The quick answer
For a 20 amp circuit, use 12 AWG copper. If you're using aluminum, use 10 AWG. The equipment ground is 12 AWG copper. Twelve-gauge copper works in both NM-B "Romex" (the yellow 12/2 you'll see at the store) and conduit.
Grab 12 AWG copper — 12/2 for a normal circuit. Just don't reach for the thinner 14-gauge; that's a 15 amp wire and can't go on a 20 amp breaker. The two follow-ups most people have — 15 vs 20, and GFCI vs AFCI — are both below.
Why it must be 12-gauge, not 14 (NEC 240.4(D))
The rule is the small-conductor rule, NEC 240.4(D). It caps how much overcurrent protection thin wires can take: 14 AWG copper tops out at 15 amps, and 12 AWG at 20 amps. So 14-gauge on a 20 amp breaker isn't a judgment call — it's simply not allowed, because the breaker would let the wire carry more than it's rated for.
Putting 14-gauge wire on a 20 amp breaker is one of the most common — and most dangerous — DIY mistakes. The breaker won't trip until 20 amps, but the wire is only good for 15, so it can overheat before the breaker ever notices. If any part of a circuit is 14-gauge, the breaker must be 15 amps.
The reverse is always fine: 12-gauge on a 15 amp breaker is allowed and common. Larger wire is never a problem — the breaker protects the wire, so a bigger conductor just has extra margin.
15 amp vs 20 amp: wire, breaker, and outlets
These two household circuits are easy to mix up, so here's the clean split:
| 15 amp circuit | 20 amp circuit | |
|---|---|---|
| Copper wire | 14 AWG | 12 AWG |
| Breaker | 15 A | 20 A |
| Receptacles allowed | 15 A | 15 A or 20 A |
| Common use | bedrooms, lights | kitchen, bath, garage |
One surprise: a 20 amp circuit can use ordinary 15 amp receptacles — you don't need the special T-slot 20 amp outlet on every plug. The 20 amp receptacle is only required when it's the single receptacle on a dedicated circuit. Multiple 15 amp outlets on a 20 amp circuit is standard and fully to code.
20 amp wire size chart
Every conductor for a 20 amp circuit, in one place:
| Conductor | Copper | Aluminum |
|---|---|---|
| Circuit wire (Romex or conduit) | 12 AWG | 10 AWG |
| Equipment ground (NEC 250.122) | 12 AWG | 10 AWG |
| Typical cable you'd buy | 12/2 or 12/3 NM-B | (rare in branch circuits) |
Aluminum needs 10 AWG for 20 amps, because 240.4(D) caps 12 AWG aluminum at only 15 amps. In practice almost every household 20 amp circuit is copper — aluminum is uncommon at this small size.
12/2 vs 12/3: which cable
For a normal 120-volt circuit, use 12/2 with ground — one hot, one neutral, and a ground. You'd reach for 12/3 with ground in two cases: a multi-wire branch circuit (two 120V circuits sharing one neutral, which saves a wire), or a 20 amp 240V load that also needs a neutral. For the everyday receptacle run, 12/2 is what you want.
Does a 20 amp circuit need GFCI or AFCI?
Usually one, sometimes both — and it depends on the room. These are two different protections: GFCI guards against shock near water, AFCI guards against arcing faults that start fires.
- GFCI (NEC 210.8) — kitchen countertops, bathrooms, garages, outdoors, laundry areas, unfinished basements, and anywhere near a sink.
- AFCI (NEC 210.12) — most living areas: kitchens, living and dining rooms, bedrooms, hallways, laundry, and similar rooms.
So a kitchen or laundry circuit needs both — handled by a dual-function breaker. A bathroom, garage, or outdoor receptacle needs GFCI (not AFCI). A bedroom or living room needs AFCI. The picker at the top of this guide walks each room. These requirements have expanded over recent code cycles, so confirm your area's adopted edition and AHJ before you buy breakers.
20 amps at 240 volts
Most 20 amp circuits are 120 volts, but a 20 amp 240V circuit does exist — a small window AC, some baseboard heat, a small tool. The wire doesn't change: it's still 12 AWG copper. A straight 240V load with no neutral uses 12/2 (both conductors are hot); the ground is still 12 AWG. Only the voltage and the breaker (a double-pole 20A) differ.
Aluminum and long runs
Aluminum for 20 amps is 10 AWG, but it's rare on a household branch circuit — copper 12-gauge is the norm. Voltage drop matters more here than you'd think, because most 20 amp circuits are 120 volts, and 120V runs out of headroom fast.
12 AWG at 120V → only about 46 ft.
12 AWG at 240V → about 91 ft. For a long run to a garage or outdoor outlet, that ~46 ft at 120V is why electricians often bump to 10-gauge.
The Voltage Drop Calculator checks your exact run, and the AWG ampacity chart has the numbers behind all of this.
Common 20 amp wiring mistakes
The big one is 14-gauge on a 20 amp breaker — the wire can't take it. Right behind it is skipping GFCI or AFCI where the room requires it, which fails inspection. Others: running 12/2 where a multi-wire branch needs 12/3; overloading a 20 amp circuit (its continuous limit is 80%, or 16 amps); and forgetting voltage drop on a long 120V run. As always, receptacle and circuit work is often permit-and-inspection territory, so confirm with your AHJ.
Frequently asked questions
Wire & CircuitWhat size wire do I need for a 20 amp breaker?
Code & SafetyCan I use 14-gauge wire on a 20 amp breaker?
Wire & CircuitIs 12-gauge wire good for 20 amps?
Wire & CircuitCan I use 12-gauge wire on a 15 amp breaker?
Wire & CircuitDo I use 12/2 or 12/3 for a 20 amp circuit?
Code & SafetyDoes a 20 amp circuit need GFCI or AFCI?
Wire & CircuitWhat size wire for a 20 amp 240V circuit?
Wire & CircuitWhat size ground wire for a 20 amp circuit?
This guide is for general information based on the National Electrical Code. GFCI and AFCI requirements and other rules vary by NEC edition and your local authority having jurisdiction (AHJ). Electrical work can be dangerous, so when in doubt, hire a licensed electrician.