Quick reference for calculating voltage drop

Why voltage drop matters on the job

Voltage drop isn't a code violation in most cases, it's a performance issue that becomes a callback. The NEC treats branch circuit and feeder voltage drop as a recommendation, not a requirement. See the Informational Notes at 210.19(A) for branch circuits and 215.2(A)(2) for feeders: 3% on the branch, 5% combined feeder and branch.

Sensitive loads, long runs, and motor starting are where it bites. Undersized conductors on a 200 foot run feeding a shop compressor will trip overloads, cook contactors, and leave you explaining to the GC why the motor won't pull. Size it right the first time.

Field tip: if the run is over 100 feet and the load is over 80% of the breaker rating, run the numbers before you pull wire. Copper is cheaper than a return trip.

The two formulas you actually use

Forget the academic derivations. Two formulas cover 95% of what you'll size in the field. Both assume a balanced load and use the resistance method, which is accurate enough for runs where reactance isn't dominant (most branch circuits and short feeders).

Single phase: VD = (2 × K × I × D) / CM

Three phase: VD = (1.732 × K × I × D) / CM

• K = 12.9 for copper, 21.2 for aluminum (ohms per circular mil foot at 75°C)
• I = load current in amps
• D = one-way distance in feet
• CM = circular mils of the conductor (see Chapter 9, Table 8)

Solve for CM when you're sizing the conductor: CM = (2 × K × I × D) / VD for single phase, swap 2 for 1.732 for three phase.

Circular mils you should memorize

You'll reference Chapter 9, Table 8 for exact values, but knowing a handful from memory speeds up rough sizing on the truck. These are the uncoated copper values.

1. 14 AWG: 4,110 CM
2. 12 AWG: 6,530 CM
3. 10 AWG: 10,380 CM
4. 8 AWG: 16,510 CM
5. 6 AWG: 26,240 CM
6. 4 AWG: 41,740 CM
7. 2 AWG: 66,360 CM
8. 1/0: 105,600 CM
9. 4/0: 211,600 CM
10. 250 kcmil: 250,000 CM

For kcmil sizes, multiply the number by 1,000. A 350 kcmil conductor is 350,000 circular mils. Simple.

Worked example: 120V branch circuit

You're feeding a 16 amp continuous load on a 20 amp circuit, 150 feet one-way, 120V single phase, copper. Target 3% drop, which is 3.6 volts.

CM = (2 × 12.9 × 16 × 150) / 3.6 = 17,200 circular mils. That's bigger than 10 AWG (10,380 CM) and just over 8 AWG (16,510 CM). Round up. Pull 8 AWG copper, land it on a 20 amp breaker per 240.4(D) exceptions and the terminal ratings on the device. Don't forget the grounded conductor sizing per 250.122 if you're upsizing for voltage drop, see 250.122(B).

Field tip: 250.122(B) requires the equipment grounding conductor to be increased proportionally when you upsize ungrounded conductors for voltage drop. Miss this and you're failing inspection.

Worked example: 480V three-phase feeder

Feeder to a rooftop unit, 60 amp load, 280 feet, 480V three phase, copper. Customer wants to keep total drop under 3%, which is 14.4 volts. Assume the branch circuits downstream are short, so you can spend most of that budget on the feeder. Budget 2.5% here, or 12 volts.

CM = (1.732 × 12.9 × 60 × 280) / 12 = 31,268 circular mils. That's between 6 AWG (26,240) and 4 AWG (41,740). Pull 4 AWG copper THHN, protected per 240.4 and the RTU nameplate MOCP. Ampacity at 75°C from Table 310.16 is 85 amps, comfortably above the 60 amp load.

If the client is cost sensitive, aluminum is an option. Recalculate with K = 21.2 and you'll need roughly 51,400 CM, which lands you at 2 AWG aluminum. Check terminal ratings on both ends before you commit.

Common field mistakes

The top four voltage drop errors that cause callbacks or inspection failures:

• Using total run length instead of one-way distance in the 2× formula. The 2 already accounts for the return path.
• Forgetting to upsize the EGC per 250.122(B) when conductors are increased for voltage drop.
• Ignoring motor inrush. Locked rotor current can be 6 to 8 times FLA; excessive drop during start will drop out contactors.
• Applying the 3% and 5% NEC recommendations as if they were mandatory. They're not, unless adopted locally or required by equipment specs. Check the job specs and AHJ.

When you're unsure, run the numbers both directions: calculate the drop for the conductor you planned to pull, and calculate the minimum CM for your target drop. If the planned conductor falls short, go up one size and move on.