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.

Wire size by run length for 300 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	350 kcmil	350 kcmil	350 kcmil
75 ft	350 kcmil	350 kcmil	350 kcmil
100 ft	350 kcmil	350 kcmil	350 kcmil
150 ft	350 kcmil	350 kcmil	350 kcmil
200 ft	500 kcmil	350 kcmil	350 kcmil
250 ft	600 kcmil	350 kcmil	350 kcmil
300 ft	750 kcmil	350 kcmil	350 kcmil

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 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.

Where 300 A circuits are actually used

300 A is a service and main-distribution rating: commercial buildings, apartment house services, large process equipment, and multi-panel industrial distribution. It is nearly always fed with parallel sets or a bus rather than a single conductor per phase.

Voltage drop is a function of current, conductor area, one-way length, material and system voltage — there is no drop rule specific to 300 A. What makes it worth checking early here is scale: feeders at this rating are usually long and run in parallel sets, so a size that satisfies ampacity can still exceed the 3% guideline, and the remedy is upsizing every set or moving the distribution point closer. Running the drop calculation before selecting the raceway is cheaper than repulling. Voltage drop limits are recommendations in the code, not enforceable requirements.

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.