30A × 0.8 = 24A
The maximum continuous load on a 30-amp circuit is 24 amps. A water heater or EV charger pulling a steady 24 amps is the most this circuit should ever see for hours at a time.
THE WIRE GUIDE — CHICAGO
What Gauge Wire for a 30-Amp Circuit? (Dryers, Water Heaters, AC Units)
The short answer is 10 AWG copper. But the 80% rule, the length of the run, and whether the wire is copper or aluminum all shape the real answer.
Part of The Wire Guide — every wire topic, explained plainly.
THE ANSWER UP FRONT
10 AWG copper on a 30-amp breaker
For a standard 30-amp, 240-volt circuit — a dryer, a water heater, a central AC unit — the wire is 10 AWG copper, protected by a 30-amp breaker. The code and the ampacity table agree on this one: the NEC caps 10 AWG copper at 30 amps of overcurrent protection (NEC 240.4(D)), and 10 AWG copper carries 30 amps in the 60°C column of the ampacity table. No derating math needed for a normal residential run.
If you're sizing other circuits too, the wire gauge chart is the master reference — every gauge, every amperage, in one table. This page is the project guide for the 30-amp case specifically.
THE 80% RULE
A 30-amp circuit carries 24 amps continuously
Here's the rule that actually governs the circuit: a breaker can only carry 80% of its rating as a continuous load (anything running 3 hours or more). The NEC phrases it the other way — size the circuit at 125% of the continuous load — but it's the same math:
Check the nameplate
Appliances are sized by their nameplate amps × 125%. A 24-amp nameplate load needs a 30-amp circuit. A 28-amp nameplate needs a 35-amp circuit — and bigger wire.
Where this bites: a 30-amp dryer circuit is fine because a dryer cycles — it's not a continuous load. But a 30-amp circuit feeding something that runs for hours (a big water heater, a 24-amp EV charger) is running at its continuous limit. If the load's nameplate says more than 24 amps continuous, 30 amps of circuit isn't enough regardless of the wire.
THE DIAGRAM
A 30-amp circuit, drawn simply
Breaker, wire, receptacle — and the one rule written on the drawing:
COPPER VS. ALUMINUM
The aluminum equivalent is 8 AWG
Aluminum carries less current per gauge than copper, so the 30-amp answer in aluminum is 8 AWG — which lands at 30 amps in the 60°C column. Same breaker, one size larger wire.
Two conditions come with aluminum, and they're non-negotiable: the breaker and every device on the circuit must be rated for aluminum terminations (look for the AL/CU or CO/ALR marking), and the connections get made up properly — aluminum expands and contracts more than copper, so a sloppy termination loosens over time. Modern 8000-series aluminum alloy is a different animal from the problematic 1960s branch-circuit aluminum, and it's fine for this use when terminated right. Here's the full copper vs. aluminum story.
DISTANCE
Long runs change the answer — upsize
10 AWG copper is the right wire for a 30-amp circuit at normal residential distances. But wire has resistance, and on a long run — a detached garage, a far corner of a big house, a barn — the voltage sagging along the wire (voltage drop) can push past the 3% guideline and leave the appliance starved.
The fix is simple: go up a size. A long 30-amp run often lands on 8 AWG copper instead of 10 AWG. Upsizing is always allowed — the code sets minimums, not maximums. For the full explanation with the distance table and a worked 100-foot example, here's voltage drop, explained simply.
WHAT RUNS ON 30 AMPS
The usual suspects
Electric dryers
The classic 30-amp load. Nameplate is usually 24 amps or less — a 10 AWG copper circuit on a 30-amp breaker with a 4-wire receptacle.
Water heaters
Standard residential electric water heaters land on 30-amp circuits. They run for hours, so the 24-amp continuous limit is the number that matters.
Central AC / heat pumps
Many residential condensers call for 30 amps. Size to the nameplate's minimum circuit ampacity — some want 25, some want the full 30.
24-amp EV chargers
A 24-amp charger is exactly the 80% continuous load of a 30-amp circuit. (Bigger chargers need bigger circuits — check the chart.)
WHAT GOES WRONG
Undersized wire on a 30-amp breaker
The dangerous combination is a 30-amp breaker protecting wire too small for it — typically 12 AWG where 10 AWG belongs. The breaker won't trip until current passes 30 amps, but 12 AWG is only good for 20. Between 20 and 30 amps, the wire overheats while the breaker sits there happily closed. Insulation melts, connections cook, and you've built a slow heater inside your walls.
This isn't a gray area: NEC 240.4(D) explicitly caps 12 AWG copper at 20 amps of protection. A 30-amp breaker on 12 AWG wire fails inspection everywhere, and it's one of the first things an inspector or home inspector flags. If you find this in a house you're buying — especially in Chicago's older housing stock — it's a rewire-the-circuit job, not a shrug.
The safety line: this page is general information, not an electrical plan. Your local code and a licensed electrician have the final word on any circuit you install or modify. Sizing wire from an article is fine for understanding the job — the actual work belongs to someone licensed.
Sized the circuit and now you've got half a roll of 10 AWG left? We buy usable leftover wire — anywhere in the country. Fast, free quote.
FREQUENTLY ASKED QUESTIONS
30-amp circuit questions, answered.
Can I use 12 AWG wire on a 30-amp breaker?
No. NEC 240.4(D) caps 12 AWG copper at 20 amps of overcurrent protection. A 30-amp breaker on 12 AWG wire is a code violation and a fire risk — the wire can overheat without ever tripping the breaker.
Can I use 8 AWG wire on a 30-amp circuit?
Yes. Upsizing is always allowed — the code sets minimum wire sizes, not maximums. 8 AWG copper on a 30-amp breaker is common on long runs where voltage drop is a concern.
What size wire for a 30-amp dryer?
10 AWG copper on a 30-amp two-pole breaker, with a 4-wire (hot-hot-neutral-ground) receptacle. Check the dryer's nameplate — if it calls for more than 24 amps continuous, the circuit needs to be bigger.
What's the aluminum equivalent of 10 AWG copper for 30 amps?
8 AWG aluminum. Every termination must be rated for aluminum (AL/CU or CO/ALR marked), and the connections need to be made up properly.
Is 10 AWG enough for a 30-amp circuit at 100 feet?
Borderline — voltage drop over 100 feet on 10 AWG at 30 amps runs past the 3% guideline. For long runs, upsizing to 8 AWG copper is the standard answer. See the voltage-drop guide for the math.
Does a 30-amp water heater circuit need a neutral?
Usually no — a standard 240V water heater needs two hots and a ground. A dryer needs the neutral too (for its 120V controls and drum light). It depends on the appliance, not the amperage.
IRONCREST KNOWS WIRE
I know wire. That's how I know what it's worth.
10 AWG on a 30-amp breaker, 12 AWG on a 20 — sizing is the first thing we check when a roll comes in, because the gauge is half the value. Know your wire, and you know what it's worth.