Voltage drop calculator
Enter the wire size, the one-way length, the load and the source voltage. You get the drop in volts and percent, the voltage at the load, and the longest run that stays under 3 %.
- Copper or aluminium (aluminum in the US)
- Single-phase or three-phase
- 14 AWG to 500 kcmil
Drag the bar and the length changes to match, so you can read off the longest run for any drop.
The voltage drop formula
The calculator uses the standard formula for the resistance of a conductor, the same one the app uses.
VD = m × K × I × L ÷ CM
- VD
- Voltage drop, in volts
- m
- 2 for single-phase, 1.732 (√3) for three-phase
- K
- The metal's resistance in Ω·cmil/ft: 12.9 for copper and 21.2 for aluminium, at 75 °C
- I
- Load current, in amps
- L
- One-way length of the run, in feet
- CM
- Conductor area in circular mils. 12 AWG is 6,530 and 10 AWG is 10,380.
Percent drop is VD ÷ source voltage × 100.
Worked example
A 24 A load on 240 V single-phase, 140 ft from the panel, on 10 AWG copper:
2 × 12.9 × 24 × 140 ÷ 10,380 = 8.35 V
That's 8.35 ÷ 240 = 3.48 %, a little over 3 %, and the load sees 231.6 V. To stay under 3 %, keep the run to about 120 ft, or go up to 8 AWG, which drops 5.25 V (2.19 %).
How far can you run each wire size?
Roughly the longest one-way run at a 3 % drop, for copper on a single-phase circuit carrying the full load shown. The calculator gives the exact figure for your job.
| Wire | Load | 120 V | 240 V |
|---|---|---|---|
| 14 AWG | 15 A | 38 ft | 76 ft |
| 12 AWG | 20 A | 46 ft | 91 ft |
| 10 AWG | 30 A | 48 ft | 97 ft |
| 8 AWG | 40 A | 58 ft | 115 ft |
| 6 AWG | 50 A | 73 ft | 146 ft |
| 4 AWG | 70 A | 83 ft | 166 ft |
| 2 AWG | 100 A | 93 ft | 185 ft |
- Half the load, twice the distance. At 80 % of the load shown, the run can be 25 % longer.
- Aluminium of the same size reaches the same drop in about 60 % of the distance.
- Three-phase goes about 15 % farther than single-phase at the same current.
- These are voltage drop limits only. They don't tell you a wire's ampacity, so size the conductor for the load and the breaker under your local code first.
Questions
How far can you run 12 AWG on a 20 A circuit?
About 46 ft one way on 120 V with the full 20 A, or 91 ft on 240 V, at a 3 % drop. At a 16 A load it's about 57 ft and 114 ft. Put your own numbers in the calculator for anything else.
What is an acceptable voltage drop?
A common target is 3 % on a branch circuit and 5 % in total from the service to the load. Some codes make that a rule and others only recommend it, so check yours.
Should I use the breaker size or the actual load?
Use the current the circuit will really carry. Using the breaker rating gives you the worst case, which is the safe choice when you don't know the load yet.
Why does it ask for the one-way length?
Current goes out on one conductor and back on the other, and the formula already allows for that. On single-phase that's the 2 in the formula. Enter the distance from the panel to the load.
How do I reduce voltage drop?
Use a bigger conductor, shorten the route, or run at a higher voltage. Moving a load from 120 V to 240 V halves the current for the same power. You can also split the load across more circuits.
Does temperature change the result?
A little. K = 12.9 and 21.2 are for conductors at 75 °C. A cooler conductor has less resistance, so the real drop is usually a bit lower than this estimate.
Is this based on the electrical code?
No. It's the physics formula, shown above. It doesn't check ampacity, conduit fill or code limits, so check your answer against your local code, the nameplate and manufacturer data.
Do it on site, with no signal.
Electrician Toolbox is made in Alberta by a former electrical engineer. It has the same voltage drop calculator, plus Ohm's law, conduit bending and six more tools, and it works offline with no account.
Electrician Toolbox
For iPhone and iPad. Free to use, and Pro is a one-time unlock.