Crash course: Ohm's Law for electricians code-compliant version (part 4)

Why Ohm's Law still runs your day

Every load calc, every voltage drop check, every breaker sizing decision traces back to V = I × R. You already use it, often without writing it down. The code doesn't spell out Ohm's Law, but NEC tables assume you know it cold.

Part 4 ties the math to what the inspector actually checks: conductor sizing under 310.16, voltage drop guidance in 210.19(A) Informational Note No. 4, and continuous load rules in 210.20(A). Get the numbers right on paper and the rough-in goes faster.

The three equations you use in the field

Memorize these and the rest is algebra:

• V = I × R ... volts equal amps times ohms.
• P = V × I ... watts equal volts times amps (single phase, unity power factor).
• P = I² × R ... heat dissipated in a conductor or resistive load.

For 3-phase, multiply by 1.732 (square root of 3): P = V × I × 1.732 × PF. Keep a calculator handy or use the wheel, but know which variable you're solving for before you punch buttons.

Resistance in a conductor is not a fixed nameplate value. It climbs with temperature and length. Chapter 9, Table 8 gives DC resistance per 1000 ft at 75°C, and Table 9 handles AC impedance. For most branch-circuit voltage drop work, Table 9 is the right column.

Voltage drop, the code-adjacent calculation

NEC doesn't mandate a voltage drop limit for most installations, but 210.19(A) Informational Note No. 4 and 215.2(A)(1) Informational Note No. 2 recommend 3% on the branch circuit, 5% total feeder plus branch. Article 647.4(D) does enforce 1.5% for sensitive electronic equipment on separately derived systems, so read the scope before you shrug it off.

Single-phase voltage drop formula:

• VD = (2 × K × I × D) / CM
• K = 12.9 for copper, 21.2 for aluminum (ohms-cmil/ft at 75°C)
• D = one-way distance in feet
• CM = circular mils of the conductor (Chapter 9, Table 8)

For 3-phase, swap the 2 for 1.732. Run the number before you pull wire, not after. Upsizing from #12 to #10 on a 120 ft run is a five minute decision at the supply house and a four hour rework in the attic.

Field tip: if the homeowner's well pump is 240V at 150 ft from the panel, start at #10 and check the drop. Nine times out of ten on a 10 to 15 amp pump you're fine, but on the tenth job you'll save the motor windings.

Sizing conductors with the heat equation

P = I² × R is why ampacity tables exist. Double the current, quadruple the heat. That's also why 310.15(C)(1) derates for more than three current-carrying conductors in a raceway, and why 310.15(B) adjusts for ambient temperature above 30°C.

Walk through a real example. A 40 amp continuous load on THHN copper:

1. 210.20(A) says size the OCPD at 125% of continuous load: 40 × 1.25 = 50 amp breaker.
2. Conductor must carry at least 50 amps at the 75°C column per 110.14(C)(1)(b) for terminations rated over 100 amps, or the 60°C column for most residential under 100 amps.
3. Table 310.16, 75°C column: #8 THHN copper is rated 50 amps. Done, unless derating kicks in.

If you've got six current-carrying conductors in the pipe, 310.15(C)(1) drops you to 80%. Now #8 at 50 × 0.80 = 40 amps, which fails. Bump to #6. The math tells you before the inspector does.

Motor and transformer shortcuts

Motors don't follow branch-circuit rules. Article 430 uses Table 430.250 for FLC, not nameplate, for conductor and overload sizing. But Ohm's Law still tells you starting inrush ... typically 6 to 8 times FLC, which is why 430.52 allows inverse-time breakers up to 250% of FLC.

For single-phase transformer secondary current: I = VA / V. A 25 kVA 120/240 transformer delivers 25000 / 240 = 104 amps on the 240V secondary. Size secondary conductors per 450.3(B) and 240.21(C).

Field tip: when you're troubleshooting a tripping breaker and the load is rated 16 amps on a 20 amp circuit, check actual draw with a clamp meter. Motors with worn bearings pull more current as R changes. Ohm's Law catches what the nameplate hides.

Common mistakes that cost money

Three that show up on inspection reports and service calls:

• Using 90°C ampacity for the final sizing. 110.14(C) ties you to the terminal rating, almost always 60°C or 75°C. The 90°C column is only for derating math.
• Ignoring voltage drop on long runs, then wondering why LED drivers chatter or well pumps short-cycle.
• Forgetting the 125% continuous-load multiplier on both the OCPD (210.20(A)) AND the conductor (210.19(A)(1)).

Ohm's Law is not a classroom exercise. It's the reason your terminations don't cook, your breakers hold, and your drops land within spec. Run the numbers, cite the article, move on to the next rough-in.