The quick answer
For a standard 30 amp circuit, run 10 AWG copper wire. If you're using aluminum, step up to 8 AWG aluminum. That's the size that legally pairs with a 30 amp breaker, and it's what the wire behind almost every 30 amp dryer, water heater, and RV outlet in the country is made of.
Most homes use copper, so grab 10 AWG — usually sold as 10/2 or 10/3 "Romex" (NM-B) cable. Which of those two you need depends on the appliance, and that's the part worth two minutes below. If your run is very long, keep reading to the voltage-drop section before you buy.
Why 30 amps means 10-gauge wire
The rule comes from NEC 240.4(D), the "small-conductor" rule. It sets a hard cap on how much overcurrent protection small wires are allowed, no matter what their insulation is rated for. The caps are simple: 14 AWG copper tops out at 15 amps, 12 AWG at 20 amps, and 10 AWG at 30 amps.
This is why you can't just use 12-gauge wire and call it close enough — 12 AWG is capped at 20 amps, so it physically isn't allowed on a 30 amp breaker. Go the other way and 10 AWG is the smallest wire that reaches 30.
If you look at the full AWG ampacity chart, 10 AWG copper shows 35 amps in the 75°C column and 40 amps at 90°C. Those higher numbers do not let you use a bigger breaker — 240.4(D) overrides them and holds 10 AWG to 30 amps. The higher ampacity only matters for derating math, never for the breaker size.
Aluminum plays by the same rule but sits one step lower: 240.4(D) caps 10 AWG aluminum at 25 amps, so for a 30 amp circuit aluminum has to move up to 8 AWG.
30 amp wire size chart
Here's every conductor in a 30 amp circuit at a glance — the hot/neutral wires that carry the load, and the equipment ground that rides along with them.
| Conductor | Copper | Aluminum |
|---|---|---|
| Circuit wire (hot & neutral) | 10 AWG | 8 AWG |
| Equipment ground (NEC 250.122) | 10 AWG | 8 AWG |
| Typical cable you'd buy | 10/2 or 10/3 NM-B | (AL rare in NM-B) |
One reason 10 AWG works so cleanly here: standard NM-B "Romex" is rated at the 60°C column, where 10 AWG copper lands at exactly 30 amps. The cable and the breaker are a matched pair by design.
What a 30 amp circuit usually powers
A 30 amp circuit sits in a useful middle ground — bigger than the 15 and 20 amp circuits that feed your rooms and outlets, but smaller than the 40 to 50 amps a kitchen range or EV charger pulls. Knowing which appliance you're wiring is what actually decides your voltage and cable, so it's worth naming the common ones.
The classic 30 amp load is an electric clothes dryer (240V). Right behind it are electric water heaters — a typical 4500-watt tank unit draws about 19 amps and is wired with 10 AWG on a 240V circuit, which is why our water heater breaker guide keeps landing on this same wire. You'll also see 30 amps on RV and camper outlets (120V TT-30), larger window and through-wall air conditioners on a dedicated circuit, some mini-split condensers, well pumps, and lighter air compressors or welders. Every one of those flows back to the same two questions: 120 or 240 volts, and 10/2 or 10/3.
10/2 vs 10/3: which cable you actually need
This is where most 30 amp projects go sideways, and it has nothing to do with the gauge. "10/2" means two insulated conductors plus a bare ground. "10/3" means three insulated conductors plus a ground. Same 10-gauge copper — the difference is whether there's a neutral.
The rule of thumb:
- 240V with no neutral (electric water heater, baseboard heat) → 10/2 with ground: two hots and a ground.
- 240V that needs a neutral (a modern electric dryer) → 10/3 with ground: two hots, a neutral, and a ground.
- 120V (an RV outlet, or a 120V 30A load) → 10/2 with ground: one hot, one neutral, and a ground.
If you match the cable to the appliance below, you'll never buy the wrong one:
| 30A device | NEMA plug | Volts | Cable |
|---|---|---|---|
| Electric dryer (modern) | 14-30 | 240V | 10/3 |
| Electric dryer (old 3-prong) | 10-30 | 240V | 10/3 * |
| Electric water heater | hardwired | 240V | 10/2 |
| RV / camper | TT-30 | 120V | 10/2 |
| Generator inlet (240V) | L14-30 | 240V | 10/3 |
* A legacy 3-prong 10-30 outlet carries two hots and a neutral with no separate ground. New installations aren't allowed to use it — code now requires the 4-prong 14-30 (two hots, neutral, and a ground), which is exactly why 10/3 with ground is the modern dryer standard.
