What size wire do I need for a 300 amp breaker?

350 kcmil copper is the smallest conductor our model estimates at or above 300 A using a 75 °C insulation rating; 500 kcmil is required if the terminations are limited to 60 °C. In aluminium the equivalent is 500 kcmil. Verify against the code edition adopted locally before installing.

Can I use aluminium wire on a 300 amp circuit?

Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 300 A circuit steps up to 500 kcmil and uses K = 21.2 in the voltage-drop formula instead of 12.9.

How far can a 300 amp circuit run?

On 350 kcmil copper the 3% voltage-drop guideline is reached near 162 ft one-way at 120 V and 325 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.

Is a 300 amp breaker sized at 100% of the load?

Only for non-continuous loads. A continuous load is designed at 125%, so 300 A of continuous current becomes 375 A of design current and lands on a 400 A device with conductors sized to match.

What is typically on a 300 amp circuit?

Commonly commercial feeders and distribution panels. The load type also drives the receptacle and disconnect ratings, not just the conductor size.

What Size Wire for a 300 Amp Breaker?

Conductor size, aluminium equivalent, derating and maximum run length for a 300 amp circuit — all derived from published conductor geometry and the voltage-drop formula.

Quick answer

A 300 amp circuit needs 350 kcmil copper with 75 °C terminations, or 500 kcmil aluminium. 500 kcmil copper applies where equipment terminals are limited to 60 °C. Voltage drop reaches 3% at roughly 325 ft one-way on 240 V and 162 ft on 120 V.

Formula estimate

Copper @ 75 °C

350 kcmil

Copper @ 60 °C

500 kcmil

Aluminium @ 75 °C

500 kcmil

3% drop @ 240 V

325ft

Conductor options for 300 A

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

Material / rating	Minimum size	Estimated ampacity	Margin over 300 A
Copper, 60 °C	500 kcmil	341 A	+41 A
Copper, 75 °C	350 kcmil	319 A	+19 A
Copper, 90 °C	300 kcmil	326 A	+26 A
Aluminium, 75 °C	500 kcmil	314 A	+14 A
Aluminium, 90 °C	400 kcmil	306 A	+6 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 350 kcmil copper

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

System	3% limit	5% limit	Drop @ 100 ft
120 V 1Ø	162 ft	271 ft	1.84%
240 V 1Ø	325 ft	542 ft	0.92%
208 V 3Ø	325 ft	543 ft	0.92%
480 V 3Ø	751 ft	1,253 ft	0.4%

Source & method: VD = 2 × K × I × L / cmil single phase, 1.732 × K × I × L / cmil three phase, with K = 12.9 for copper. 350 kcmil is 350,000 circular mils and 0.0296 Ω per 1000 ft at 20 °C.

Hot attics, conduit runs and bundling

What a 300 A circuit actually needs once ambient temperature and conductor count are applied.

Ambient	3 conductors	4–6 conductors	7–9 conductors
30 °C	350 kcmil	500 kcmil	600 kcmil
40 °C	400 kcmil	600 kcmil	750 kcmil
50 °C	500 kcmil	750 kcmil	—
60 °C	750 kcmil	—	—

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 300 A.

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. 300 A continuous becomes 375 A of design current, which lands on a 400 A overcurrent device and 500 kcmil copper.

Frequently asked

What size wire do I need for a 300 amp breaker?: 350 kcmil copper is the smallest conductor our model estimates at or above 300 A using a 75 °C insulation rating; 500 kcmil is required if the terminations are limited to 60 °C. In aluminium the equivalent is 500 kcmil. Verify against the code edition adopted locally before installing.
Can I use aluminium wire on a 300 amp circuit?: Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 300 A circuit steps up to 500 kcmil and uses K = 21.2 in the voltage-drop formula instead of 12.9.
How far can a 300 amp circuit run?: On 350 kcmil copper the 3% voltage-drop guideline is reached near 162 ft one-way at 120 V and 325 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.
Is a 300 amp breaker sized at 100% of the load?: Only for non-continuous loads. A continuous load is designed at 125%, so 300 A of continuous current becomes 375 A of design current and lands on a 400 A device with conductors sized to match.
What is typically on a 300 amp circuit?: Commonly commercial feeders and distribution panels. The load type also drives the receptacle and disconnect ratings, not just the conductor size.

Wire and breakers for a 300 A circuit

350 kcmil copper wire

Matched to the 300 A sizing on this page.

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