What wire size do I need for 125 amps?

For a 125-amp circuit, use 1/0 AWG copper wire or 2/0 AWG aluminum wire. This is per NEC Table 310.16 at 75°C insulation temperature. Common uses: service entrance and large main panels.

What breaker do I need for 125 amps?

A 125-amp circuit uses a 125A double-pole circuit breaker. The breaker protects the 1/0 AWG copper (or 2/0 AWG aluminum) wire from overload. Double-pole breakers are used for 240V circuits and occupy two slots in the panel.

What is 1/0 AWG wire used for?

1/0 AWG (one-aught) copper wire is rated for 150A at 75°C per NEC 310.16. It is commonly used for 100–125A service entrances, feeders to large sub-panels, and main breaker connections. Aluminum 1/0 AWG rates at 120A.

What is the difference between 100A and 125A panels?

A 125A main breaker provides 25% more capacity than a 100A breaker. Both use similar wire sizes (1 AWG or 1/0 AWG copper), but 125A service allows more simultaneous loads and provides headroom for EV chargers or additional appliances without immediately requiring a panel upgrade.

What Wire Size for 125 Amps?

Copper Wire

1/0 AWG

8.2515 mm · 53.4751 mm²

Aluminum Wire

2/0 AWG

9.2658 mm · 67.4309 mm²

Breaker: 125A · Based on NEC Table 310.16 (75°C insulation)

Common Uses for 125A Circuits

Service entrance, large main panels

1/0 AWG Copper — Key Specs

Diameter	8.2515 mm (0.324861")
Area	53.4751 mm²
Resistance at 20°C	0.3224 Ω/km
Weight	479.137 kg/km
Ampacity (75°C)	150 A

Important: For long wire runs, you may need to upsize the wire to compensate for voltage drop. NEC recommends keeping voltage drop under 3% for branch circuits. Use our voltage drop calculator to check.

Voltage Drop by Distance — 1/0 AWG Copper at 125A

One-way run	Drop	% of 240V	Verdict
25 ft	0.61 V	0.3%	Within the 3% branch-circuit target
50 ft	1.23 V	0.5%	Within the 3% branch-circuit target
75 ft	1.84 V	0.8%	Within the 3% branch-circuit target
100 ft	2.46 V	1.0%	Within the 3% branch-circuit target
150 ft	3.69 V	1.5%	Within the 3% branch-circuit target
200 ft	4.92 V	2.0%	Within the 3% branch-circuit target

Single-phase, 240V, full 125A load, 0.0983 Ω/1000 ft DC resistance (NEC Chapter 9, Table 8) at 75°C terminations. NEC 210.19(A) Informational Note recommends keeping branch-circuit drop at or below 3%, and 5% total across feeder plus branch circuit. Voltage drop is a performance recommendation, not a code requirement — but exceeding it causes dim lights, slow motor starts, and wasted energy.

Aluminum Feeder Specifications

Aluminum carries about 61% of the current of copper for the same cross-section, so a 125A feeder that takes 1/0 AWG copper needs 2/0 AWG aluminum — typically one to two gauge sizes larger.

Aluminum conductor	2/0 AWG
Ampacity (75°C, Table 310.16)	135 A
Copper equivalent	1/0 AWG (150 A)
Resistance	0.1198 Ω/1000 ft
Voltage drop at 100 ft, 125A	3.00 V (1.2% of 240V)

Installing aluminum correctly

Modern aluminum building wire must be AA-8000 series alloy (NEC 310.14(A)(1)). The 1960s-era AA-1350 alloy behind aluminum’s bad reputation is no longer permitted for branch circuits.
Terminate only on lugs and breakers marked AL/CU or AL9CU. Copper-only terminals are the most common failure point.
Apply an antioxidant compound to the stripped conductor before landing it, unless the termination is listed as not requiring it.
Torque every connection to the manufacturer’s value with a calibrated torque tool — NEC 110.14(D) makes this mandatory. Loose aluminum terminations are the primary cause of feeder failures.
Aluminum runs roughly half the weight and a third to half the cost of copper at feeder sizes, which is why it dominates service entrance and sub-panel feeders.

Circuit breaker size chart → Wire types and insulation ratings → NEC Table 250.122 — ground wire size → 1/0 AWG full specifications → Full AWG chart → Wire size calculator → Ampacity tables →