Two checks decide wire size
- Ampacity. The conductor must carry the current without overheating. The calculator uses the 75°C column of NEC Table 310.16 and the small-conductor limits in 240.4(D): 14 AWG copper is limited to 15 A, 12 AWG to 20 A and 10 AWG to 30 A.
- Voltage drop. On long runs, a wire that is safe for the current can still lose too much voltage. The calculator increases the size until the drop is at or below your limit, using the same formula as the voltage drop calculator.
The recommended size is whichever check needs the larger wire.
Copper and aluminum ampacity at 75°C
| Size | Copper (A) | Aluminum (A) |
|---|---|---|
| 14 AWG | 15* | n/a |
| 12 AWG | 20* | 15* |
| 10 AWG | 30* | 25* |
| 8 AWG | 50 | 40 |
| 6 AWG | 65 | 50 |
| 4 AWG | 85 | 65 |
| 2 AWG | 115 | 90 |
| 1/0 | 150 | 120 |
| 2/0 | 175 | 135 |
| 3/0 | 200 | 155 |
| 4/0 | 230 | 180 |
* Limited by NEC 240.4(D) overcurrent protection rules.
The table does not apply derating for more than three current-carrying conductors in a raceway or for high ambient temperatures. Those adjustments can require a larger wire.
Wire size for 12 V and 24 V DC
Choose "DC or single phase AC" and enter 12 or 24 as the voltage. On low-voltage DC, voltage drop almost always decides the size, not ampacity. Example: a 20 A load at 12 V, 10 ft from the battery, with a 3% limit (0.36 V). 12 AWG is enough for the current, but only 8 AWG keeps the drop under 3%. The 12V wire gauge chart lists the answer for common currents and distances. Wiring that is not covered by the NEC, such as vehicles and boats, has its own standards, so check them for your project.
How many amps can 12 gauge wire handle?
12 AWG copper is limited to 20 amps under NEC 240.4(D), even though the 75°C ampacity table lists 25 A. 14 AWG copper is limited to 15 A and 10 AWG copper to 30 A. Those limits set the largest breaker the wire can be on.
What size wire for 20, 40, 50 and 100 amps?
With copper and 75°C terminations: 12 AWG for 20 A, 8 AWG for 40 A, 8 AWG minimum for 50 A (6 AWG is common for long or continuous loads), and 3 AWG for a 100 A feeder. A 100 A dwelling service may use 4 AWG copper or 2 AWG aluminum under NEC Table 310.12. Long runs can need a larger size for voltage drop.
Frequently asked questions
What size wire for 20 amps?
12 AWG copper for runs up to roughly 50 ft at 120 V. Beyond that, voltage drop usually pushes you to 10 AWG.
What size wire for a 50 amp circuit?
8 AWG copper meets 75°C ampacity for 50 A, but many installers use 6 AWG copper for EV chargers and ranges because terminals, length and continuous loading add up.
Can I use the 90°C column?
Only for derating calculations. Final ampacity is limited by the lowest-rated termination, which is usually 75°C for equipment over 100 A and 60°C or 75°C below that.
Does this include derating?
No. Bundling more than three current-carrying conductors or running through hot attics reduces ampacity, so apply NEC 310.15 adjustment factors separately.
Sources
Reference charts
- Wire Gauge Chart
- AWG to mm Chart
- Conduit Fill Chart
- Watts to Amps Chart
- 12V Wire Gauge Chart
- Amps per Wire Size
- Breaker to Wire Size Chart
- Wire Size for 200 Amp Service
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