What Size Wire for a 125 Amp Breaker?

Conductor options for 125 A

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

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 1 AWG copper

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

Source & method: VD = 2 × K × I × L / cmil single phase, 1.732 × K × I × L / cmil three phase, with K = 12.9 for copper. 1 AWG is 83,693 circular mils and 0.1239 Ω per 1000 ft at 20 °C.

Wire size by run length for 125 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.

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 125 A circuit actually needs once ambient temperature and conductor count are applied.

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 125 A.

Where 125 A circuits are actually used

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. 125 A continuous becomes 156.3 A of design current, which lands on a 175 A overcurrent device and 2/0 AWG copper.

Frequently asked

What size wire do I need for a 125 amp breaker?

1 AWG copper is the smallest conductor our model estimates at or above 125 A using a 75 °C insulation rating; 2/0 AWG is required if the terminations are limited to 60 °C. In aluminium the equivalent is 2/0 AWG. Verify against the code edition adopted locally before installing.

Can I use aluminium wire on a 125 amp circuit?

Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 125 A circuit steps up to 2/0 AWG and uses K = 21.2 in the voltage-drop formula instead of 12.9.

How far can a 125 amp circuit run?

On 1 AWG copper the 3% voltage-drop guideline is reached near 93 ft one-way at 120 V and 186 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.

Is a 125 amp breaker sized at 100% of the load?

Only for non-continuous loads. A continuous load is designed at 125%, so 125 A of continuous current becomes 156.3 A of design current and lands on a 175 A device with conductors sized to match.

What is typically on a 125 amp circuit?

Commonly subpanels in larger homes. The load type also drives the receptacle and disconnect ratings, not just the conductor size.

Other breaker sizes

• 15 A
• 20 A
• 25 A
• 30 A
• 35 A
• 40 A
• 45 A
• 50 A
• 60 A
• 70 A
• 80 A
• 90 A
• 100 A
• 110 A
• 150 A
• 175 A
• 200 A
• 225 A
• 250 A
• 300 A
• 350 A
• 400 A

More on 1 AWG

• 1 AWG voltage drop
• 1 AWG conduit fill
• 1 AWG breaker size

Related charts & calculators

• Allen Wrench Size ChartHex key sizes with SAE and metric cross-reference.
• Voltage Drop CalculatorPercent drop, end voltage and minimum conductor for any run length.
• Conduit Fill CalculatorFill percentage and maximum conductor count for EMT, RMC, IMC and PVC.
• Breaker Size CalculatorOvercurrent device rating and minimum conductor from load current.
• Wire Ampacity CalculatorDerated ampacity for ambient temperature and bundled conductors.
• Box Fill CalculatorVolume allowances for conductors, clamps, devices and grounds per NEC 314.16.