What size wire do I need for a 250 amp breaker?

250 kcmil copper is the smallest conductor our model estimates at or above 250 A using a 75 °C insulation rating; 350 kcmil is required if the terminations are limited to 60 °C. In aluminium the equivalent is 400 kcmil. Verify against the code edition adopted locally before installing.

Can I use aluminium wire on a 250 amp circuit?

Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 250 A circuit steps up to 400 kcmil and uses K = 21.2 in the voltage-drop formula instead of 12.9.

How far can a 250 amp circuit run?

On 250 kcmil copper the 3% voltage-drop guideline is reached near 139 ft one-way at 120 V and 279 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.

Is a 250 amp breaker sized at 100% of the load?

Only for non-continuous loads. A continuous load is designed at 125%, so 250 A of continuous current becomes 312.5 A of design current and lands on a 350 A device with conductors sized to match.

What is typically on a 250 amp circuit?

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

What Size Wire for a 250 Amp Breaker?

Conductor size, aluminium equivalent, derating and maximum run length for a 250 amp circuit — all derived from published conductor geometry and the voltage-drop formula.

Quick answer

A 250 amp circuit needs 250 kcmil copper with 75 °C terminations, or 400 kcmil aluminium. 350 kcmil copper applies where equipment terminals are limited to 60 °C. Voltage drop reaches 3% at roughly 279 ft one-way on 240 V and 139 ft on 120 V.

Formula estimate

Copper @ 75 °C

250 kcmil

Copper @ 60 °C

350 kcmil

Aluminium @ 75 °C

400 kcmil

3% drop @ 240 V

279ft

Conductor options for 250 A

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

Material / rating	Minimum size	Estimated ampacity	Margin over 250 A
Copper, 60 °C	350 kcmil	271 A	+21 A
Copper, 75 °C	250 kcmil	257 A	+7 A
Copper, 90 °C	4/0 AWG	260 A	+10 A
Aluminium, 75 °C	400 kcmil	272 A	+22 A
Aluminium, 90 °C	300 kcmil	254 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 250 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	139 ft	232 ft	2.15%
240 V 1Ø	279 ft	465 ft	1.08%
208 V 3Ø	279 ft	465 ft	1.07%
480 V 3Ø	644 ft	1,074 ft	0.47%

Source & method: VD = 2 × K × I × L / cmil single phase, 1.732 × K × I × L / cmil three phase, with K = 12.9 for copper. 250 kcmil is 250,000 circular mils and 0.0415 Ω per 1000 ft at 20 °C.

Wire size by run length for 250 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	250 kcmil	250 kcmil	250 kcmil
75 ft	250 kcmil	250 kcmil	250 kcmil
100 ft	250 kcmil	250 kcmil	250 kcmil
150 ft	300 kcmil	250 kcmil	250 kcmil
200 ft	400 kcmil	250 kcmil	250 kcmil
250 ft	500 kcmil	250 kcmil	250 kcmil
300 ft	600 kcmil	300 kcmil	250 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 250 A circuit actually needs once ambient temperature and conductor count are applied.

Ambient	3 conductors	4–6 conductors	7–9 conductors
30 °C	250 kcmil	350 kcmil	500 kcmil
40 °C	300 kcmil	500 kcmil	600 kcmil
50 °C	400 kcmil	600 kcmil	750 kcmil
60 °C	600 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 250 A.

Where 250 A circuits are actually used

250 A serves commercial services, large multi-family feeders, machine and process equipment feeds, and elevator or fire-pump distribution. On the residential side it is the size of an all-electric home with heat pumps, an EV and a large PV interconnection.

At 250 A the conductor is normally kcmil rather than AWG, and the design questions shift to raceway family, pull length and available fault current. The overcurrent device must also have an interrupting rating at or above the available fault current at its point of installation — a requirement that applies to overcurrent devices generally, but one that is far more likely to be the binding constraint on service and feeder equipment of this size than on a small branch circuit.

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. 250 A continuous becomes 312.5 A of design current, which lands on a 350 A overcurrent device and 350 kcmil copper.

Frequently asked

What size wire do I need for a 250 amp breaker?: 250 kcmil copper is the smallest conductor our model estimates at or above 250 A using a 75 °C insulation rating; 350 kcmil is required if the terminations are limited to 60 °C. In aluminium the equivalent is 400 kcmil. Verify against the code edition adopted locally before installing.
Can I use aluminium wire on a 250 amp circuit?: Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 250 A circuit steps up to 400 kcmil and uses K = 21.2 in the voltage-drop formula instead of 12.9.
How far can a 250 amp circuit run?: On 250 kcmil copper the 3% voltage-drop guideline is reached near 139 ft one-way at 120 V and 279 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.
Is a 250 amp breaker sized at 100% of the load?: Only for non-continuous loads. A continuous load is designed at 125%, so 250 A of continuous current becomes 312.5 A of design current and lands on a 350 A device with conductors sized to match.
What is typically on a 250 amp circuit?: Commonly commercial feeders. The load type also drives the receptacle and disconnect ratings, not just the conductor size.