Voltage Drop: Why It Matters and How to Calculate It
Every conductor has resistance, and every foot of wire costs you a little voltage. Here's the working electrician's guide to voltage drop: the formulas, the NEC recommendations, and the field shortcuts — plus our calculator that does it for you.
The 30-second version
Wire has resistance. Current through resistance drops voltage (E = I × R) and makes heat. Too much drop means dim lights, hot motors, nuisance tripping, and equipment that dies young. The NEC's informational notes recommend limiting drop to 3% on a branch circuit or feeder, 5% total from service to the last outlet.
Try the voltage drop calculator to run the numbers instantly.
The formulas
Single-phase:
VD = (2 × K × I × D) / CM
Three-phase:
VD = (1.732 × K × I × D) / CM
Where:
- VD = voltage drop (volts)
- K = resistivity constant — approximately 12.9 for copper, 21.2 for aluminum (ohms-cmil/ft at 75°C)
- I = load current (amps)
- D = one-way circuit length (feet)
- CM = conductor area in circular mils (Chapter 9, Table 8)
Percent drop = VD ÷ source voltage × 100.
Worked example
120 V circuit, 16 A load, 150 ft one-way, #12 copper (6,530 CM):
VD = (2 × 12.9 × 16 × 150) ÷ 6,530 = 9.5 volts — about 7.9%. Way over the recommendation.
Same load on #8 copper (16,510 CM): VD = 3.75 V = 3.1%. That's the difference wire size makes on a long run: two sizes, not one, is the usual answer.
Field rules of thumb
- Every wire size up cuts drop roughly in half per two sizes (CM roughly doubles every two AWG sizes).
- On 120 V, drop hurts twice as fast as on 240 V for the same load and distance — one more reason to run 240 V to outbuildings.
- Size for the actual load, but think about what the circuit will feed in ten years. Nobody ever complained the wire was too big.
- Aluminum needs roughly two sizes larger than copper for the same drop.
Common mistakes
Using round-trip distance in the formula (the 2 and 1.732 already account for it — D is one-way), forgetting that the drop percentage applies at the load's operating voltage, and ignoring the starting current on motor loads at the end of long runs.
Frequently asked questions
Is voltage drop an NEC requirement or a recommendation?
For most branch circuits and feeders it's an informational note (a recommendation of 3% branch / 5% total), not an enforceable requirement — but there are exceptions where it IS mandatory, such as fire pumps and sensitive equipment sections. Some jurisdictions and specs also make it binding. Design to the recommendation regardless; it's cheap insurance for equipment life.
When does voltage drop actually bite in the field?
Long runs are the classic case: well pumps, gate operators, detached garages and shops, parking lot lighting, and EV chargers at the far end of a house. Motors are the most sensitive loads — low voltage means high current, heat, and shortened motor life.
Should I use DC resistance or AC impedance for the calculation?
For typical branch-circuit sizes (below about 1/0) at 60 Hz, DC resistance from Chapter 9 Table 8 is close enough and slightly conservative. For large conductors, conduit type matters (reactance), and Table 9 effective impedance is the better tool.