What size wire do I need for a 225 amp breaker?

4/0 AWG copper is the smallest conductor our model estimates at or above 225 A using a 75 °C insulation rating; 300 kcmil is required if the terminations are limited to 60 °C. In aluminium the equivalent is 300 kcmil. Verify against the code edition adopted locally before installing.

Can I use aluminium wire on a 225 amp circuit?

Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 225 A circuit steps up to 300 kcmil and uses K = 21.2 in the voltage-drop formula instead of 12.9.

How far can a 225 amp circuit run?

On 4/0 AWG copper the 3% voltage-drop guideline is reached near 131 ft one-way at 120 V and 262 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.

Is a 225 amp breaker sized at 100% of the load?

Only for non-continuous loads. A continuous load is designed at 125%, so 225 A of continuous current becomes 281.3 A of design current and lands on a 300 A device with conductors sized to match.

What is typically on a 225 amp circuit?

Commonly large residential and light commercial service. The load type also drives the receptacle and disconnect ratings, not just the conductor size.

What Size Wire for a 225 Amp Breaker?

Conductor size, aluminium equivalent, derating and maximum run length for a 225 amp circuit — all derived from published conductor geometry and the voltage-drop formula.

Quick answer

A 225 amp circuit needs 4/0 AWG copper with 75 °C terminations, or 300 kcmil aluminium. 300 kcmil copper applies where equipment terminals are limited to 60 °C. Voltage drop reaches 3% at roughly 262 ft one-way on 240 V and 131 ft on 120 V.

Formula estimate

Copper @ 75 °C

4/0 AWG

Copper @ 60 °C

300 kcmil

Aluminium @ 75 °C

300 kcmil

3% drop @ 240 V

262ft

Conductor options for 225 A

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

Material / rating	Minimum size	Estimated ampacity	Margin over 225 A
Copper, 60 °C	300 kcmil	245 A	+20 A
Copper, 75 °C	4/0 AWG	230 A	+5 A
Copper, 90 °C	4/0 AWG	260 A	+35 A
Aluminium, 75 °C	300 kcmil	225 A	+0 A
Aluminium, 90 °C	250 kcmil	226 A	+1 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 4/0 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	131 ft	218 ft	2.29%
240 V 1Ø	262 ft	437 ft	1.14%
208 V 3Ø	262 ft	437 ft	1.14%
480 V 3Ø	606 ft	1,010 ft	0.49%

Source & method: VD = 2 × K × I × L / cmil single phase, 1.732 × K × I × L / cmil three phase, with K = 12.9 for copper. 4/0 AWG is 211,600 circular mils and 0.0490 Ω per 1000 ft at 20 °C.

Wire size by run length for 225 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	4/0 AWG	4/0 AWG	4/0 AWG
75 ft	4/0 AWG	4/0 AWG	4/0 AWG
100 ft	4/0 AWG	4/0 AWG	4/0 AWG
150 ft	250 kcmil	4/0 AWG	4/0 AWG
200 ft	350 kcmil	4/0 AWG	4/0 AWG
250 ft	500 kcmil	4/0 AWG	4/0 AWG
300 ft	500 kcmil	250 kcmil	4/0 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 225 A circuit actually needs once ambient temperature and conductor count are applied.

Ambient	3 conductors	4–6 conductors	7–9 conductors
30 °C	4/0 AWG	300 kcmil	400 kcmil
40 °C	250 kcmil	350 kcmil	500 kcmil
50 °C	350 kcmil	500 kcmil	600 kcmil
60 °C	500 kcmil	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 225 A.

Where 225 A circuits are actually used

225 A is a panelboard and feeder rating common in light commercial and multi-family distribution, and the standard bus rating on many main-lug load centres. Residentially it appears where a 200 A service has been upgraded to handle EV charging plus electric heat.

Parallel conductors and aluminium feeders become standard practice at this size. Conductors run in parallel must be the same size, material, length and termination method in every set, or the current divides unevenly and one set overheats while the other loafs. Termination temperature and lug quantity at the panel are again the physical limits, not the ampacity arithmetic.

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. 225 A continuous becomes 281.3 A of design current, which lands on a 300 A overcurrent device and 300 kcmil copper.

Frequently asked

What size wire do I need for a 225 amp breaker?: 4/0 AWG copper is the smallest conductor our model estimates at or above 225 A using a 75 °C insulation rating; 300 kcmil is required if the terminations are limited to 60 °C. In aluminium the equivalent is 300 kcmil. Verify against the code edition adopted locally before installing.
Can I use aluminium wire on a 225 amp circuit?: Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 225 A circuit steps up to 300 kcmil and uses K = 21.2 in the voltage-drop formula instead of 12.9.
How far can a 225 amp circuit run?: On 4/0 AWG copper the 3% voltage-drop guideline is reached near 131 ft one-way at 120 V and 262 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.
Is a 225 amp breaker sized at 100% of the load?: Only for non-continuous loads. A continuous load is designed at 125%, so 225 A of continuous current becomes 281.3 A of design current and lands on a 300 A device with conductors sized to match.
What is typically on a 225 amp circuit?: Commonly large residential and light commercial service. The load type also drives the receptacle and disconnect ratings, not just the conductor size.