What size wire do I need for a 350 amp breaker?

500 kcmil copper is the smallest conductor our model estimates at or above 350 A using a 75 °C insulation rating; 600 kcmil is required if the terminations are limited to 60 °C. In aluminium the equivalent is 600 kcmil. Verify against the code edition adopted locally before installing.

Can I use aluminium wire on a 350 amp circuit?

Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 350 A circuit steps up to 600 kcmil and uses K = 21.2 in the voltage-drop formula instead of 12.9.

How far can a 350 amp circuit run?

On 500 kcmil copper the 3% voltage-drop guideline is reached near 199 ft one-way at 120 V and 398 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.

Is a 350 amp breaker sized at 100% of the load?

Only for non-continuous loads. A continuous load is designed at 125%, so 350 A of continuous current becomes 437.5 A of design current and lands on a 450 A device with conductors sized to match.

What is typically on a 350 amp circuit?

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

What Size Wire for a 350 Amp Breaker?

Conductor size, aluminium equivalent, derating and maximum run length for a 350 amp circuit — all derived from published conductor geometry and the voltage-drop formula.

Quick answer

A 350 amp circuit needs 500 kcmil copper with 75 °C terminations, or 600 kcmil aluminium. 600 kcmil copper applies where equipment terminals are limited to 60 °C. Voltage drop reaches 3% at roughly 398 ft one-way on 240 V and 199 ft on 120 V.

Formula estimate

Copper @ 75 °C

500 kcmil

Copper @ 60 °C

600 kcmil

Aluminium @ 75 °C

600 kcmil

3% drop @ 240 V

398ft

Conductor options for 350 A

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

Material / rating	Minimum size	Estimated ampacity	Margin over 350 A
Copper, 60 °C	600 kcmil	384 A	+34 A
Copper, 75 °C	500 kcmil	403 A	+53 A
Copper, 90 °C	350 kcmil	360 A	+10 A
Aluminium, 75 °C	600 kcmil	354 A	+4 A
Aluminium, 90 °C	500 kcmil	354 A	+4 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 500 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	199 ft	332 ft	1.51%
240 V 1Ø	398 ft	664 ft	0.75%
208 V 3Ø	398 ft	664 ft	0.75%
480 V 3Ø	920 ft	1,534 ft	0.33%

Source & method: VD = 2 × K × I × L / cmil single phase, 1.732 × K × I × L / cmil three phase, with K = 12.9 for copper. 500 kcmil is 500,000 circular mils and 0.0207 Ω per 1000 ft at 20 °C.

Wire size by run length for 350 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	500 kcmil	500 kcmil	500 kcmil
75 ft	500 kcmil	500 kcmil	500 kcmil
100 ft	500 kcmil	500 kcmil	500 kcmil
150 ft	500 kcmil	500 kcmil	500 kcmil
200 ft	600 kcmil	500 kcmil	500 kcmil
250 ft	750 kcmil	500 kcmil	500 kcmil
300 ft	—	500 kcmil	500 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 350 A circuit actually needs once ambient temperature and conductor count are applied.

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

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 350 A.

Where 350 A circuits are actually used

350 A appears on commercial services, industrial machine feeds, chiller and compressor plants, and switchboard sections. It is chosen from a calculated load that fell between the 300 A and 400 A standard devices, usually with parallel conductor sets.

Mechanical constraints often bind before electrical ones do. Bending radius, sidewall pressure at each 90, lug torque and pulling tension apply to any large-conductor pull, not only at 350 A — they simply become harder to satisfy as conductors get stiffer and sets multiply. The same holds for ampacity adjustment: it is triggered by having more than three current-carrying conductors in a raceway or bundle at any rating, but with parallel sets at this size the conductor count crosses that threshold routinely, and the adjustment can erase margin that a single-set calculation appeared to show.

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. 350 A continuous becomes 437.5 A of design current, which lands on a 450 A overcurrent device and 600 kcmil copper.

Frequently asked

What size wire do I need for a 350 amp breaker?: 500 kcmil copper is the smallest conductor our model estimates at or above 350 A using a 75 °C insulation rating; 600 kcmil is required if the terminations are limited to 60 °C. In aluminium the equivalent is 600 kcmil. Verify against the code edition adopted locally before installing.
Can I use aluminium wire on a 350 amp circuit?: Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 350 A circuit steps up to 600 kcmil and uses K = 21.2 in the voltage-drop formula instead of 12.9.
How far can a 350 amp circuit run?: On 500 kcmil copper the 3% voltage-drop guideline is reached near 199 ft one-way at 120 V and 398 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.
Is a 350 amp breaker sized at 100% of the load?: Only for non-continuous loads. A continuous load is designed at 125%, so 350 A of continuous current becomes 437.5 A of design current and lands on a 450 A device with conductors sized to match.
What is typically on a 350 amp circuit?: Commonly commercial service and switchgear feeders. The load type also drives the receptacle and disconnect ratings, not just the conductor size.