Wire Gauge Calculator
Enter the voltage, the load in amps or watts and the one-way length of the run. The calculator finds the smallest wire that keeps the voltage drop within your limit and can carry the current, in AWG or mm², for copper or aluminum.
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This calculator gives an estimate for planning only. Electrical work must follow your local wiring rules and be checked by a qualified electrician.
How to use it
- Choose AWG or mm² sizes, copper or aluminum, and the type of circuit: DC, single-phase or three-phase AC.
- Enter the voltage and the load, in amps or in watts.
- Enter the one-way length of the run from the source to the load, not the total length of wire, and the maximum voltage drop you accept (3% is common).
- Read the recommended size, what decides it, and the voltage drop and rating of every size in the table.
Why use this tool
AWG and mm²
US gauges from 14 AWG to 500 kcmil, or metric sizes from 1.5 to 240 mm², with lengths in feet or meters.
Voltage drop and ampacity
The recommended size meets both your voltage drop limit and a reference current rating, and the calculator tells you which one decides.
DC, single- and three-phase
Works for 12 V and 24 V DC systems, household circuits and three-phase supplies, with the correct formula for each.
Amps or watts
Enter the current directly or the power in watts, with a power factor for motors and other AC loads.
Maximum distance
See how far you can run the recommended wire before the voltage drop goes over your limit.
Private
Everything is calculated in your browser. Nothing you enter is sent or stored.
How to calculate voltage drop
Single-phase AC and DC: voltage drop = 2 × length × current × resistance per unit length. The 2 is there because current flows out on one wire and back on the other.
Three-phase AC: voltage drop = √3 × length × current × resistance per unit length, compared with the line-to-line voltage.
Resistance per meter = resistivity ÷ cross-section. Copper has a resistivity of 0.0172 Ω·mm²/m at 20 °C and aluminum 0.0283 Ω·mm²/m, and both rise by about 0.4% per °C as the wire warms up.
Example: a 20 A load at 120 V, 100 feet from the panel, in copper at 75 °C. 12 AWG drops 7.7 V (6.4%), 10 AWG drops 4.9 V (4.0%) and 8 AWG drops 3.1 V (2.5%). For a 3% limit you need 8 AWG, even though 12 AWG is rated for 20 A.
Wire size by amps (NEC Table 310.16)
Allowable ampacity of insulated conductors at the 75 °C column, with no more than three current-carrying conductors in a raceway or cable at an ambient temperature of 30 °C (86 °F). For 14, 12 and 10 AWG, NEC 240.4(D) limits the breaker size.
| Wire size | Copper (A) | Aluminum (A) | Cross-section |
|---|---|---|---|
| 14 AWG | 15 (breaker limit) | – | 2.08 mm² |
| 12 AWG | 20 (breaker limit) | 15 (breaker limit) | 3.31 mm² |
| 10 AWG | 30 (breaker limit) | 25 (breaker limit) | 5.26 mm² |
| 8 AWG | 50 | 40 | 8.37 mm² |
| 6 AWG | 65 | 50 | 13.3 mm² |
| 4 AWG | 85 | 65 | 21.2 mm² |
| 2 AWG | 115 | 90 | 33.6 mm² |
| 1/0 AWG | 150 | 120 | 53.5 mm² |
| 4/0 AWG | 230 | 180 | 107 mm² |
Maximum distance for a 3% voltage drop at 120 V
One-way distance in feet before a copper circuit at 75 °C drops more than 3% (3.6 V) of 120 V. At 240 V the distances double. A dash means the wire is not rated for that current.
| Wire size | 10 A | 15 A | 20 A | 25 A | 30 A |
|---|---|---|---|---|---|
| 14 AWG | 58 ft | 39 ft | – | – | – |
| 12 AWG | 93 ft | 62 ft | 46 ft | – | – |
| 10 AWG | 148 ft | 98 ft | 74 ft | 59 ft | 49 ft |
| 8 AWG | 235 ft | 157 ft | 117 ft | 94 ft | 78 ft |
| 6 AWG | 374 ft | 249 ft | 187 ft | 149 ft | 124 ft |
| 4 AWG | 595 ft | 397 ft | 297 ft | 238 ft | 198 ft |
How much voltage drop is acceptable?
The NEC recommends, in an informational note, a voltage drop of no more than 3% on a branch circuit and 5% in total for the feeder and branch circuit together. IEC 60364-5-52 suggests 3% for lighting and 5% for other uses when you are supplied from the public low-voltage network.
For 12 V and 24 V DC systems, such as solar, boats and vehicles, a few tenths of a volt matter, so many installers use 3% or less, and 1% to 2% for sensitive electronics.
Common wire sizes in US homes
These copper sizes are typical for household branch circuits at normal lengths. Your cable type, the local code and the appliance instructions decide the final size:
- 15 A lighting and general outlets: 14 AWG.
- 20 A kitchen, bathroom and laundry outlets: 12 AWG.
- 30 A water heater or dryer: 10 AWG.
- 40 A to 50 A range or EV charger: 8 to 6 AWG, depending on the cable type.
- Long runs: go up one or more sizes to keep the voltage drop under 3%.
- Aluminum: usually two sizes larger than copper for the same circuit, with devices rated AL or CU/AL.
Frequently asked questions
What size wire do I need for 20 amps?
12 AWG copper for a 20 A circuit at normal lengths. On longer runs, voltage drop decides: at 120 V and 100 feet, you need 8 AWG copper to stay under 3%.
What size wire for 30 amps at 100 feet?
10 AWG copper is rated for a 30 A breaker, but at 120 V it drops about 7.3 V (6.1%) over 100 feet. For 3% you need 6 AWG at 120 V, or 8 AWG at 240 V.
Is the length one way or round trip?
Enter the one-way length from the source to the load. The calculator doubles it for DC and single-phase circuits, because the current goes out and comes back.
Why is the recommended size bigger than the ampacity chart says?
Ampacity only tells you how much current the wire can carry without overheating. On a long run, the wire also has to be thick enough that the voltage at the far end does not drop too much. The calculator shows which of the two decides.
Does this follow the NEC?
It uses NEC Table 310.16 (75 °C column) and the limits of NEC 240.4(D) for the current rating. It does not apply corrections for ambient temperature, more than three conductors or continuous loads. Your electrician and local inspector decide what is required.
How do I convert AWG to mm²?
14 AWG is 2.08 mm², 12 AWG is 3.31 mm², 10 AWG is 5.26 mm², 8 AWG is 8.37 mm² and 6 AWG is 13.3 mm². Every three gauge numbers smaller, the area roughly doubles.
Is my data sent anywhere?
No. The calculation runs in your browser. The page only remembers your chosen sizes, conductor and circuit type on this device.
How to Choose the Right Wire Size
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