Crash course: Ohm's Law for electricians top tips edition (part 4)

Ohm's Law in 10 seconds

V = I × R. Voltage equals current times resistance. Rearrange as needed: I = V/R, R = V/I. Power rides alongside: P = V × I, which expands to P = I²R and P = V²/R.

On the truck you do not need a textbook. You need to know which variable you have and which one you are hunting. The rest is arithmetic.

Memorize the wheel once. After that, treat it like a tape measure. Pull it out, take the reading, put it away.

Voltage drop without the calculator dance

NEC 210.19(A) Informational Note 4 recommends keeping branch circuit voltage drop at 3% or less, and 5% combined with the feeder. It is a recommendation, not a hard rule, but inspectors and customers both notice when motors hum or LEDs flicker.

The field shortcut 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 from Chapter 9, Table 8.

• #12 Cu = 6,530 CM
• #10 Cu = 10,380 CM
• #8 Cu = 16,510 CM
• #6 Cu = 26,240 CM

For three phase, swap the 2 for 1.732. Same K, same distance, same conductor table.

Sizing for the load you actually have

Ohm's Law tells you what current a known resistive load will draw at a given voltage. A 1,500W heater on 120V pulls 12.5 amps. Same heater on 240V pulls 6.25 amps. Half the current, quarter the voltage drop loss, because losses scale with I².

This is why service upgrades to 240V equipment pay off on long runs. The math is in your pocket. Use it when a customer asks why you are recommending the bigger wire or the higher voltage option.

If a homeowner is fighting a long run to a detached garage, price out the 240V subpanel option. The wire savings often beat the panel cost on runs over 100 feet.

Reading a meter and trusting the math

Your Fluke does not lie, but it also does not think. If you measure 118V at the panel and 109V at the receptacle under load, that 9V drop is telling you something. Plug the numbers into V = I × R and you can back into the resistance of the run, including every splice and termination.

Loose neutrals show up here. So do undersized conductors that someone "made work." A megger gives you insulation resistance, but a loaded voltage reading gives you the real world story.

1. Measure no load voltage at the source.
2. Measure under full load at the farthest point.
3. Subtract. Divide by the load current. That is your circuit resistance.
4. Compare to the expected resistance from Chapter 9, Table 8. If the measured value is significantly higher, you have a connection problem.

Power, heat, and why terminations fail

P = I²R is the equation that burns down houses. Double the current and you quadruple the heat at every connection. A loose lug with 0.05 ohms of resistance carrying 30 amps is dissipating 45 watts in a space the size of your thumbnail. That is a soldering iron tip clamped to your bus bar.

This is why NEC 110.14(D) requires torque values from the manufacturer's instructions, and why 110.14(B) is picky about splices. The code is not bureaucracy. It is Ohm's Law enforcement.

If a breaker is warm to the touch and the load is within rating, pull it and inspect the stab and the lug. Heat means resistance. Resistance at a connection means failure is coming.

Quick field reference

Keep these ratios in your head. They cover 80% of the residential and light commercial work you will hit in a week.

• 120V, 15A circuit: max continuous load 12A (80% per NEC 210.19 and 210.20)
• 120V, 20A circuit: max continuous 16A
• 240V, 30A dryer circuit: 7,200W max
• 240V, 50A range circuit: 12,000W max
• 1 horsepower at 240V single phase: roughly 5A from NEC Table 430.248

When the numbers in front of you do not match these, stop and recheck. Either the nameplate is wrong, the circuit is wrong, or someone before you took a shortcut. Ohm's Law does not negotiate.

Carry the wheel, trust your meter, and respect the heat. That is the whole job.