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This wire size calculator recommends the AWG wire size for a circuit. Enter the current, voltage, one-way distance and conductor type, and it checks NEC ampacity and voltage drop to find the smallest wire that carries the load safely with an acceptable voltage drop.
How to Use the Wire Size Calculator
- Enter the load current in amps.
- Tick continuous load for loads running three hours or more, such as EV chargers; these are sized at 125%.
- Choose the voltage and enter the one-way distance from the panel to the load.
- Choose copper or aluminum and the insulation temperature rating.
- Set the maximum voltage drop (3% for branch circuits is the common recommendation).
Wire Ampacity Chart (NEC Table 310.16)
| AWG | Copper 60°C | Copper 75°C | Aluminum 60°C | Aluminum 75°C |
|---|---|---|---|---|
| 14 | 15* | 15* | — | — |
| 12 | 20* | 20* | 15* | 15* |
| 10 | 30* | 30* | 25* | 25* |
| 8 | 40 | 50 | 35 | 40 |
| 6 | 55 | 65 | 40 | 50 |
| 4 | 70 | 85 | 55 | 65 |
| 3 | 85 | 100 | 65 | 75 |
| 2 | 95 | 115 | 75 | 90 |
| 1 | 110 | 130 | 85 | 100 |
| 1/0 | 125 | 150 | 100 | 120 |
| 2/0 | 145 | 175 | 115 | 135 |
| 3/0 | 165 | 200 | 130 | 155 |
| 4/0 | 195 | 230 | 150 | 180 |
*Limited by the NEC small-conductor rule to these breaker sizes. Values assume no more than three current-carrying conductors at 30°C (86°F). NM-B (Romex) cable must use the 60°C column.
Voltage Drop Formula
Voltage drop = 2 × K × I × L ÷ Circular mils
K is 12.9 for copper and 21.2 for aluminum, I is the current, L is the one-way length in feet, and circular mils is the conductor area (10,380 for 10 AWG, for example).
Example
A 20 A load at 120 V, 50 ft away, could use 12 AWG copper for ampacity, but the voltage drop would be 3.3%. Moving up to 10 AWG brings the drop down to 2.1%.
Safety
- Breaker size must protect the wire: never put a larger breaker on a smaller wire.
- Derate for more than three current-carrying conductors in a conduit or hot ambient temperatures.
- Electrical work must meet local code and often needs a permit and a licensed electrician.
Sources
Frequently Asked Questions
What size wire for 20 amps?
12 AWG copper is the minimum for a 20 A circuit. For 20 A runs longer than about 40 ft at 120 V, 10 AWG keeps voltage drop under 3%.
What size wire for a 50 amp circuit?
Typically 6 AWG copper with NM-B cable (60°C), or 8 AWG copper THHN in conduit with 75°C terminals.
What size wire for 100 feet?
It depends on the current. A 15 A, 120 V load 100 ft away needs 8 AWG copper to stay under 3% voltage drop; 10 AWG would drop about 3.1%.
What is acceptable voltage drop?
The NEC recommends no more than 3% on a branch circuit and 5% in total from the service to the load.
Read the guide: What Size Wire Do I Need? Amps, Distance and Voltage Drop
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