125 Amp Wire Size: What Gauge Wire Do You Need?
NEC-based wire gauge for a 125 ampbreaker — copper and aluminum sizes, ground wire, conduit, and when to upsize for voltage drop.
For a 125 amp circuit or feeder, use #1 AWG copper or 2/0 AWG aluminum wire (THWN/THHN at 75°C).
Based on NEC Table 310.16 (75°C column) and, where noted, Table 310.12 for dwelling services. Confirm termination ratings and local amendments before installing.
125 Amp Wire Size at a Glance
| Copper wire size | #1 AWG |
|---|---|
| Aluminum wire size | 2/0 AWG |
| Breaker size | 125 amp |
| Ground wire | #6 AWG copper |
| Typical conduit | 1-1/2" EMT (four #1 THHN conductors) |
Verify conduit fill for your exact conductor count and insulation type with the conduit fill calculator.
Common Uses for a 125 amp Circuit
- Larger sub panels for shops with welders and machine tools
- ADU and guest house feeders with electric heat
- 125 amp service upgrades on smaller homes
- Feeders for multi-unit panelboards
- Large hot tub/pool equipment combination panels
When to Upsize for Voltage Drop
At 240V and a full 125 amp load, #1 copper stays under 3% voltage drop to roughly 105 feet, and 2/0 aluminum performs similarly. For long feeders, 2/0 copper or 3/0 aluminum buys you 130+ feet. As always, calculate with the realistic demand load — a 125A feeder loafing at 60A can run twice as far.
The NEC recommends (in informational notes to 210.19 and 215.2) keeping voltage drop under 3% on a branch circuit or feeder and 5% total. Ampacity tables ignore distance entirely, so on any run much past 100 feet, check the numbers with the voltage drop calculator before buying wire.
NEC References and Code Notes
- #1 AWG copper is rated 130A at 75°C in Table 310.16, covering a 125 amp breaker. 2/0 AWG aluminum is rated 135A at 75°C.
- For a 125A whole-dwelling service or feeder, NEC Table 310.12 (83% rule) permits #2 AWG copper or 1/0 AWG aluminum.
- The equipment grounding conductor jumps to #6 AWG copper at 125 amps per Table 250.122, because the breaker exceeds the table’s 100A row.
- 125A is a common panelboard bus rating; check that the feeder breaker, conductors, and panel bus ratings all align.
See the NEC code reference for related articles. Always confirm requirements with your local authority having jurisdiction.
Frequently Asked Questions
What size wire do I need for a 125 amp sub panel?
Use #1 AWG copper or 2/0 AWG aluminum feeder conductors on a 125 amp breaker, with a #6 AWG copper equipment ground per Table 250.122. Run it as a 4-wire feeder with neutrals and grounds separated at the sub panel. 2/0 aluminum SER cable is the usual budget choice.
Can #2 copper carry 125 amps?
#2 AWG copper is rated 115A at 75°C — short of a 125 amp breaker for a normal feeder. The exception is a dwelling service or whole-dwelling feeder, where NEC Table 310.12 lists #2 copper as acceptable for 125A under the 83% rule. For a standard sub panel feeder, step up to #1 copper.
What ground wire does a 125 amp feeder need?
A #6 AWG copper (or #4 AWG aluminum) equipment grounding conductor, per NEC Table 250.122. This is one size larger than the #8 copper used at 100 amps — the table steps at "over 100A," so 110A, 125A, and 150A breakers all take #6 copper grounds.
Is 125 amp service enough for a house?
Often, yes. A 1,500-2,500 sq ft home with gas heat and cooking typically calculates well under 125A using the NEC 220.82 optional method. All-electric homes with EV charging, heat pumps, and electric ranges push loads higher — run a load calculation before choosing 125A over a 200A service, since the labor cost is nearly identical.
Related Tools and Wire Size Guides
Wire Sizing Calculator
Calculate the exact wire gauge for any load, voltage, and distance per NEC tables.
Calculate Wire Size →Voltage Drop Calculator
Check whether your run length forces a larger conductor than ampacity alone requires.
Calculate Voltage Drop →Wire Ampacity Chart
Full NEC Table 310.16 ampacities at 60°C, 75°C, and 90°C for copper and aluminum.
View Ampacity Chart →Breaker Size Calculator
Find the right breaker for a given load, including the 125% continuous-load rule.
Size a Breaker →150 Amp Wire Size: What Gauge Wire Do You Need?
For a 150 amp circuit or feeder, use 1/0 AWG copper or 3/0 AWG aluminum wire (THWN/THHN at 75°C).
Read the Guide →200 Amp Service Wire Size: What Gauge Wire Do You Need?
For a 200 amp residential service, use 2/0 AWG copper or 4/0 AWG aluminum wire (per NEC Table 310.12, the 83% rule for dwelling services).
Read the Guide →30 Amp Wire Size: What Gauge Wire Do You Need?
For a 30 amp circuit, use #10 AWG copper or #8 AWG aluminum wire (THWN/THHN rated at 75°C).
Read the Guide →