How to use this wire size calculator
- Enter the circuit voltage and the load current in amps.
- Enter the one-way distance from the panel to the load.
- Keep 3% maximum voltage drop for branch circuits (the NEC recommends 3% branch, 5% total).
- Choose copper or aluminum and single- or three-phase.
Example
A 240 V, 40 A circuit running 60 ft needs 8 AWG copper wire. The voltage drop is 3.75 V (1.56%), within the 3% target.
What to know before you buy
AWG explained
American Wire Gauge numbers get smaller as the wire gets thicker: 14 AWG is thin, 4/0 AWG is very thick. Each drop of 3 gauge numbers roughly doubles the cross-sectional area. Common household circuits use 14 AWG (15 A), 12 AWG (20 A) and 10 AWG (30 A) copper.
Copper or aluminum?
Copper conducts better and is used for most branch circuits. Aluminum is lighter and cheaper and common for large feeders and service entrances, but it needs a larger size for the same current and connectors rated for aluminum.
Copper wire size, 120 V single-phase, 3% max drop
| Load and distance | Recommended wire size | Voltage drop |
|---|---|---|
| 15 A, 50 ft | 12 AWG | 2.96 V (2.47%) |
| 20 A, 50 ft | 10 AWG | 2.49 V (2.07%) |
| 20 A, 100 ft | 8 AWG | 3.13 V (2.6%) |
| 30 A, 100 ft | 6 AWG | 2.95 V (2.46%) |
| 15 A, 150 ft | 8 AWG | 3.52 V (2.93%) |
Formula
Required circular mils = 2 × K × I × L ÷ allowed volt drop (√3 instead of 2 for three-phase), with K = 12.9 for copper and 21.2 for aluminum. The result must also meet the NEC Table 310.16 ampacity at 75 °C.
Tips from the pros
- Size the breaker to protect the wire, not the appliance.
- For continuous loads (3 hours or more) the NEC requires circuits sized at 125% of the load.
- Aluminum wire needs connectors and devices rated for aluminum.
Common mistakes to avoid
- Using round-trip distance: enter the one-way length; the formula already accounts for both conductors.
- Ignoring voltage drop on long runs to sheds, pumps and outbuildings.
Frequently asked questions
What size wire for 30 amps?
10 AWG copper for short runs; longer runs may need 8 AWG or larger to limit voltage drop.
Is this calculator a substitute for an electrician?
No. Wire sizing also depends on insulation type, temperature, conduit fill and local code. Have work checked by a licensed electrician.
What size wire for a 50 amp circuit?
6 AWG copper for most runs; longer runs may need 4 AWG to limit voltage drop.
What size wire for a 100 ft run at 20 amps?
At 120 V with 3% drop, 8 AWG copper; at 240 V, 10 AWG may be enough.