What size wire do I need for a 30 amp breaker?

10 AWG copper is the smallest conductor our model estimates at or above 30 A using a 75 °C insulation rating; 8 AWG is required if the terminations are limited to 60 °C. In aluminium the equivalent is 8 AWG. Verify against the code edition adopted locally before installing.

Can I use aluminium wire on a 30 amp circuit?

Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 30 A circuit steps up to 8 AWG and uses K = 21.2 in the voltage-drop formula instead of 12.9.

How far can a 30 amp circuit run?

On 10 AWG copper the 3% voltage-drop guideline is reached near 48 ft one-way at 120 V and 96 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.

Is a 30 amp breaker sized at 100% of the load?

Only for non-continuous loads. A continuous load is designed at 125%, so 30 A of continuous current becomes 37.5 A of design current and lands on a 40 A device with conductors sized to match.

What is typically on a 30 amp circuit?

Commonly electric dryers, water heaters, 240 V baseboard heat. The load type also drives the receptacle and disconnect ratings, not just the conductor size.

What Size Wire for a 30 Amp Breaker?

Conductor size, aluminium equivalent, derating and maximum run length for a 30 amp circuit — all derived from published conductor geometry and the voltage-drop formula.

Quick answer

A 30 amp circuit needs 10 AWG copper with 75 °C terminations, or 8 AWG aluminium. 8 AWG copper applies where equipment terminals are limited to 60 °C. Voltage drop reaches 3% at roughly 96 ft one-way on 240 V and 48 ft on 120 V.

Formula estimate

Copper @ 75 °C

10 AWG

Copper @ 60 °C

8 AWG

Aluminium @ 75 °C

8 AWG

3% drop @ 240 V

96ft

Conductor options for 30 A

Estimated ampacity of each candidate size, so you can see the margin you are buying.

Material / rating	Minimum size	Estimated ampacity	Margin over 30 A
Copper, 60 °C	8 AWG	37 A	+7 A
Copper, 75 °C	10 AWG	32 A	+2 A
Copper, 90 °C	10 AWG	37 A	+7 A
Aluminium, 75 °C	8 AWG	34 A	+4 A
Aluminium, 90 °C	8 AWG	39 A	+9 A

Ampacity estimated as I = 0.0670 × cmil^0.6505, scaled for insulation rating and material.

Source & method: Conductor areas come from the ASTM B258 relationship d = 5 × 92^((36 − n) / 39). Ampacity is a fitted physical model, not a reproduction of any code table, and runs within roughly ±8% of published values.

Maximum run length on 10 AWG copper

One-way distance before voltage drop passes the 3% and 5% guidelines.

System	3% limit	5% limit	Drop @ 100 ft
120 V 1Ø	48 ft	80 ft	6.21%
240 V 1Ø	96 ft	160 ft	3.11%
208 V 3Ø	96 ft	161 ft	3.1%
480 V 3Ø	223 ft	371 ft	1.34%

Source & method: VD = 2 × K × I × L / cmil single phase, 1.732 × K × I × L / cmil three phase, with K = 12.9 for copper. 10 AWG is 10,383 circular mils and 0.9988 Ω per 1000 ft at 20 °C.

Wire size by run length for 30 A

Ampacity sets the floor; distance is what actually forces the upsize on long runs. Smallest copper conductor that carries the load and still holds 3% drop.

One-way run	120 V 1Ø	240 V 1Ø	480 V 3Ø
50 ft	8 AWG	10 AWG	10 AWG
75 ft	8 AWG	10 AWG	10 AWG
100 ft	6 AWG	8 AWG	10 AWG
150 ft	4 AWG	8 AWG	10 AWG
200 ft	3 AWG	6 AWG	10 AWG
250 ft	2 AWG	4 AWG	8 AWG
300 ft	2 AWG	4 AWG	8 AWG

Sized on both criteria at once: estimated 75 C ampacity and 3% voltage drop at the stated one-way distance.

Source & method: A dash means no listed size holds 3% at that distance and voltage — split the load, raise the system voltage, or move the panel closer.

Hot attics, conduit runs and bundling

What a 30 A circuit actually needs once ambient temperature and conductor count are applied.

Ambient	3 conductors	4–6 conductors	7–9 conductors
30 °C	10 AWG	8 AWG	8 AWG
40 °C	8 AWG	8 AWG	6 AWG
50 °C	8 AWG	6 AWG	6 AWG
60 °C	6 AWG	4 AWG	4 AWG

Source & method: Ambient correction F = √((Tc − Ta) / (Tc − 30)) with Tc = 75 °C; bundling adjustment uses the standard stepped schedule (1.00 / 0.80 / 0.70). The conductor shown is the smallest copper size whose corrected ampacity still carries 30 A.

Where 30 A circuits are actually used

30 A covers electric water heaters, clothes dryers, well pumps, baseboard heat runs, small subpanels and 30 A RV service. It is the smallest rating that regularly gets its own disconnect and its own dedicated cable, which is why 10 AWG is stocked on every truck.

The consideration that most often decides a 30 A circuit is the small-conductor cap: 10 AWG copper is held to a 30 A device, so 30 A is the ceiling of that conductor rather than a comfortable point in its range. Water heaters and dryers are also sized against nameplate rather than measured draw, and a 30 A dryer circuit with a long basement-to-laundry run is a common place where voltage drop, not ampacity, forces 8 AWG.

Continuous loads

A continuous load — one expected to run three hours or more, like commercial lighting or an EV charger — is designed at 125% of the load current. 30 A continuous becomes 37.5 A of design current, which lands on a 40 A overcurrent device and 8 AWG copper.

Frequently asked

What size wire do I need for a 30 amp breaker?: 10 AWG copper is the smallest conductor our model estimates at or above 30 A using a 75 °C insulation rating; 8 AWG is required if the terminations are limited to 60 °C. In aluminium the equivalent is 8 AWG. Verify against the code edition adopted locally before installing.
Can I use aluminium wire on a 30 amp circuit?: Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 30 A circuit steps up to 8 AWG and uses K = 21.2 in the voltage-drop formula instead of 12.9.
How far can a 30 amp circuit run?: On 10 AWG copper the 3% voltage-drop guideline is reached near 48 ft one-way at 120 V and 96 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.
Is a 30 amp breaker sized at 100% of the load?: Only for non-continuous loads. A continuous load is designed at 125%, so 30 A of continuous current becomes 37.5 A of design current and lands on a 40 A device with conductors sized to match.
What is typically on a 30 amp circuit?: Commonly electric dryers, water heaters, 240 V baseboard heat. The load type also drives the receptacle and disconnect ratings, not just the conductor size.