Crash course: Ohm's Law for electricians what journeymen forget (part 3)

Why Part 3 Exists

Parts 1 and 2 covered the triangle, the math, and voltage drop on branch circuits. Part 3 is the stuff that bites you on the job: the places where Ohm's Law quietly governs a decision you thought was a code lookup. If you have been in the trade five years or twenty, these are the spots where muscle memory replaces the math, and muscle memory gets it wrong.

Keep a calculator on your phone. Keep the formulas on your brain.

Voltage Drop Is Not Just 3 Percent

Every journeyman remembers the 3 percent branch, 5 percent total FPN from NEC 210.19(A) Informational Note No. 4 and 215.2(A)(1) Informational Note No. 2. What gets forgotten: those are recommendations, not mandates, except where the Code specifically requires a calculation, like fire pumps (NEC 695.7) or sensitive electronic equipment per the manufacturer.

The real trap is assuming the run is short enough to skip the math. A 120V, 20A kitchen circuit pulling 16A continuous through 90 feet of #12 copper is already sitting around 4.3 percent drop one-way at the receptacle. Add a long tap, a loose backstab, and you are cooking the load.

• VD = (2 × K × I × L) / CM, single phase
• K = 12.9 for copper, 21.2 for aluminum at 75°C
• CM for #12 Cu = 6,530; #10 Cu = 10,380
• Three phase: multiply by 1.732 / 2, or use (√3 × K × I × L) / CM

Temperature Changes Resistance, and Resistance Changes Everything

Conductor resistance climbs roughly 0.4 percent per degree C above 20°C for copper. A conductor sized at 75°C ampacity runs hotter under full load, which raises resistance, which raises voltage drop, which raises heat. Chapter 9 Table 8 gives you DC resistance at 75°C. Table 9 gives AC resistance and reactance at 75°C for typical raceway fill. Use Table 9 for anything over 100 feet or above 100A.

This is why NEC 110.14(C) terminations matter so much. A 90°C conductor terminated on a 75°C lug is limited by the lug, not the wire. Ignore that and your "derated" calculation is fiction.

Field tip: if a lug feels warm enough to make you pull your hand back, you are losing real voltage across that connection. Shut it down, torque to the label, and re-check with a clamp meter under load.

Ground Fault Current: The Math Nobody Runs

NEC 250.4(A)(5) requires the equipment grounding conductor path to be low impedance enough to facilitate the operation of the overcurrent device. That is Ohm's Law, not a vibe. If your available fault current at the panel is 10,000A and the return path through the EGC, conduit, and bonding has 2 ohms of impedance on a 240V circuit, you get 120A of fault current. Your 20A breaker sees that as a 6x overload, not a short circuit. It might trip in 10 seconds. It might not.

Run the numbers on long EMT runs, long feeders to detached structures (NEC 250.32), and anything with flexible metal conduit used as the EGC under NEC 250.118(6). A separate copper EGC is cheap insurance.

1. Get available fault current from the POCO or the transformer nameplate
2. Calculate circuit impedance, line and return
3. Divide voltage by impedance for fault current magnitude
4. Cross-check against the breaker's time-current curve

Continuous Loads and the 80 Percent Rule

NEC 210.19(A)(1) and 215.2(A)(1) require conductors to be sized at 125 percent of continuous load plus 100 percent of non-continuous. That is not Ohm's Law directly, but Ohm's Law is why the rule exists: sustained current times resistance equals sustained heat, and heat degrades insulation over time.

The forgotten piece: the breaker also has to be rated for 125 percent unless listed for 100 percent continuous per NEC 210.20(A). And the terminations get hot too. A 100A continuous load on a 100A breaker with standard terminations is a callback waiting to happen.

Field tip: if a panel schedule shows loads that run more than three hours, treat the whole circuit as continuous even if the spec sheet is ambiguous. Signage, EVSE (NEC 625.41), and commercial lighting almost always qualify.

Parallel Conductors and Series Mistakes

NEC 310.10(G) lets you parallel conductors 1/0 AWG and larger. Ohm's Law says two identical conductors in parallel have half the resistance, so they split the current evenly. But only if they are identical in length, material, CSA, insulation type, and termination. One run six inches longer than the other, and the shorter run carries more current. Under fault conditions, that imbalance can be dramatic.

Series resistance matters on the small end too. A loose neutral at a splice adds resistance only on the neutral, unbalancing a multiwire branch circuit (NEC 210.4) and sending 240V across 120V loads when the neutral opens. Ohm's Law on a broken neutral is how you burn out every plug-in device in a kitchen.

What To Carry On The Truck

Memorize the four forms: V = IR, P = VI, P = I²R, P = V²/R. Keep Chapter 9 Table 8 and Table 9 bookmarked. Keep a voltage drop calculator that handles three phase and parallel sets. And never trust a number you did not either measure or calculate.

Part 4 will get into power factor, harmonic currents on the neutral, and why the 1.732 in three-phase math is not optional even when the load looks balanced.