Voltage Drop Chart: How Far Can You Run Wire by Size and Amps?
Part of the project planners: Sizing a circuit: watts, amps, wire and breaker and Planning backup power, batteries and solar.
Every wire loses some voltage along its length, and a long run on thin wire loses a lot. As a guide, keep the drop to 3% or less. For example 12 AWG copper carrying 15 A on a 120 V circuit stays within 3% for only about 61 feet one way. These charts show the longest run for each wire size, and the page explains how to fix a run that drops too much.
Longest one-way run for a 3% drop at 120 V
Copper wire, single-phase. A shorter run keeps the drop below 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 |
At 240 V the same wire goes twice as far
The same wire and current lose the same volts, which is half the percentage on a 240 V system:
| Wire | 20 A | 30 A | 40 A | 50 A |
|---|---|---|---|---|
| 14 AWG | 57 | 38 | 29 | 23 |
| 12 AWG | 91 | 61 | 46 | 36 |
| 10 AWG | 145 | 97 | 72 | 58 |
| 8 AWG | 230 | 154 | 115 | 92 |
| 6 AWG | 366 | 244 | 183 | 146 |
| 4 AWG | 582 | 388 | 291 | 233 |
At 12 V, runs are very short
Solar, RV, boat and low-voltage lighting circuits lose the same volts as a 120 V circuit, but that is ten times the percentage. Feet of one-way run for a 3% drop at 12 V DC:
| Wire | 5 A | 10 A | 20 A | 30 A | 50 A |
|---|---|---|---|---|---|
| 14 AWG | 11.5 | 5.7 | 2.9 | 1.9 | 1.1 |
| 12 AWG | 18.2 | 9.1 | 4.6 | 3.0 | 1.8 |
| 10 AWG | 29.0 | 14.5 | 7.2 | 4.8 | 2.9 |
| 8 AWG | 46.1 | 23.0 | 11.5 | 7.7 | 4.6 |
| 6 AWG | 73.2 | 36.6 | 18.3 | 12.2 | 7.3 |
| 4 AWG | 116.5 | 58.2 | 29.1 | 19.4 | 11.6 |
Use the voltage drop calculator for your own wire, current, distance and voltage, and for aluminum or three-phase circuits.
How it is calculated
The wire's resistance per foot is the resistivity of the metal (12.9 ohm-circular-mils per foot for copper at 75 °C) divided by the wire's cross-section in circular mils. The current has to go out and come back, so a single-phase or DC circuit has twice the one-way distance in wire.
For 12 AWG (6,530 circular mils), 15 A and 100 feet that is 2 × 15 × 12.9 × 100 ÷ 6,530 = 5.9 V, which is 4.9% of 120 V.
Fixing a run that drops too much
- Use a thicker wire. Each gauge number lower cuts the resistance by about 20%, so 12 to 10 AWG cuts it by about 37%. The calculator names the smallest size that stays within 3%.
- Shorten the run, or move the source closer to the load.
- Raise the voltage. The same power at 240 V draws half the current of 120 V, and loses a quarter of the voltage in percentage terms.
- Reduce the current by splitting the load across two circuits.
Safety
Voltage drop is separate from a wire's current rating (ampacity). A wire has to meet both, and the breaker has to protect the wire. Local code decides the rules for your area and the work needs to be done to code. Have a licensed electrician do or check any household wiring.
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% (10 AWG drops 4.1%). At 240 V, 10 AWG will do.
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