How the wire size calculator works
- Ampacity. It finds the smallest copper or aluminum wire whose rated current-carrying capacity (NEC Table 310.16) meets your load, using the column for your insulation and terminal rating. Small conductors are also capped by NEC 240.4(D): 14 AWG copper at 15 A, 12 AWG at 20 A and 10 AWG at 30 A.
- Continuous loads. Loads running three hours or more (EV chargers, water heaters, some heaters) are sized at 125% of the current.
- Voltage drop. Long runs lose voltage. The calculator increases the wire size until the drop over your one-way distance stays within your limit — 3% is the common recommendation for branch circuits.
Voltage drop formula
Three-phase: Circular mils = 1.732 × K × I × L ÷ Allowed drop (V)
K = 12.9 for copper, 21.2 for aluminum · L = one-way length in feet
Worked example
A 20 A, 120 V circuit run 100 ft in copper NM cable: ampacity alone allows 12 AWG. But a 3% drop is only 3.6 V, which needs 14,333 circular mils — so the calculator recommends 8 AWG, which drops about 3.13 V (2.60%).
Wire ampacity chart (NEC 310.16)
Allowable amps for not more than three current-carrying conductors at 30 °C ambient. Small copper conductors are further limited by 240.4(D) to 15 A (14 AWG), 20 A (12 AWG) and 30 A (10 AWG).
| Size | Copper 60 °C | Copper 75 °C | Aluminum 60 °C | Aluminum 75 °C | Area (mm²) |
|---|---|---|---|---|---|
| 14 AWG | 15 | 20 | — | — | 2.08 |
| 12 AWG | 20 | 25 | 15 | 20 | 3.31 |
| 10 AWG | 30 | 35 | 25 | 30 | 5.26 |
| 8 AWG | 40 | 50 | 35 | 40 | 8.37 |
| 6 AWG | 55 | 65 | 40 | 50 | 13.30 |
| 4 AWG | 70 | 85 | 55 | 65 | 21.15 |
| 3 AWG | 85 | 100 | 65 | 75 | 26.66 |
| 2 AWG | 95 | 115 | 75 | 90 | 33.63 |
| 1 AWG | 110 | 130 | 85 | 100 | 42.41 |
| 1/0 AWG | 125 | 150 | 100 | 120 | 53.51 |
| 2/0 AWG | 145 | 175 | 115 | 135 | 67.44 |
| 3/0 AWG | 165 | 200 | 130 | 155 | 85.03 |
| 4/0 AWG | 195 | 230 | 150 | 180 | 107.22 |
Common household circuits (copper, short runs)
| Breaker | Typical use | Typical wire |
|---|---|---|
| 15 A | Lighting, general outlets | 14 AWG |
| 20 A | Kitchen, bath, laundry outlets | 12 AWG |
| 30 A | Dryer, water heater, small AC | 10 AWG |
| 40 A | Range, cooktop | 8 AWG |
| 50 A | Range, EV charger (40 A continuous) | 6 AWG |
| 60 A | Subpanel, large EV charger | 6 AWG (75 °C) or 4 AWG (60 °C) |
These assume short runs; for longer distances, check voltage drop with the calculator above. Appliance installation instructions may specify a size — follow them.
What else affects wire size
- Bundling: more than three current-carrying conductors in a raceway or cable bundle require derating.
- Ambient temperature: hot attics and rooftops reduce ampacity.
- Terminal ratings: breakers and devices are often rated 60 °C or 75 °C — the lowest rating in the circuit sets the column you can use.
- Cable type: NM-B (Romex) is limited to the 60 °C column for sizing, even though its conductors are rated 90 °C.
Frequently asked questions
What size wire for a 20 amp circuit?
12 AWG copper for typical residential runs. At a full 20 A load on 120 V, runs longer than about 45 ft one way need 10 AWG to keep voltage drop under 3%; at lighter real-world loads 12 AWG reaches further.
What size wire for a 30 amp circuit?
10 AWG copper for most residential 30 A circuits such as dryers and water heaters, increasing for long runs.
What size wire for a 50 amp circuit?
6 AWG copper is commonly used where the 75 °C column applies (e.g. individual conductors in conduit). With NM-B cable, sized at 60 °C, 6 AWG is rated 55 A. Check terminal ratings and local code.
How far can I run 12 gauge wire on 20 amps?
At 120 V with a 3% voltage-drop target, about 45 ft one way at a full 20 A load. At lower actual loads the distance is longer.
What is acceptable voltage drop?
The NEC recommends (in informational notes) no more than 3% on a branch circuit and 5% total for feeder plus branch circuit. Sensitive or motor loads may need less.
Is aluminum wire safe?
Modern aluminum alloy conductors are widely used for feeders and service entrances when installed with properly rated (AL/CU) terminals and correct torque. Aluminum needs a larger size than copper for the same current.