The voltage drop formula
This calculator uses the K method taught by Mike Holt and the IAEI, the same one used on electrical exams:
Single-phase and DC: VD = 2 × K × I × L ÷ CM
Three-phase: VD = 1.732 × K × I × L ÷ CM
K is the resistance of a circular-mil foot of wire: 12.9 for copper and 21.2 for aluminum, at 75 °C. I is the load in amps, L the one-way length in feet and CM the wire's area in circular mils.
Example: 20 A at 120 V, single-phase, 150 ft of copper. CM needed for 3% (3.6 V) = 2 × 12.9 × 20 × 150 ÷ 3.6 = 21,500 cmil. The smallest size at least that big is 6 AWG (26,240 cmil), which drops 2.95 V, or 2.46%.
How much voltage drop is allowed?
The NEC's informational notes recommend no more than 3% on a branch circuit or feeder and 5% in total from the service to the farthest outlet. They are recommendations, not code requirements, but most designers and many specs hold to them. Some circuits, such as fire pumps and sensitive electronic equipment, have their own mandatory limits.
Things this calculator checks for you
It never suggests wire smaller than 14 AWG copper or 12 AWG aluminum, the smallest allowed for building wire, and parallel sets are only sized at 1/0 and larger. If the wire is upsized for voltage drop, the equipment grounding conductor has to be upsized in the same proportion (NEC 250.122(B)). On AC runs of 2/0 and larger, the real drop is a little higher than this method shows, because it leaves out reactance.
Voltage drop is only one check. The wire must also be rated for the load current and protected by the right breaker.
Circular mils by wire size
| Size | Circular mils |
|---|---|
| 14 AWG | 4,110 |
| 12 AWG | 6,530 |
| 10 AWG | 10,380 |
| 8 AWG | 16,510 |
| 6 AWG | 26,240 |
| 4 AWG | 41,740 |
| 2 AWG | 66,360 |
| 1/0 | 105,600 |
| 2/0 | 133,100 |
| 4/0 | 211,600 |
Standard areas for each size (ASTM B258). kcmil sizes are their own value: 250 kcmil = 250,000 cmil.
Voltage drop, wire size, amps and conduit bending in one app, with no signal.
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