Voltage Drop Calculator

Single-phase voltage drop is VD = 2 × K × I × L ÷ CM: K is 12.9 for copper or 21.2 for aluminum, I is amps, L is one-way feet and CM is circular mils. A 40 A, 240 V load 150 ft away on 8 AWG copper drops 9.4 V (3.9%); 6 AWG brings it under 3%.

How do you calculate voltage drop?

Multiply the current by the resistance of the full circuit path. For a single-phase circuit the current goes out and back, so the length is doubled; for three-phase the factor is √3 (1.732). Resistance comes from the conductor's cross-section in circular mils and the K constant for its metal. Divide the drop by the source voltage for a percentage.

VD = 2 × K × I × L ÷ CM  (single phase)   VD = 1.732 × K × I × L ÷ CM  (three phase)

To find the minimum size for a target instead, rearrange it: CM = 2 × K × I × L ÷ (allowed volts), then pick the next larger conductor in the table. The calculator above does both.

1. 1

   Size for ampacity first.

   Pick the conductor the load and the NEC require (Article 310 ampacity tables, terminal ratings, continuous-load factors). That's the floor.

2. 2

   Measure the one-way run.

   Panel to the load along the actual wire route, not as the crow flies. Don't double it; the formula does that.

3. 3

   Look up circular mils.

   From NEC Chapter 9, Table 8 or the table on this page. 12 AWG is 6,530 CM; 6 AWG is 26,240.

4. 4

   Run the formula.

   VD = 2 × K × I × L ÷ CM. Divide by the source voltage for percent.

5. 5

   Upsize if needed.

   If the drop is above your target (commonly 3% for a branch circuit), step up until it isn't. Check that the larger conductor still fits the terminals and raceway.

What does the NEC say about voltage drop?

For most circuits, the NEC recommends rather than requires a limit. Informational Notes to 210.19(A) (branch circuits) and 215.2(A) (feeders) say that conductors sized to keep voltage drop at the farthest outlet to 3%, with a total of 5% for feeders and branch circuits combined, provide reasonable efficiency of operation. Under NEC 90.5(C), Informational Notes are explanatory material and are not enforceable. Two places do set enforceable limits: 647.4(D) for sensitive electronic equipment (1.5% branch, 2.5% combined) and 695.7 for fire pump motor circuits. Energy codes and local amendments may impose their own limits, so check with the authority having jurisdiction.

Voltage drop by wire size

How far can you run wire before voltage drop is a problem?

Copper vs. aluminum

Aluminum's K of 21.2 is about 64% higher than copper's 12.9, so at the same size it drops about 64% more voltage. Aluminum feeders are usually one or two sizes larger than the copper equivalent for ampacity anyway, which closes most of the gap. Use the calculator to compare the actual sizes you'd install, with listed terminations rated for the metal you choose.

Common voltage drop mistakes

1. Doubling the length yourself. The 2 in the formula already accounts for the return path. Enter the one-way distance.
2. Using breaker size instead of load. Drop depends on the current that actually flows. A 20 A circuit feeding a 6 A load drops a third as much.
3. Treating 3% as code. It's an Informational Note, not a requirement, except where 647.4(D), 695.7 or a local amendment applies. Many designers still hold to it because motors, LEDs and electronics behave better.
4. Sizing for drop before ampacity. Ampacity, terminal temperature ratings and overcurrent protection set the minimum. Drop only ever increases it.
5. Ignoring reactance on big wire. For 2/0 and larger on AC, the Table 9 impedance method is more accurate than the K approximation.
6. Forgetting the feeder. A 2% drop on the feeder plus 3% on the branch is 5% at the receptacle. Check the whole path.