What Size Wire for a 20 Amp Circuit?
Short answer: a 20 amp circuit needs 12 AWG copper (12/2 NM-B with ground, typically the yellow-jacketed cable) or 10 AWG aluminum. Never use 14 AWG copper on a 20 A breaker. On runs longer than about 50 feet at full load, step up to 10 AWG copper.
20 amp wire size by wiring method
| Wiring method | Copper | Aluminum |
|---|---|---|
| NM-B or UF cable | 12 AWG (12/2 or 12/3 w/ ground) | 10 AWG |
| THHN/THWN-2 in conduit | 12 AWG | 10 AWG |
Ground wire: 12 AWG copper or 10 AWG aluminum.
NEC 240.4(D) limits 14 AWG copper to a 15 A breaker and 12 AWG copper to 20 A. That's why a 20 A breaker on 14 AWG wire is one of the most common violations home inspectors find. 12 AWG aluminum is limited to 15 A by the same rule, so aluminum has to go up to 10 AWG.
20 amp wire size by distance (120 V)
| Conductor | 25 ft | 50 ft | 75 ft | 100 ft |
|---|---|---|---|---|
| Copper | 12 AWG (1.6%) | 10 AWG (2.1%) | 8 AWG (2.0%) | 8 AWG (2.6%) |
| Aluminum | 10 AWG (1.7%) | 8 AWG (2.1%) | 6 AWG (2.0%) | 6 AWG (2.7%) |
Smallest conductor that meets both the 75°C ampacity for a 20 A circuit and a 3% voltage drop at 20 A (100% of the rating), 120 V single-phase, one-way distance. Percent drop in brackets. If the circuit uses NM-B cable or 60°C terminations, start from the 60°C column instead.
At a full 20 A load on 120 V, 12 AWG copper reaches the 3% guideline at about 45 feet one way. Most 20 A receptacle circuits never carry a steady 20 A, but for a shop, garage or outdoor circuit with a heavy tool at the far end, use 10 AWG.
Maximum one-way run at 3% drop
| Load | 12 AWG, 120 V | 10 AWG, 120 V | 12 AWG, 240 V | 10 AWG, 240 V |
|---|---|---|---|---|
| 16 A (80% continuous) | 57 ft | 90 ft | 114 ft | 181 ft |
| 20 A (full rating) | 45 ft | 72 ft | 91 ft | 145 ft |
The same conductor runs twice as far at 240 V because the percentage drop is measured against a voltage twice as large. That's one reason 240 V baseboard heat and mini-splits tolerate long runs on 12 AWG.
Worked example: table saw in a detached garage
A 120 V table saw draws 15 A (nameplate). The receptacle is 85 feet from the panel on a 20 A circuit.
- 12 AWG copper: 2 × 12.9 × 15 × 85 ÷ 6,530 = 5.04 V, or 4.2%. The saw would start slowly and run hot under load.
- 10 AWG copper: 2 × 12.9 × 15 × 85 ÷ 10,380 = 3.17 V, or 2.6%. Good.
Result: 10/2 NM-B or UF (or 10 AWG THHN in conduit) on the 20 A breaker. A 20 A breaker on 10 AWG is always allowed; the breaker protects the wire, and the wire is simply oversized for distance. Check your own numbers in the voltage drop calculator.
Which circuits must be 20 amps
The NEC requires 20 A branch circuits for:
- Kitchen small-appliance circuits (at least two, 210.11(C)(1)).
- Laundry receptacle circuit (210.11(C)(2)).
- Bathroom receptacle circuit (210.11(C)(3)).
- Garage receptacles in dwellings: at least one 120 V, 20 A branch circuit (210.11(C)(4)).
Other common 20 A circuits: dishwasher and disposal (per manufacturer), microwave, window AC units, 240 V baseboard heaters and small mini-split heat pumps (check the nameplate). At 240 V, a 20 A circuit carries 20 × 0.8 = 16 A continuous, or 16 × 240 = 3,840 W of baseboard heat.
12/3 and multi-wire branch circuits
A 12/3 cable can carry two 20 A circuits that share one neutral, a common way to feed the two kitchen countertop circuits. The two hots must land on opposite legs of the panel, and the breakers must disconnect both hots together (a two-pole breaker or listed handle tie, 210.4(B)). With both hots on the same leg, the shared neutral can carry up to 40 A on a 12 AWG conductor.
12 AWG in conduit and boxes
- ½" EMT holds up to 9 12 AWG THHN; ¾" EMT holds 16 and ¾" PVC Schedule 40 holds 15.
- More than three current-carrying conductors in a raceway need an adjustment from Table 310.15(C)(1).
- Each 12 AWG conductor counts as 2.25 in³ of box volume, more than 14 AWG, so size boxes with the box fill calculator.
Common mistakes inspectors flag
- 14 AWG anywhere on a 20 A circuit, including short pigtails.
- A single 15 A receptacle on an individual 20 A branch circuit (it must be rated 20 A).
- Multi-wire branch circuits without a handle tie or with both hots on the same leg.
- Kitchen countertop receptacles fed from a lighting circuit.
Frequently asked questions
Can I use 14 gauge wire on a 20 amp breaker?
No. 14 AWG copper is limited to 15 A overcurrent protection. If a 20 A breaker is on 14 AWG wire, change the breaker to 15 A or rewire with 12 AWG. See 15 amp wire size.
Can I use 12 gauge wire on a 15 amp circuit?
Yes. Larger wire is always allowed. It costs more and is stiffer to work with, but it reduces voltage drop.
Can a 20 amp circuit have 15 amp receptacles?
Yes. On a 20 A circuit with more than one receptacle, 15 A receptacles are permitted (NEC Table 210.21(B)(3)). A single receptacle on an individual 20 A circuit must be rated 20 A.
How many outlets can go on a 20 amp circuit?
The NEC doesn't set a limit for dwellings. For commercial work, each general-purpose receptacle is calculated at 180 VA, which works out to 13 receptacles on a 20 A, 120 V circuit (2,400 ÷ 180, rounded down).
How many amps can 12 gauge wire handle?
Its breaker is limited to 20 A by 240.4(D). Table 310.16 lists 20 A at 60°C, 25 A at 75°C and 30 A at 90°C; the higher values matter only for derating (see the ampacity chart). For 25–30 A loads, go to 10 AWG and a 30 A circuit.
Last updated: October 6, 2026. Based on NEC 2023. Check results against the code edition and local amendments your jurisdiction enforces and with the AHJ; electrical work should be done by qualified people.