What Wire Size for 125 Amps?

Copper Wire

1 AWG

7.3481 mm · 42.4077 mm²

Aluminum Wire

2/0 AWG

9.2658 mm · 67.4309 mm²

Breaker: 125A · Based on NEC Table 310.16, 75°C column (NEC 110.14(C) above 100A)

125-Amp Wire Size by Termination Rating

Rule	Copper	Aluminum
75°C column — NEC Table 310.16	1 AWG	2/0 AWG
Dwelling service or main feeder — NEC 310.12 (83% = 103.75A)	2 AWG	1/0 AWG

Above 100A, NEC 110.14(C)(1)(b) allows the 75°C column when terminations are rated 75°C, which is standard for this equipment.

Common Uses for 125A Circuits

Service entrance, large main panels

1 AWG Copper — Key Specs

Diameter	7.3481 mm (0.289297")
Area	42.4077 mm²
Resistance at 20°C	0.4065 Ω/km
Weight	379.973 kg/km
Ampacity (75°C)	130 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 AWG Copper at 125A

One-way run	Drop	% of 240V	Verdict
25 ft	0.96 V	0.4%	Within the 3% branch-circuit target
50 ft	1.93 V	0.8%	Within the 3% branch-circuit target
75 ft	2.89 V	1.2%	Within the 3% branch-circuit target
100 ft	3.85 V	1.6%	Within the 3% branch-circuit target
150 ft	5.78 V	2.4%	Within the 3% branch-circuit target
200 ft	7.70 V	3.2%	Over 3% — upsize one gauge

Single-phase, 240V, full 125A load, 0.154 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 has about 61% of copper’s conductivity, 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 AWG (130 A)
Resistance	0.1198 Ω/1000 ft
Voltage drop at 100 ft, 125A	3.98 V (1.7% 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.

Frequently Asked Questions

What wire size do I need for 125 amps?

For a 125-amp circuit, NEC Table 310.16 at 75°C calls for 1 AWG copper (130A) or 2/0 AWG aluminum (135A). If it is the service or main feeder for a dwelling, NEC 310.12 allows sizing at 83% (103.75A): 2 AWG copper or 1/0 AWG aluminum.

What breaker do I need for 125 amps?

A 125-amp circuit uses a 125A double-pole circuit breaker. The breaker protects the 1 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 AWG wire used for?

1 AWG copper is rated 130A at 75°C per NEC Table 310.16 (110A at 60°C), which makes it the minimum copper size for a 125A circuit above 100A. It is used for 125A feeders, large sub-panels and, under NEC 310.12, 150A dwelling services. 1 AWG aluminum is rated 100A.

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

A 125A main breaker provides 25% more capacity than a 100A breaker. The conductors step up too: a 125A circuit needs 1 AWG copper or 2/0 aluminum under Table 310.16, while a 100A feeder needs 1 AWG copper or 1/0 aluminum at 60°C. The extra capacity gives headroom for an EV charger or additional appliances.

Wire size for other amperages

15A 20A 30A 40A 50A 60A 70A 80A 100A 150A 200A

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