For a 100 amp breaker, you need 3 AWG copper wire or 1 AWG aluminum wire, assuming 75°C rated terminations. This is the exact minimum based on the 75°C column of NEC Table 310.16. Never downsize to 4 AWG copper or 2 AWG aluminum; they will overheat and degrade before the breaker clears a fault.
Baseline Sizing Rules and Assumptions
Wire sizing is not a guessing game; it is a strict thermal calculation. The numbers provided above are the absolute code-minimums for standard residential and commercial installations. Before pulling any wire, verify that your installation matches the baseline assumptions below. If your environment deviates from these parameters, the required wire size will increase.
- Material: Copper (primary), Aluminum (secondary)
- Temperature Column: 75°C (Standard for modern breakers, lugs, and panels)
- Ambient Temperature: 30°C (86°F)
- Conduit/Raceway: Standard conduit or cable assembly (e.g., SER cable, THHN in PVC)
- Bundling: 3 or fewer current-carrying conductors in a single raceway
A common mistake on the jobsite is using the 90°C column for THHN wire to justify a smaller gauge. While THHN insulation is rated for 90°C, the National Electrical Code (NEC) mandates that you must size the wire based on the lowest temperature rating of any connected termination, device, or conductor. Since almost all standard 100A breakers and panel lugs are rated for 75°C, your ampacity lookup must stop at the 75°C column.
Ampacity Data and Voltage Drop Checks
The table below maps out the exact ampacity thresholds for common wire sizes surrounding the 100A mark. Notice the hard stop at 3 AWG Copper and 1 AWG Aluminum.
| Wire Size (AWG) | Material | 75°C Ampacity | 90°C Ampacity (THHN) | Use for 100A Breaker? |
|---|---|---|---|---|
| 4 AWG | Copper | 85A | 95A | No (Undersized) |
| 3 AWG | Copper | 100A | 110A | Yes (Code Minimum) |
| 2 AWG | Copper | 115A | 130A | Yes (VD Buffer) |
| 2 AWG | Aluminum | 90A | 100A | No (Undersized at 75°C) |
| 1 AWG | Aluminum | 100A | 115A | Yes (Code Minimum) |
| 1/0 AWG | Aluminum | 120A | 135A | Yes (VD Buffer) |
The Voltage Drop Reality Check
NEC ampacity tables tell you what size wire will prevent a fire. They do not guarantee your equipment will run efficiently. The NEC recommends a maximum voltage drop of 3% for branch circuits and feeders. Let's run a voltage drop check for a 240V circuit pulling an 80A continuous load using 3 AWG Copper, referencing standard manufacturer voltage drop calculators.
- At 50 feet: 1.96V drop (0.8%) — Perfectly fine.
- At 100 feet: 3.92V drop (1.6%) — Well within the 3% limit.
- At 150 feet: 5.88V drop (2.45%) — Acceptable, but getting warm.
- At 200 feet: 7.84V drop (3.2%) — Fails the 3% recommendation.
If your run from the main panel to the subpanel or load exceeds 150 feet, you must upsize to 2 AWG Copper or 1/0 AWG Aluminum to maintain power quality and prevent motor burnout or dimming lights.
Derating Factors That Force an Upsize
The baseline assumptions rarely survive contact with a real-world jobsite. Here is what changes the answer and forces you to buy thicker wire.
1. Conductor Bundling
If you pull more than three current-carrying conductors through a single conduit, the trapped heat requires you to derate the wire's ampacity per NEC 310.15(C)(1). If you have 4 to 6 conductors in a pipe, you must multiply the 90°C ampacity by 80%.
Example: 3 AWG THHN has a 90°C ampacity of 110A. Multiplied by 0.80, your new ampacity is 88A. This is now too small for a 100A breaker. You must upsize to 2 AWG or 1 AWG copper.
2. High Ambient Temperatures
If your conduit runs through an unventilated attic in a southern climate where ambient temperatures regularly exceed 30°C (86°F), you must apply temperature correction factors. At 41°C to 45°C (105°F to 113°F), the correction factor for 75°C insulation is 0.82. A 3 AWG copper wire (100A) derates to 82A, requiring an upsize to 1 AWG copper.
3. Aluminum vs. Copper Terminations
Never treat aluminum and copper interchangeably. Aluminum expands and contracts at a different rate than copper and is prone to oxidation. If you use 1 AWG Aluminum, you must use an anti-oxidant compound (like Noalox) on the stripped ends and torque the lugs to the manufacturer's exact specification using a calibrated torque screwdriver, as mandated by NEC 110.14(D).
When an Engineer or the AHJ Must Confirm
While the rules above cover 95% of residential and light commercial 100A feeder or branch circuits, you must pull a permit and have the local Authority Having Jurisdiction (AHJ) or a licensed electrical engineer review your plans under these conditions:
- You are sizing conductors for a 100% rated breaker (rare in standard residential panels, common in industrial switchgear).
- The run involves utility transformer secondary lugs, which often have strict 60°C limitations or specific aluminum-only sizing requirements.
- You are running parallel conductors (not permitted for sizes smaller than 1/0 AWG per NEC 310.10(G)).
- The calculated continuous load is exactly 100A, which legally requires a 125A breaker and 1 AWG Copper / 2/0 Aluminum wire.
Frequently Asked Questions
Can I use 4 AWG copper wire on a 100 amp breaker if my actual load is only 80 amps?
No. While 4 AWG copper is rated for 85A at 75°C, NEC 240.4(B) allows you to round up to the next standard breaker size to protect the wire. The standard breaker sizes are 80A, 90A, and 100A. The next standard size up from 85A is 90A. Therefore, 4 AWG copper can be protected by a 90A breaker, but it cannot legally be protected by a 100A breaker. You must use 3 AWG copper minimum.
What size ground wire do I need for a 100 amp breaker?
According to NEC Table 250.122, the minimum equipment grounding conductor (EGC) size for a 100A overcurrent protection device is 8 AWG copper or 6 AWG aluminum. If you upsized your current-carrying conductors for voltage drop (e.g., using 1 AWG copper instead of 3 AWG), NEC 250.122(B) requires you to proportionally upsize the ground wire as well to maintain the same ratio of resistance.
Does the 100 amp breaker wire size change if I run it underground in PVC conduit?
The baseline ampacity does not change simply because the conduit is underground, as earth temperature generally stabilizes around 20°C (68°F), which is cooler than the 30°C ambient baseline. However, if you are using direct burial cable (like UF-B or USE-2) instead of THHN in conduit, you must check the specific ampacity table for that cable type (NEC Table 310.10(A)(2)), as direct burial cables often have lower ampacity ratings due to different thermal dissipation properties in soil.
Why do some online calculators say I need 2 AWG or 1 AWG copper for 100 amps?
This confusion usually stems from two sources. First, many calculators default to a 3% voltage drop limit over a long distance (like 100+ feet), which automatically bumps 3 AWG up to 2 AWG. Second, if you tell a calculator you have a "100 Amp Load" rather than a "100 Amp Breaker", the software applies the NEC 125% continuous load rule. A 100A continuous load requires a wire rated for 125A, which pushes you to 1 AWG Copper and a 125A breaker. Always distinguish between your breaker size and your actual calculated load.
