For a standard 100-amp circuit, use 3 AWG copper wire or 1 AWG aluminum wire with a 100A breaker. This assumes THHN/THWN-2 insulation, 75°C termination ratings, 30°C ambient temperature, and no more than three current-carrying conductors in the raceway.
- Material: Copper (unless aluminum is explicitly stated)
- Termination Rating: 75°C (Standard for modern breakers and lugs per NEC 110.14(C))
- Ambient Temperature: 30°C (86°F)
- Conduit Type: Standard raceway (PVC, EMT) with max 3 current-carrying conductors
- Code Basis: NEC 2023/2026 cycle guidelines (always defer to your local AHJ)
The Direct Answer: Sizing for 100 Amps
When wiring a 100-amp subpanel, a Level 2 EV charger, or a large hot tub, the breaker and the wire must be matched perfectly to the load and the termination limits of your equipment. The most common mistake I see on jobsites is pulling wire based on the 90°C column of the ampacity table, only to fail inspection because the breaker lugs are only rated for 75°C.
For copper, 3 AWG is your baseline. For aluminum, you must step up to 1 AWG. Never present these materials interchangeably; aluminum has a higher resistance and different thermal expansion profile, requiring a larger cross-sectional area to carry the same current safely.
NEC Ampacity Table: Why 3 AWG and Not Smaller?
Let's look at the actual numbers from standard ampacity charts derived from NEC Table 310.16. You might wonder why we can't use 4 AWG copper, which is cheaper and easier to pull.
| Wire Size (AWG) | 60°C Column (Amps) | 75°C Column (Amps) | 90°C Column (Amps) |
|---|---|---|---|
| 4 AWG Copper | 70A | 85A | 95A |
| 3 AWG Copper | 85A | 100A | 110A |
| 2 AWG Copper | 95A | 115A | 130A |
| 1 AWG Aluminum | 65A | 100A | 115A |
Notice that 4 AWG copper maxes out at 85A in the 75°C column. Under NEC 240.4(B) (the "next size up" rule), you can protect an 85A conductor with a 90A breaker, but not a 100A breaker. Therefore, 4 AWG is physically and legally insufficient for a 100A circuit. You must use 3 AWG copper, which lands exactly on 100A in the 75°C column.
Voltage Drop: The 100-Foot Reality Check
Ampacity tells you what the wire can handle without melting. Voltage drop tells you if your equipment will actually run correctly at the end of the line. The NEC recommends a maximum 3% voltage drop on branch circuits and feeders for reasonable efficiency.
Let's run the math for a 100A load at 240V over a 100-foot run using 3 AWG copper:
- Formula: VD = (2 × K × I × D) / Circular Mils
- K (Copper): 12.9
- I (Current): 100A
- D (Distance): 100 ft
- CM (3 AWG): 52,620
VD = (2 × 12.9 × 100 × 100) / 52,620 = 4.9 Volts.
4.9V / 240V = 2.04% drop. This is well under the 3% threshold, meaning 3 AWG is perfectly fine for a 100-foot run.
If your subpanel is 150 feet away, that same 3 AWG wire yields a 3.06% drop, crossing the recommended threshold. For a 150-foot run, you must upsize to 2 AWG copper to keep the drop at 2.04%, or 1 AWG copper if you are pushing the full 100A continuously. Always use a voltage drop calculator before pulling wire for long runs.
Variables That Change Your Wire Size
The baseline answer assumes a perfect, short run in a cool basement. Real-world conditions often force you to upsize. Use this decision tree to adjust your wire gauge based on your specific installation environment.
| Installation Condition | Base Wire Size | Required Adjustment | NEC Reference |
|---|---|---|---|
| Standard run (< 100ft), 3 conductors | 3 AWG Cu | None (Keep 3 AWG) | 310.16 |
| Long run (> 125ft) at full 100A load | 3 AWG Cu | Up to 2 AWG or 1 AWG Cu | Chapter 9, Note 4 |
| 4 to 6 current-carrying conductors in conduit | 3 AWG Cu | Up to 2 AWG Cu (Derating) | 310.15(C)(1) |
| Ambient temp 41°C - 45°C (105°F - 113°F) | 3 AWG Cu | Up to 2 AWG Cu (Temp Correction) | Table 310.15(B)(1) |
| Using Aluminum (SER cable in attic) | N/A | Must use 1 AWG Al minimum | 310.16 |
The Bundling Trap: If you are pulling two 120V circuits and a neutral in the same conduit (4 current-carrying conductors), NEC 310.15(C)(1) requires you to derate the ampacity to 80%. While 3 AWG THHN has a 90°C rating of 110A, 80% of 110A is only 88A. That fails the 100A requirement. You must upsize to 2 AWG (130A × 0.80 = 104A) to pass inspection.
When an Engineer or AHJ Must Confirm
While this guide covers 95% of residential and light commercial 100A feeder and branch circuit scenarios, there are specific situations where you must consult a licensed professional engineer or your local Authority Having Jurisdiction (AHJ):
- Continuous Loads: If your 100A load will run for 3 hours or more continuously (like a massive server room HVAC or commercial kiln), NEC 210.20(A) requires the breaker to be sized at 125% of the continuous load (125A). This completely changes your wire and breaker sizing.
- Service Entrance Conductors: The rules for the main feed from the utility meter to your main panel are different than subpanel feeders. The National Fire Protection Association (NFPA) outlines specific service entrance rules in NEC 310.12 that sometimes allow smaller wire for dwelling services.
- High Fault Currents: If your utility transformer supplies massive available fault current, the "let-through current" and bracing requirements might dictate specific cable types or busbar configurations over standard THHN in conduit.
Frequently Asked Questions
What gauge wire for 100 amp service entrance vs. subpanel?
This is the most common point of confusion. Under NEC 310.12(A), the minimum wire size for a 100-amp residential service entrance (the main feed from the utility to your main disconnect) is 4 AWG copper or 2 AWG aluminum. However, this special allowance applies only to service entrance conductors for dwelling units. If you are wiring a 100-amp subpanel, an EV charger, or a hot tub, you are wiring a feeder or branch circuit, and you must use the standard 3 AWG copper or 1 AWG aluminum rules outlined in Table 310.16.
Can I use 4 gauge wire for a 100 amp breaker?
No, not for a standard feeder or branch circuit. 4 AWG copper is rated for 85A at 75°C. The NEC "next size up" rule (240.4(B)) allows you to protect an 85A conductor with the next standard breaker size, which is 90A. It does not allow you to jump to a 100A breaker. If you pull 4 AWG wire and terminate it on a 100A breaker, the inspector will fail it, and you risk overheating the breaker lugs during a sustained 95A load.
What size ground wire do I need for a 100 amp circuit?
According to NEC Table 250.122, the minimum size equipment grounding conductor (EGC) for a 100-amp overcurrent protection device is 8 AWG copper or 6 AWG aluminum. Note that if you had to upsize your ungrounded (hot) conductors for voltage drop over a long distance, NEC 250.122(B) requires you to increase the size of your ground wire proportionately. If you bumped your hots from 3 AWG to 1 AWG for a long run, your ground must also increase in cross-sectional area to maintain the same safety ratio.
