What Size Wire for a 40 Amp Breaker? 8 AWG Copper (the Easy One)

The quick answer

For a 40 amp circuit, use 8 AWG copper. If you're using aluminum, use 6 AWG. The equipment ground is 10 AWG copper. That's the whole answer — and unlike the bigger breakers, it doesn't come with an asterisk about your wiring method.

Why 40 amps is the easy one (no Romex trap)

If you've just read our 50- or 60-amp guides, you know the catch there: 6-gauge copper only hits those amps as conduit wire, and the Romex version comes up short. At 40 amps, that problem disappears.

8 AWG copper is rated 40 amps at the 60°C column (which is where NM-B "Romex" is read) and 50 amps at 75°C (conduit). Since a 40 amp breaker only needs 40 amps of ampacity, 8 AWG Romex is legal — it lands exactly on the number. That makes 40 amps the largest common breaker where ordinary 8-gauge Romex works without stepping up. One size bigger, at 50 amps, and you're back to watching the wire type.

40 amp wire size chart

Every conductor for a 40 amp circuit, in one place:

On aluminum, 8 AWG is the bare 75°C minimum (45 amps), but 6 AWG aluminum is the common choice — it carries 40 amps with margin and works in 60°C-rated cable too, so it's the safer default.

8/3 vs 8/2: which cable you need

Same 8-gauge copper — the only question is whether the appliance needs a neutral. "8/3" has two hots, a neutral, and a ground; "8/2" has two hots and a ground.

• Needs a neutral (120V controls) — a range or wall oven with a clock, light, or electronic touch panel → 8/3 with ground.
• Straight 240V, no neutral — an AC condenser, a small water heater, baseboard heat → 8/2 with ground.

Cooktops sit on the fence: many modern ones have 120V electronic controls and need 8/3, but a simple 240V-only cooktop takes 8/2. This is exactly why the golden rule is to check the appliance's installation manual — it states plainly whether a neutral is required, so you buy the cable once and wire it right.

What a 40 amp circuit usually powers

Forty amps is the sweet spot for a lot of kitchen and comfort equipment: electric cooktops and smaller ranges and wall ovens, a 32-amp EV charger, AC condensers and heat pumps (many land at a 40A max breaker), smaller electric water heaters, hot tubs, and air compressors. The interactive picker above walks the common ones; the EV case is worth its own note.

Is 40 amps enough for an electric range?

Sometimes yes, sometimes no — and the answer is printed right on the appliance. A separate cooktop, a compact or apartment-size range, or many single wall ovens are commonly a 40 amp circuit, and 40A / 8 AWG is correct for them.

A full-size 30-inch freestanding range, especially with a big oven or a double oven, often calls for a 50 amp circuit instead — which means 6 AWG copper, not 8. The rule is simple: go by the nameplate or the installation manual, never by the burner count. If the plate says 40A max, you're on 8 AWG; if it says 50A, step up to a 50 amp circuit and 6 AWG. Sizing the wire to the appliance's rating, not a guess, is what keeps it both safe and to code.

30, 40 and 50 amp wire compared

If you're deciding between breaker sizes, here's how the three most common mid-range circuits stack up. Notice 40 amps sits in the clean middle — 8-gauge copper works in any wiring method, and it's only at 60 amps that the conduit-vs-Romex split begins.

See the neighbors for the full picture: 30 amp wire (10 AWG, the dryer and RV size) and 50 amp wire (where the 6-vs-8 gauge question starts).

40 amps for an EV charger (the 32A case)

A 32-amp Level 2 charger is a continuous load, so NEC 625.42 sizes its circuit at 125% of the charger's amps: 32 × 1.25 = a 40 amp circuit, on 8 AWG copper. It's the tier just below the popular 48A/60A setup, and a great fit for homes that don't need the fastest charge. For the full plug-vs-hardwired and 48-amp picture, see our EV charger wire guide.

Copper vs aluminum for 40 amps

Copper is 8 AWG; aluminum is 6 AWG (the common, margin-friendly pick). At 40 amps the cost gap between the two is small enough that most branch circuits are copper — the savings from aluminum only really add up on longer runs and larger feeders. For a subpanel-style run you'll sometimes see aluminum as 6 AWG SER cable; for a branch circuit it's usually copper NM-B. If you go aluminum, use aluminum-rated terminals, antioxidant compound, and torque to spec — loose aluminum connections are the main way these fail. For the full ampacity behind these numbers, see the AWG ampacity chart.

Long runs and voltage drop

Over distance, keep the voltage drop under 3% (an Informational Note under 210.19(A) — advisory, not a hard limit).

Here's the math for a run that crosses the line:

1. Running 40A at 240V over 140 ft on 8 AWG copper.
2. Voltage drop = 2 × K × I × length ÷ circular mils = 2 × 12.9 × 40 × 140 ÷ 16,510 = 8.75 V.
3. 8.75 V ÷ 240 V = 3.6% — over the 3% target.
4. Step up to 6 AWG (26,240 circular mils) and the same run drops to about 2.3% ✓.

The Voltage Drop Calculator runs your exact length, load, and material in one step.

Common 40 amp wiring mistakes

The most common slip is using 10 AWG — that's a 30 amp wire, and it can't sit on a 40 amp breaker. Others: reading the 90°C column to justify undersizing (your terminals are 60 or 75°C); running 8/2 where a cooktop needs the neutral in 8/3; and putting a 40-amp or 48-amp EV charger on a 40 amp circuit when only a 32-amp charger fits (a bigger charger needs a 50 or 60 amp circuit). And on long runs, forgetting voltage drop. As always, confirm with your local AHJ.

Frequently asked questions

Dr. Artie Vance

Technical reviewer · Ph.D. in Physics, MIT · 14 years' experience · Cross-checked against NEC 2026 & 2026

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.