Crash course: Ohm's Law for electricians what journeymen forget (part 5)

Ohm's Law is not a classroom trick

Every journeyman memorized V=IR for the test, then stopped using it the day after. That is the mistake. Ohm's Law tells you why a 14 AWG extension cord melts at 20 amps, why your voltage drop pushes a motor into nuisance trips, and why that buzzing ballast is pulling more current than the nameplate says. It is not theory. It is the math behind every callback you have ever made.

Part 5 of this crash course is about the stuff that gets forgotten between apprentice school and year ten in the field. The formulas are simple. The application is where guys get burned, sometimes literally.

The wheel, and the one line that matters

You learned the Ohm's Law wheel: V=IR, P=VI, P=I²R, P=V²/R. Fine. In the field, the one you reach for is P=I²R, because heat is what kills conductors, terminations, and breakers. Current squared, times resistance, equals watts of heat dumped into whatever you just wired.

Double the current, quadruple the heat. That is why a loose lug on a 200 amp feeder does not just run warm, it runs hot enough to carbonize the insulation and arc across the enclosure. A 0.01 ohm connection at 100 amps dissipates 100 watts in a spot the size of a dime.

• V=IR for sizing and voltage drop
• P=I²R for heat, terminations, and why things fail
• P=VI for load calcs and verifying nameplate data
• R=V/I for troubleshooting a suspect circuit

Voltage drop: the calc nobody runs until the inspector asks

NEC 210.19(A) Informational Note 4 recommends branch circuit voltage drop not exceed 3 percent, with total drop (feeder plus branch) capped at 5 percent. It is a recommendation, not a rule, but the engineer on the job has likely made it a spec. Run the numbers before you pull wire on a long run.

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 circular mils from NEC Chapter 9 Table 8. For three phase, swap the 2 for 1.732. Do the math at the load current, not the breaker size.

Tip: on any home run over 100 feet at 20 amps, upsize to 10 AWG by default. You will spend an extra twenty bucks in copper and save yourself a return trip when the receptacle at the end reads 108 volts.

Resistance is not just the wire

Apprentices think of resistance as a property of the conductor. Journeymen know the real resistance in a circuit lives at the terminations. A properly torqued lug is milliohms. A backstabbed receptacle that has been cycled a thousand times? Tenths of an ohm. Run P=I²R on that and you see why backstabs start fires.

NEC 110.14(D) now requires terminations to be torqued to manufacturer spec, and that spec is usually printed on the device or in the instructions. Most guys eyeball it. A calibrated torque screwdriver costs less than one callback.

1. Strip to the gauge mark, not by feel
2. Torque to spec, in inch pounds, not "tight enough"
3. Re-torque aluminum feeders per the lug manufacturer's schedule
4. Thermal scan panels under load if you own the IR camera

Motor loads lie to you

A motor nameplate gives you full load amps at rated voltage. Drop the voltage 10 percent and current climbs to hold torque, because P=VI and the load does not care what the meter reads. That is why brownouts cook motors. Ohm's Law, running in reverse, dumping heat into the windings.

NEC 430.22 sizes branch conductors at 125 percent of motor FLA for continuous duty. That margin exists because real world voltage sags, harmonics, and starting currents push the steady state number around. Size to the code, but understand why the code is written that way.

Tip: if a motor is tripping on overload and the nameplate FLA looks normal on your clamp meter, check voltage at the motor terminals under load. Nine times out of ten you are feeding it 208 on a 230 volt motor, and the current is running high to compensate.

Troubleshooting with the formula, not the parts cannon

A circuit that reads 120 volts open and 90 volts under load has a resistance problem. R=V/I on the voltage drop tells you where. Thirty volts lost at 15 amps is 2 ohms of unwanted resistance somewhere between the panel and the load. That is a bad splice, a corroded neutral, or a loose breaker lug. It is not the fixture.

Guys throw parts at problems because it feels like progress. The meter and the math find the fault in ten minutes. A clamp meter, a multimeter, and the four formulas above solve 80 percent of residential and light commercial service calls without pulling a single device.

What to keep in your head

You do not need to memorize the wheel. You need to know which formula answers which question. Heat problem, reach for P=I²R. Long run, reach for VD. Motor acting up, reach for P=VI and check voltage under load. Dead circuit with weird readings, R=V/I.

The journeymen who forget Ohm's Law are the ones still guessing at year fifteen. The ones who kept it sharp are the ones the foreman sends when the apprentice cannot figure out why the panel is warm. Be the second guy.