The 120V vs 240V trap (RV vs dryer)
Here's the single most misunderstood point in this whole topic: a 30 amp RV plug (NEMA TT-30) is 120 volts, not 240. It has one hot leg, one neutral, and a ground. People see "30 amps" and assume it's a big 240V connection like a dryer — it isn't.
A 30 amp dryer or generator plug (NEMA 10-30, 14-30, or L14-30) is 240 volts, with two hot legs. Same amperage rating, completely different circuit.
Wiring a TT-30 RV outlet as if it were 240V can send 240 volts into equipment built for 120 — an expensive and dangerous mistake. It also changes your distance limit: a 120V circuit hits the voltage-drop wall at roughly half the run length of a 240V one, so RV runs need more attention on long distances.
What size ground wire for a 30 amp circuit?
The equipment grounding conductor for a 30 amp circuit is 10 AWG copper (or 8 AWG aluminum), set by NEC Table 250.122. If you're pulling NM-B cable, the ground is already inside it and already the right size — you only need to think about this when you're running individual conductors in conduit.
When 10 AWG isn't enough: long runs and voltage drop
240.4(D) gives you the minimum legal wire. But over a long distance, voltage "drops" along the wire, and by the time it reaches the load there may not be enough left. The NEC recommends keeping a branch circuit's voltage drop under 3% (an Informational Note under 210.19(A) — a recommendation, not a hard rule, but a good one to follow).
At a full 30 amp load, here's roughly how far 10 AWG copper gets you before it crosses that 3% line:
240V circuit → good to about 100 ft. Past that, step up to 8 AWG.
120V circuit (RV) → good to only about 50 ft, because there's half the voltage to spare.
Here's the math for a run that pushes past the limit, so you can see how it works:
- Say you're running 30A at 240V over 120 ft.
- Voltage drop = 2 × K × I × length ÷ circular mils = 2 × 12.9 × 30 × 120 ÷ 10,380 = 8.95 V.
- 8.95 V ÷ 240 V = 3.7% — over the 3% target, so 10 AWG is too small here.
- Bump up to 8 AWG (16,510 circular mils) and the same run drops to about 2.3% ✓.
These thresholds shift with the actual load — a circuit that rarely pulls the full 30 amps has more room. The Wire Size Calculator does this for your exact length, load, and material in one step.
Voltage drop is about distance. There's a second reason 10 AWG can fall short: heat. If you bundle four or more current-carrying conductors in one conduit, or run through a hot attic, NEC ampacity-derating factors cut the wire's rating — and that can push a 30 amp circuit onto 8 AWG even on a short run. The AWG ampacity chart walks through that math in full.
Using aluminum for a 30 amp circuit
If you go aluminum, size up to 8 AWG — 240.4(D) caps 10 AWG aluminum at 25 amps, so it can't sit on a 30 amp breaker. Aluminum also needs a little more care at the connections: use terminals and breakers rated for aluminum, apply an antioxidant compound, and torque the lugs to spec. Because of that extra handling, most short 30 amp branch circuits in homes are copper; aluminum tends to show up on longer runs and feeders where the cost savings add up.
Common 30-amp wiring mistakes
The most frequent one is putting 12 AWG on a 30 amp breaker — it's capped at 20 amps and is a genuine code violation, not a gray area. A close second is reading the 90°C column (40 amps) and assuming you can undersize; the 240.4(D) cap governs the breaker, full stop. After that it's cable mix-ups — grabbing 10/2 when a dryer needs the neutral in 10/3, or wiring an RV outlet as 240V when it's a 120V connection. And on longer runs, forgetting voltage drop, or skipping the derating math when several conductors share a hot conduit or attic.
Frequently asked questions
Wire & CircuitCan I use 12-gauge wire for a 30 amp breaker?
⚡ Wire Size calculator →
Wire & CircuitIs 10-gauge wire good for 30 amps?
Wire & CircuitDo I need 10/2 or 10/3 for a 30 amp dryer?
Code & SafetyIs a 30 amp RV plug 120 or 240 volts?
Code & SafetyWhat size ground wire do I need for a 30 amp circuit?
Wire & CircuitHow far can I run 10-gauge wire on a 30 amp circuit?
⚡ Full AWG ampacity chart →
Wire & CircuitWhat gauge wire do I need for a 30 amp 240V circuit?
This guide is for general information based on the National Electrical Code. Local codes and your authority having jurisdiction (AHJ) may differ. Electrical work can be dangerous, so when in doubt, hire a licensed electrician.