Crash course: Ohm's Law for electricians what journeymen forget (part 2)

Ohm's Law on the Job: The Real Failure Modes

Part 1 covered V=IR and P=IE. Part 2 is what journeymen forget after a decade in the field: the second-order effects that turn a "clean" calculation into a callback. Voltage drop, temperature derating, and the way real loads behave nothing like the nameplate.

The math is trivial. The habit of applying it before you pull wire is what separates a clean rough-in from a warranty call.

Voltage Drop Is Ohm's Law in Disguise

NEC 210.19(A) Informational Note 4 recommends no more than 3% drop on branch circuits, 5% combined feeder and branch. It is a recommendation, not a requirement, but fail it and your motors run hot, your LEDs flicker, and your resistive heaters underperform.

Voltage drop is just V=IR applied to the conductor itself. The wire is a resistor. Push current through it and you lose voltage across it. The longer the run, the smaller the conductor, the bigger the loss.

• Single phase: VD = 2 x K x I x D / CM
• Three phase: VD = 1.732 x K x I x D / CM
• K = 12.9 for copper, 21.2 for aluminum (approximate, at 75C)
• D = one-way distance in feet
• CM = circular mils from Chapter 9 Table 8

A 20A circuit at 120V running 150 feet on #12 copper drops about 5.8V, roughly 4.8%. Out of spec before the breaker even trips. Bump to #10 and you are back under 3%.

Temperature Changes Resistance, Which Changes Everything

Copper resistance rises roughly 0.4% per degree C. That 75C ampacity column in NEC Table 310.16 is not arbitrary. Load a conductor to its rated current in a 40C ambient attic and the conductor itself heats further, resistance climbs, I squared R losses climb, and the insulation ages faster than the spec sheet suggests.

This is why NEC 310.15(B) ambient correction factors exist, and why 310.15(C)(1) conduit fill adjustments matter. Nine current carrying conductors in a pipe on a hot roof is not the same circuit you designed at the desk.

Tip: if you can't hold your hand on the raceway for five seconds, your load calc missed something. Pull an amp reading and compare to the corrected ampacity, not the table value.

Power Factor: Why Your Clamp Meter Lies

P=IE works cleanly for resistive loads. For inductive loads, motors, ballasts, VFDs, transformers, it breaks down. Apparent power (VA) is what the conductor carries. Real power (watts) is what the load consumes. The ratio is power factor.

A 10 HP motor at 240V single phase is not 7460W / 240V = 31A. Nameplate FLA from NEC Table 430.248 puts it at 50A. The difference is power factor and motor efficiency, both baked into the table. NEC 430.6(A)(1) is explicit: use the tables for conductor sizing, not the nameplate, not the calculator.

• Resistive load (heater, incandescent): PF ~ 1.0, math works clean
• Motor under load: PF 0.8 to 0.9
• Lightly loaded motor: PF can drop below 0.5
• Modern LED driver: PF usually 0.9+ by design

Series vs Parallel: Where Techs Get Burned

Loose neutrals on a multiwire branch circuit are the classic trap. A shared neutral carries the imbalance between two hots. Lose that neutral and the two loads end up in series across 240V, and Ohm's Law does exactly what it is supposed to do. The smaller load sees the larger share of the voltage.

A 100W bulb and a 1500W space heater on opposite legs, neutral open: the bulb sees roughly 230V and pops. The heater sees about 10V and barely warms. NEC 210.4(B) requires simultaneous disconnect for this reason, and 300.13(B) prohibits depending on device terminals for neutral continuity in multiwire branch circuits.

Tip: when troubleshooting bizarre voltages on a MWBC (one outlet reading 180V, another reading 60V) stop chasing the hots. The neutral is open somewhere upstream. Find it before something expensive dies.

Short Circuit Current: Ohm's Law at Fault

Available fault current is V divided by total impedance from source to fault. At a 25 kVA 120/240V transformer with 2% impedance, secondary fault current is roughly 25000 / 240 / 0.02, about 5200A. Add service conductor impedance and it drops, but the principle holds.

NEC 110.9 requires interrupting rating equal to or greater than available fault current. NEC 110.10 requires the components in the circuit to withstand the let-through energy. A 10 kAIC breaker on a 22 kA service is a bomb waiting for a bolted fault.

Run the calc, or pull it from the utility. Do not assume the last guy sized the gear correctly, especially on service upgrades where the transformer got bigger but the panel did not.

The Takeaway

Ohm's Law does not stop at V=IR. It shows up in every voltage drop complaint, every nuisance trip, every open neutral, every underrated breaker. The journeymen who stay sharp are the ones who still do the arithmetic before they pull the wire, not after the inspector red tags it.