The minimum correct copper wire size for a 100-amp circuit is 3 AWG when terminating on standard 75°C-rated breakers and lugs, or 1 AWG if you are pulling NM-B (Romex) cable limited to the 60°C column. Ampacity is the maximum continuous current a specific conductor can carry safely without exceeding its insulation and termination temperature ratings.
If you are asking what size copper for 100 amp feeds because you are wiring a subpanel, an EV charger, or a heavy workshop tool, getting this wrong means tripped breakers at best, and melted terminal lugs at worst. This guide breaks down the exact National Electrical Code (NEC) requirements, the math behind voltage drop, and the real-world traps that cause DIYers to fail their rough-in inspections.
The Direct Answer: Sizing Copper for 100 Amps
To safely carry 100 amps, your wire must have an allowable ampacity of at least 100A in the temperature column that matches your equipment's terminations. Most modern residential breakers and panelboard lugs are rated for 75°C. Therefore, you must use the 75°C column of NEC Table 310.16.
• 3 AWG THHN/THWN-2 (in conduit): 100A @ 75°C
• 2 AWG THHN/THWN-2 (in conduit): 115A @ 75°C (Recommended for long runs)
• 1 AWG NM-B (Romex): 110A @ 60°C
• 4 AWG Copper: 85A @ 75°C (Code violation on a 100A breaker)
What people commonly confuse this with is aluminum sizing or the 90°C insulation column. Many online forums incorrectly recommend 4 AWG copper for 100 amps. They are either looking at the 90°C column (where 4 AWG is rated 95A—still not 100A) or they are confusing copper with aluminum, where 1/0 AWG is required for 100A. What this changes in a real circuit is the thermal bottleneck: even if your THHN wire insulation can handle 90°C, the breaker lug cannot. The weakest link dictates the legal ampacity.
| Material | Wire Type | Minimum AWG for 100A | Actual Ampacity | Approx. Cost per Foot (2026) |
|---|---|---|---|---|
| Copper | THHN/THWN-2 | 3 AWG | 100 Amps | $2.10 - $2.80 |
| Copper | NM-B (Romex) | 1 AWG | 110 Amps | $4.50 - $6.00 |
| Aluminum | XHHW-2 / THWN-2 | 1/0 AWG | 120 Amps | $0.80 - $1.20 |
The Terminal Temperature Trap (NEC 110.14)
The most frequent reason a 100-amp subpanel feed fails inspection is a violation of NEC 110.14(C). This rule states that the ampacity of a conductor must be selected based on the lowest temperature rating of any connected termination, conductor, or device.
Imagine water flowing through a pipe that narrows at the valve. The valve restricts the total flow, regardless of how wide the pipe is. Similarly, THHN wire has a 90°C insulation rating, but standard square D or Eaton breaker lugs are stamped 'AL/CU 75°C'. If you try to push 100 amps through 4 AWG copper (rated 95A at 90°C), the wire itself might not melt, but the 75°C breaker lug will overheat, oxidize, and eventually fail. You must size the wire using the 75°C column, making 3 AWG the absolute minimum for copper.
Where You Meet This in Practice
You will typically need to size copper for a 100-amp circuit in the following scenarios:
- Detached Garage Subpanels: Feeding a 100A main lug subpanel from a 200A main service to run welders, compressors, and lighting.
- Level 2 EV Chargers: Hardwired 80A continuous chargers (like the Tesla Wall Connector or ChargePoint Home Flex) require a 100A breaker due to the NEC 125% continuous load rule.
- Hot Tubs and Spa Panels: Large dual-motor spas with inline heaters frequently require a 100A GFCI-protected disconnect.
- Whole-Home Backup Generators: 22kW to 26kW air-cooled standby generators often utilize a 100A automatic transfer switch (ATS).
Real-World Scenario: The 150-Foot EV Charger Run
The Setup: A homeowner decided to install an 80A continuous hardwired EV charger in a detached garage, 150 feet from the main panel. The continuous load requires a 100A breaker (80A x 1.25 = 100A). To save money, the installer pulled 4 AWG THHN copper in PVC conduit, relying on an outdated internet chart claiming 4 AWG was sufficient for 100A.
The Numbers: 4 AWG copper has an ampacity of 85A at 75°C. Furthermore, at 150 feet, voltage drop becomes a factor. Using the Southwire Voltage Drop Calculator, pushing 80A through 150 feet of 4 AWG copper yields a voltage drop of 7.39V (3.08% on a 240V circuit).
The Outcome & What Went Wrong: Two things failed here. First, the 4 AWG wire was legally only rated for 85A. The NEC 240.4(B) 'next size up' rule allows you to round up to the next standard breaker size if your ampacity doesn't match a standard breaker, but the next size up from 85A is 90A, not 100A. Putting 4 AWG on a 100A breaker is a direct fire hazard. Second, the 3.08% voltage drop caused the EV charger's internal contactors to chatter and throw under-voltage fault codes during peak summer heat when the wire's resistance increased.
The Fix: We pulled new 2 AWG copper. At 115A (75°C), it easily handles the 100A breaker requirement, and the voltage drop at 80A over 150 feet shrank to 1.9%, well under the NEC recommended 3% limit for branch circuits.
Sizing Steps for Your Specific Installation
Follow this sequence to ensure your 100-amp copper feed is safe and code-compliant:
- Calculate the Actual Load: Is the load continuous (on for 3+ hours, like an EV charger)? If yes, multiply the load by 1.25. An 80A continuous load requires 100A of capacity.
- Verify Termination Ratings: Check the breaker and panelboard labels. If they say 75°C, use the 75°C column. If you are using NM-B cable, you are legally forced into the 60°C column regardless of the breaker rating.
- Select Base Wire Size: Choose 3 AWG copper for conduit runs (75°C) or 1 AWG copper for NM-B (60°C).
- Run the Voltage Drop Math: Use the formula: VD = (2 x Length x Resistance per 1000ft x Load Amps) / 1000. If the drop exceeds 3% for a branch circuit or 5% total from the utility transformer, bump up one wire size (e.g., from 3 AWG to 2 AWG).
- Size the Equipment Ground: Per NEC 250.122, the ground wire scales with the breaker, not the ungrounded conductors. For a 100A breaker, you need an 8 AWG copper equipment grounding conductor, even if you bumped your hot wires up to 2 AWG for voltage drop.
Frequently Asked Questions
Can I use 4 AWG copper for a 100 amp breaker?
No. Under the 2023 and 2026 NEC, 4 AWG copper is rated for a maximum of 85 amps at 75°C. The 'next size up' rule only allows you to protect 85A wire with a 90A breaker. A 100A breaker will not trip before the 4 AWG wire overheats, creating a severe fire risk.
What size aluminum wire do I need for 100 amps?
You need 1/0 AWG aluminum (rated 120A at 75°C) or 2 AWG aluminum (rated 90A at 75°C, which can be rounded up to 100A per NEC 240.4(B) if the exact load is calculated below 90A). However, for a standard 100A feeder, 1/0 AWG aluminum is the safest, most universally accepted choice that avoids inspector pushback.
Do I need to pull 4 wires for a 100A subpanel?
Yes. A modern 100A subpanel feed requires two hot wires (3 AWG copper), one neutral (3 AWG copper, must be white or gray), and one separate equipment ground (8 AWG copper). You cannot bond the neutral and ground at a subpanel; they must remain isolated on separate busbars.
