100 Amp Wire Size: What Gauge Wire Do You Need?
NEC-based wire gauge for a 100 ampbreaker — copper and aluminum sizes, ground wire, conduit, and when to upsize for voltage drop.
For a 100 amp feeder or circuit, use #3 AWG copper or #1 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.
100 Amp Wire Size at a Glance
| Copper wire size | #3 AWG |
|---|---|
| Aluminum wire size | #1 AWG |
| Breaker size | 100 amp |
| Ground wire | #8 AWG copper |
| Typical conduit | 1-1/4" EMT (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
- Sub panels in detached garages, shops, and additions
- 100 amp main service upgrades (see the 83% rule note)
- Large workshop and barn feeders
- Accessory dwelling unit (ADU) feeders
- Commercial branch panels
When to Upsize for Voltage Drop
At 240V with a full 100 amp load, #3 copper passes 3% voltage drop around 100 feet, and #1 aluminum behaves similarly. For long feeders to detached buildings — 150 feet and beyond — step up to #1 copper or 2/0 aluminum. Real-world feeder loads rarely sit at 100A continuously, so calculate with your realistic demand load before paying for the upsize.
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
- #3 AWG copper is rated exactly 100A at 75°C in Table 310.16; #1 AWG aluminum is also rated 100A at 75°C.
- #4 AWG copper (85A at 75°C) is only permitted for 100 amps where NEC 310.12 applies — the 83% rule for single-phase dwelling services and feeders that carry the entire dwelling load. Table 310.12 lists #4 Cu / #2 Al for a 100A dwelling service.
- For a standard 100A feeder or sub panel that does not supply the whole dwelling, the 83% rule does NOT apply — use #3 copper or #1 aluminum.
- The equipment grounding conductor for a 100 amp circuit is #8 AWG copper per Table 250.122.
- Aluminum SER cable (#1 Al, often sold as 1-1-1-3 SER) is the most economical choice for 100A feeders; copper THHN in conduit wins where physical protection or space is tight.
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 wire for 100 amps?
Only in one specific case. #4 AWG copper is rated 85A at 75°C, so it normally cannot serve a 100 amp breaker. NEC 310.12 makes an exception for 120/240V single-phase dwelling services and feeders that supply the entire dwelling load — there, conductors may be sized at 83% (Table 310.12 lists #4 Cu for 100A). A garage sub panel does not qualify: use #3 copper.
What size aluminum wire for 100 amps?
Use #1 AWG aluminum, rated exactly 100A at 75°C in Table 310.16. For a 100A whole-dwelling service, Table 310.12 permits #2 aluminum under the 83% rule. Aluminum SER cable is the standard, cost-effective choice for 100A residential feeders — just torque the terminations properly.
What size wire for a 100 amp service?
A 100 amp residential service (service-entrance conductors carrying the whole dwelling load) may use #4 AWG copper or #2 AWG aluminum per NEC Table 310.12. This is smaller than the feeder answer because the 83% rule recognizes that whole-house demand is diverse. A 100A feeder to a sub panel, by contrast, needs #3 copper or #1 aluminum.
How far can I run 100 amp wire before upsizing?
At 240V, #3 copper or #1 aluminum stays under 3% voltage drop to roughly 100 feet at a full 100 amp load. Since most feeders see well under full load, many 130-150 foot runs still calculate fine — run the numbers with your actual demand load. Beyond that, move to #1 copper or 2/0 aluminum.
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 →100 Amp Sub Panel Wire Size: What Gauge Feeder Do You Need?
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).
Read the Guide →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 →