Every wire has some resistance, so part of the voltage is used up pushing current out to the load and back. The longer the run and the heavier the load, the bigger the loss: lights dim, motors run hot and electronics reset. The usual target is no more than 3 percent on a branch circuit and 5 percent from the source to the farthest load. The calculator starts at the first and colors every result against both.
How to use this calculator
- Choose the circuit. DC is for batteries, solar panels, vehicles and boats; single-phase covers ordinary household circuits and low-voltage landscape lighting; three-phase is for equipment fed from a three-phase panel.
- Pick copper or aluminum and the wire size in AWG. A smaller AWG number is a thicker wire with less resistance, so it loses less voltage.
- Enter the source voltage (tap one of the common values) and the current the load draws in amps, from its nameplate or your load calculation.
- Enter the one-way distance from the source to the load: the length of the cable run, not the round trip. The calculator counts the return conductor for you.
- Read the drop in volts and percent, the colored bar and the size table. Your wire is highlighted, and green rows stay within your limit. Change the limit under More options if your project calls for a tighter or looser target.
- Then check the size you picked against the breaker and the electrical code's ampacity rules. Voltage drop is only half of the job: a wire that passes here can still be too small for the current.
Worked examples
Shed fed from the house
Wire size: 10 AWG · Source voltage: 240 V · Load current: 20 A · One-way distance: 150 ft
- Voltage drop
- 7.44 volts
- Drop
- 3.10%
- Voltage at the load
- 232.6 V
- Longest run within 3%
- 145 ft
Low-voltage landscape lighting run
Wire size: 12 AWG · Source voltage: 12 V · Load current: 6 A · One-way distance: 80 ft
- Voltage drop
- 1.90 volts
- Drop
- 15.84%
- Voltage at the load
- 10.1 V
- Longest run within 3%
- 15 ft
Trolling motor or RV battery cable
Circuit: DC · Wire size: 8 AWG · Source voltage: 12 V · Load current: 20 A · One-way distance: 15 ft
- Voltage drop
- 0.47 volts
- Drop
- 3.89%
- Voltage at the load
- 11.5 V
- Longest run within 3%
- 12 ft
Well pump at the far end of the yard
Wire size: 10 AWG · Source voltage: 240 V · Load current: 10 A · One-way distance: 250 ft
- Voltage drop
- 6.20 volts
- Drop
- 2.58%
- Voltage at the load
- 233.8 V
- Longest run within 3%
- 290 ft
Three-phase shop motor
Circuit: Three-phase · Wire size: 8 AWG · Source voltage: 208 V · Load current: 30 A · One-way distance: 100 ft
- Voltage drop
- 4.04 volts
- Drop
- 1.94%
- Voltage at the load
- 204.0 V
- Longest run within 3%
- 154 ft
Longest one-way run in feet before copper wire passes the starting limit
| Copper wire | 120 V, 15 A | 120 V, 20 A | 240 V, 20 A | 240 V, 30 A |
|---|---|---|---|---|
| 14 AWG | 38 | 29 | 57 | 38 |
| 12 AWG | 61 | 45 | 91 | 61 |
| 10 AWG | 97 | 73 | 145 | 97 |
| 8 AWG | 154 | 116 | 231 | 154 |
| 6 AWG | 244 | 183 | 367 | 244 |
| 4 AWG | 390 | 292 | 584 | 390 |
Each cell: Longest run within 3% (ft). Inputs not in the table use the calculator's starting values.
How voltage drop is worked out
The drop is the current times the resistance of the conductors it flows through. On a DC or single-phase circuit the current goes out on one conductor and comes back on the other, so the resistance of the whole loop counts: the one-way length out and back, times the wire's resistance per foot, times the amps. On a balanced three-phase circuit the loop factor is the square root of three instead.
The resistance of each size comes from a published conductor table for stranded copper and aluminum at 75 °C, the temperature building wire is normally rated at. For example, 12 AWG copper is 1.98 ohms per 1,000 ft and 10 AWG copper is 1.24 ohms per 1,000 ft.
On AC circuits this leaves out the small extra effect of inductance, which matters mainly for very large conductors and long feeders. For those, an electrician or engineer will use the full AC impedance.
How much voltage drop is acceptable?
- Branch circuits: about 3 percent. This is the recommendation most wire sizing tables are built on, and the calculator's starting limit.
- Feeders: about 2 percent, so that a feeder and a branch circuit together stay within 5 percent.
- Total from the source to the farthest load: 5 percent. Some energy codes make this a requirement for commercial buildings.
- Low-voltage landscape lighting: one lighting maker asks for 10 to 14 volts at the socket of LED fixtures and 10.8 to 12 volts for incandescent ones, so what matters is the voltage left at the last fixture. The calculator reports it for supplies of low voltage.
Voltage drop and wire size are separate checks
A wire has to pass two tests. First, it must be rated for the current: the electrical code sets the smallest size for each breaker and load, based on the wire's insulation and how it is installed. Second, it must deliver enough voltage at the far end, which is what this calculator checks. On short runs the first test decides the size; on long runs the second usually does.
So use the larger of the two answers. If you are not sure what the code requires for your circuit, ask a licensed electrician or your local building department before buying wire.
Ways to cut voltage drop
- Use a thicker wire. Each step down in AWG number lowers the resistance, and the size table shows how much each step buys you.
- Shorten the run, for example by moving a subpanel or transformer closer to the loads.
- Run the same load at a higher voltage. It draws less current, so the drop in volts falls, and so does its share of the higher supply voltage.
- Split the load. Two circuits or cable runs each carrying half the current each lose half as many volts. For landscape lights, several shorter runs from the transformer beat one long daisy chain.
- Choose copper over aluminum of the same size: copper has lower resistance, so aluminum needs a larger size for the same drop.
Common questions
How do I calculate voltage drop?
Multiply the current in amps by the resistance of the wire per foot and by the one-way length out and back. On a three-phase circuit, use the square root of three in place of the out-and-back length factor. The calculator looks up the resistance for your wire size and does the rest.
How do I work out the wire size for amps?
Start with the code's ampacity rules: they give the smallest wire for your breaker and load. Then check voltage drop over your distance with this calculator. Use whichever size is larger; on long runs it is usually the voltage drop that decides.
Does voltage drop matter more at low voltage?
Yes. The same drop in volts is a much bigger share of a low supply voltage, so battery systems, solar panels, vehicles and landscape lights need thicker wire than a household circuit would for the same current and distance.
Is aluminum wire worse than copper for voltage drop?
For the same size, yes: 12 AWG aluminum is 3.25 ohms per 1,000 ft against 1.98 ohms per 1,000 ft for copper. Aluminum is lighter and cheaper, so it is common for large feeders, sized up to make up for it.
How do I measure voltage drop with a multimeter?
With the load running, measure the voltage at the source and again at the load; the difference is the drop. Measuring live circuits is dangerous, so leave it to a qualified electrician if you are not trained for it.
How much power does voltage drop waste?
The wire turns the lost voltage into heat. The power wasted is the volts dropped times the amps, so a bigger drop on a heavier load wastes more energy and warms the wire more. Thicker wire cuts both.
Why does three-phase voltage drop use the square root of three?
In a three-phase circuit the voltage is measured between two phase wires, and the two phases are a third of a cycle apart. Because of that offset, the drop between the two wires is the square root of three times the drop along one wire, where an out-and-back single-phase or DC circuit doubles it.