Ohm's Law is physics. The NEC turns physics into enforceable minimums. A few articles worth memorizing:

• 110.14(C) ... termination temperature ratings, the cap on your conductor selection.
• 210.19, 210.20, 215.2, 215.3 ... branch and feeder sizing, including the 125 percent continuous rule.
• 240.4 ... overcurrent protection matched to conductor ampacity.
• 310.15, 310.16 ... ampacity, derates, and the actual table you size from.
• Chapter 9, Table 8 and Table 9 ... conductor properties for voltage drop calculations.

Get those five into muscle memory and most sizing arguments end before they start. The inspector is not going to debate Ohm's Law with you. He is going to point at the article.

Field summary

V = I × R sizes the wire. P = I² × R finds the failure. The NEC sets the floor. Everything else on the truck is just tools to measure what the math already told you.

Part 3 will get into three-phase, power factor, and why a 480 V motor draws less current than the 240 V version of the same horsepower. Until then, run the voltage drop before you pull, and clamp the load before you blame the breaker.