100 Amp Sub Panel Wire Size: What Gauge Feeder Do You Need?
NEC-based wire gauge for a 100 amp (feeder breaker in main panel)breaker — copper and aluminum sizes, ground wire, conduit, and when to upsize for voltage drop.
For a 100 amp sub panel, use #3 AWG copper or #1 AWG aluminum feeder conductors with a #8 AWG copper equipment ground (4-wire feeder).
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.
100 Amp Wire Size at a Glance
| Copper wire size | #3 AWG |
|---|---|
| Aluminum wire size | #1 AWG |
| Breaker size | 100 amp (feeder breaker in main panel) |
| Ground wire | #8 AWG copper |
| Typical conduit | 1-1/4" EMT or Schedule 40 PVC (four #3 THHN conductors) |
Verify conduit fill for your exact conductor count and insulation type with the conduit fill calculator.
Common Uses for a 100 amp Circuit
- Detached garage and workshop panels
- Basement or addition sub panels
- Barn and outbuilding power
- ADU and in-law suite panels
- Pool equipment and outdoor kitchen panels
When to Upsize for Voltage Drop
Detached-building feeders are exactly where voltage drop bites: at 240V and full 100A load, #3 copper or #1 aluminum reaches about 100 feet before passing 3%. Most garage panels see far less than 100A of actual demand, so 150 foot runs often still calculate fine — but verify with your realistic load. For long runs, 2/0 aluminum is usually the cheapest fix.
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
- A 100A sub panel feeder needs #3 AWG copper or #1 AWG aluminum per Table 310.16 (75°C). The NEC 310.12 83% rule (#4 Cu / #2 Al) applies only when the feeder supplies the ENTIRE dwelling load — a typical sub panel feeding part of the load does not qualify.
- Sub panel feeders must be 4-wire: two hots, a neutral, and an equipment grounding conductor. The days of 3-wire sub panel feeds ended with the 2008 NEC.
- At the sub panel, neutrals and grounds must be kept separate: neutral bar isolated (bonding screw removed), grounds on a bonded ground bar per NEC 250.32(B) and 408.40.
- The feeder equipment grounding conductor is #8 AWG copper (or #6 aluminum) per Table 250.122 for a 100A overcurrent device.
- A detached building fed by a feeder needs its own grounding electrode system (ground rods) connected to the sub panel ground bar per NEC 250.32(A).
See the NEC code reference for related articles. Always confirm requirements with your local authority having jurisdiction.
Frequently Asked Questions
Can I use #4 copper for a 100 amp sub panel?
No, in almost every case. #4 copper is rated 85A at 75°C. The NEC 310.12 exception that allows #4 copper at 100A applies only to services and feeders supplying the ENTIRE dwelling load. A garage or basement sub panel feeding part of the house does not qualify — use #3 AWG copper or #1 AWG aluminum.
Does a 100 amp sub panel need 3 or 4 wires?
Four wires: two hot conductors, a neutral, and an equipment grounding conductor. The neutral and ground must remain separated at the sub panel — isolated neutral bar, bonding screw removed. Bonding them together downstream of the main panel puts normal neutral current on the ground path, which is both a code violation and a shock hazard.
What size ground wire for a 100 amp sub panel?
Run a #8 AWG copper (or #6 AWG aluminum) equipment grounding conductor with the feeder, per NEC Table 250.122 for a 100 amp breaker. If the sub panel is in a detached building, you also need ground rods at that building connected to the panel ground bar per NEC 250.32 — the rods supplement, not replace, the feeder ground wire.
Can I feed a 100 amp sub panel from a 100 amp main panel?
Yes — the sub panel breaker just cannot exceed the feeder conductor rating, and your total calculated load must fit the main service. A 100A main breaker panel can feed a 100A sub panel because loads are diversified; the main breaker protects against total overload. Run a load calculation to confirm the service can carry both panels’ demand.
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 →125 Amp Wire Size: What Gauge Wire Do You Need?
For a 125 amp circuit or feeder, use #1 AWG copper or 2/0 AWG aluminum wire (THWN/THHN at 75°C).
Read the Guide →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 →