Voltage Drop Calculator
Check conductor voltage drop against the NEC 3% branch-circuit and 5% total recommendations, and find the smallest compliant wire size.
Step-by-step
- Enter valid values to begin.
K = 12.9 (copper) and 21.2 (aluminum) Ω·cmil/ft are the approximate values at 75°C used in NEC informational notes. The AC impedance estimate uses approximate NEC Chapter 9, Table 9 resistance and reactance values (aluminum resistance scaled from copper); use the printed tables for final design.
Voltage Drop: Why the Light at the End of the Run Is Dim
Core Engineering Principles
Copper is a good conductor but it is not a perfect one. Every foot of wire has resistance, so when current flows some of the voltage is used up in the cable before it ever reaches the load. The longer the run and the heavier the current, the more you lose. At the far end a motor runs hot because it draws more current to make up for low voltage, lights look yellow, and electronics reset for no obvious reason. I’ve walked onto plenty of jobs where the “bad” equipment was perfectly fine and the 150-foot run of 12 AWG was the problem.
The shop formula uses circular mils: VD = 2 × K × I × L / CM. K is the resistivity of the metal in ohm-circular-mils per foot (about 12.9 for copper, 21.2 for aluminum at 75°C), L is the one-way distance, and the 2 accounts for the trip out and back. Three-phase swaps the 2 for 1.732. Single-phase and DC use the same formula, which is why the DC solar page uses it too. On AC the wire also has inductive reactance, especially in steel conduit, so the page adds an impedance estimate using your power factor and conduit type. For big conductors the reactance term matters more than resistance.
% drop = VD / Vsource × 100 • K(Cu) = 12.9, K(Al) = 21.2
NEC & Standard References
The NEC doesn’t make voltage drop mandatory for most circuits, which surprises a lot of people. 210.19(A) Informational Note 4 recommends no more than 3% on a branch circuit and 215.2(A) Informational Note 2 recommends 3% on a feeder, with 5% total from the service to the farthest outlet. Informational notes aren’t enforceable, but engineers write them into specs and some jurisdictions adopt them, so treat 3% as the number to hit. Where it is required: fire pump feeders (695.7), and sometimes the manufacturer’s installation instructions for the equipment itself. The resistance and reactance values live in Chapter 9, Table 9, and conductor areas in Table 8.
1. 12 AWG (6,530 CM): VD = 2 × 12.9 × 20 × 100 / 6,530 = 7.90 V = 6.58% — fails.
2. 10 AWG (10,380 CM): VD = 51,600 / 10,380 = 4.97 V = 4.14% — still fails.
3. 8 AWG (16,510 CM): VD = 51,600 / 16,510 = 3.13 V = 2.60% — passes.
The 12 AWG wire is legal for a 20 A breaker, and a compressor on the far end would still struggle to start on 112 V. Go to 8 AWG, and remember the equipment grounding conductor has to go up with it under 250.122(B).
- One-way length only. The formula already doubles it. I see people enter the round-trip distance and wonder why everything is oversized.
- Motors care about starting drop. A motor draws five to seven times its running current for a second or two. Check the drop at that current if it’s a pump or compressor.
- Upsized wire still has to fit the lugs. Check the terminal range on the breaker and equipment before you order, per 110.14(C).