How this calculator works
A wire has resistance, so it loses voltage as current flows through it. Resistance per foot is the resistivity of the metal divided by the wire's cross-section in circular mils. The current goes out and comes back, so a single-phase or DC circuit has twice the distance in wire, and a three-phase circuit has 1.732 times the one-way length in its effect.
We use a resistivity of 12.9 ohm-cmil per foot for copper and 21.2 for aluminum, both at 75 °C. Worked example: 12 AWG copper (6,530 circular mils) carrying 15 A for 100 ft on a 120 V circuit drops 2 × 15 × 12.9 × 100 ÷ 6,530 = 5.9 V, which is 4.9%. That is over 3%. A run of up to about 61 feet stays within 3% at this wire size, and 8 AWG copper does the full 100 ft (10 AWG drops 3.1%).
Longest run for a 3% drop, by wire size and current (copper, 120 V)
One-way distance in feet, single-phase. Shorter than this keeps the drop under 3%.
| Wire | 10 A | 15 A | 20 A | 30 A | 50 A |
|---|---|---|---|---|---|
| 14 AWG | 57 | 38 | 29 | 19 | 11 |
| 12 AWG | 91 | 61 | 46 | 30 | 18 |
| 10 AWG | 145 | 97 | 72 | 48 | 29 |
| 8 AWG | 230 | 154 | 115 | 77 | 46 |
| 6 AWG | 366 | 244 | 183 | 122 | 73 |
| 4 AWG | 582 | 388 | 291 | 194 | 116 |
Tips before you buy
- Low-voltage systems suffer most. 12 volts DC loses the same volts as 120 volts AC, but that is ten times the percentage, so solar, RV, boat and landscape lighting runs need thick wire or short runs.
- Raise the wire size to cut the drop. Each gauge number lower cuts the resistance by about 20%, so going from 12 to 10 AWG cuts it by about 37%.
- Aluminum needs a larger wire than copper for the same drop and the same current: about two sizes up.
- Motors and other loads draw several times their running current when they start, so a drop that looks fine at the running current can cause a stall at start-up.
- Check the ampacity too. A wire that does not drop too much voltage may still overheat if it is too small for the current in its conditions: use the table in your local code.
Tools & materials for this project
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Frequently asked questions
What is an acceptable voltage drop?
A common guide is 3% or less on a branch circuit or feeder, and 5% for the circuit as a whole. Sensitive electronics and long runs may need less.
How do I calculate voltage drop?
Multiply 2 by the current in amps, the resistivity of the metal (12.9 for copper), and the one-way distance in feet, then divide by the wire's circular mils. Divide by the system voltage for the percentage.
How far can I run 12 AWG wire at 15 amps?
About 61 feet one way on 120 V for a 3% drop, using copper. On a 240 V circuit the same wire and current can run about 121 feet.
What size wire do I need for 100 feet at 20 amps?
On 120 V, 8 AWG copper keeps the drop within 3% (at 100 ft and 20 A, 10 AWG drops 4.1%). At 240 V, 10 AWG will do.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.