You need 3 AWG copper wire and a 100-amp breaker for a standard 100-amp circuit. This assumes THHN/THWN-2 insulation in a raceway, a 75°C temperature rating, and an ambient temperature of 30°C (86°F). If using NM-B (Romex) cable, you must step up to 1 AWG copper due to its 60°C column limitation.
Baseline Assumptions for 100-Amp Sizing
Wire sizing is never a one-size-fits-all number. The 3 AWG answer relies on a specific set of jobsite conditions. If your installation deviates from these parameters, the required wire size will change. Always verify these baseline assumptions before purchasing materials:
- Conductor Material: Copper (Do not use aluminum ampacity charts for copper wire).
- Insulation Type: THHN/THWN-2 in conduit, or XHHW-2. (NM-B/Romex requires different sizing, detailed below).
- Temperature Column: 75°C column per NEC Table 310.16. This assumes your breaker and panel lugs are rated for 75°C, which is standard for modern 100A equipment per NEC 110.14(C).
- Ambient Temperature: 30°C (86°F) or lower. Attics or hot garages require temperature derating.
- Conduit Fill: 3 or fewer current-carrying conductors in the raceway. (4+ conductors require ampacity derating).
- Load Type: Non-continuous load (operating for less than 3 hours continuously).
NEC Ampacity Table: Why 3 AWG Copper?
To understand why 3 AWG is the minimum, we have to look at the actual ampacity limits defined by the National Electrical Code. A common and dangerous DIY mistake is using 4 AWG copper because it is cheaper and easier to pull through conduit. However, 4 AWG copper is only rated for 85 amps in the 75°C column. If you push 100 amps through 4 AWG wire, the insulation will degrade, melt, and eventually cause a short circuit or fire long before the 100-amp breaker trips.
Here is how the relevant rows of NEC Table 310.16 break down for copper conductors:
| AWG Size | 60°C Column (NM-B / Romex) | 75°C Column (THHN in Conduit) | 90°C Column (Derating Baseline) |
|---|---|---|---|
| 4 AWG | 70A | 85A | 95A |
| 3 AWG | 85A | 100A (Target) | 115A |
| 2 AWG | 95A | 115A | 130A |
| 1 AWG | 110A | 130A | 145A |
The NM-B (Romex) Trap: Per NEC 334.80, NM-B cable ampacity must be determined using the 60°C column, regardless of the fact that the wire insulation inside the sheath might be rated for 90°C. As the table shows, 3 AWG in the 60°C column is only good for 85A. Therefore, if you are running a 100-amp feeder using NM-B cable, you must use 1 AWG copper (rated 110A at 60°C) to safely carry the load.
Voltage Drop Check: When to Upsize
Ampacity tells you what size wire prevents a fire; voltage drop tells you what size wire ensures your equipment actually works. The NEC recommends a maximum 3% voltage drop for branch circuits and a 5% maximum for the total feeder plus branch circuit combined (NEC 215.2 Informational Note).
Let us run a voltage drop check for a 240V, 100-amp circuit using 3 AWG copper at a distance of 150 feet. Using the standard single-phase voltage drop formula and a copper K-factor of 12.9:
Calculation: VD = (2 × K × I × D) / Circular Mils
VD = (2 × 12.9 × 100A × 150ft) / 52,620 CM
VD = 387,000 / 52,620 = 7.35 Volts
Percentage: (7.35V / 240V) × 100 = 3.06%
At 150 feet, 3 AWG copper yields a 3.06% drop. While technically acceptable for a feeder under the 5% total limit, it exceeds the 3% optimal threshold. If your 100-amp subpanel is located more than 120 feet from the main panel, best practice dictates upsizing to 2 AWG copper to keep the voltage drop under 3% and prevent motor burnout or dimming lights under heavy load. You can verify your specific run using the Southwire Voltage Drop Calculator.
Decision Tree: What Changes the Wire Size?
Jobsite conditions rarely match textbook examples perfectly. Use this decision matrix to determine if your specific installation requires upsizing from the baseline 3 AWG copper.
| Installation Condition | Impact on 3 AWG | Required Action |
|---|---|---|
| Run Length > 120 ft | Voltage drop exceeds 3% at full 100A load. | Upsize to 2 AWG copper. |
| 4+ Conductors in Conduit | NEC 310.15(C)(1) requires an 80% derating factor. 100A × 0.80 = 80A capacity. | Upsize to 1 AWG copper (115A × 0.80 = 92A, still tight; consider 1/0 AWG). |
| Ambient Temp 105°F - 113°F | NEC 310.15(B)(1) requires a 0.82 correction factor. 100A × 0.82 = 82A capacity. | Upsize to 2 AWG copper (115A × 0.82 = 94.3A) or 1 AWG. |
| Continuous Load (>3 hours) | NEC 210.19 requires conductors to be sized at 125% of continuous load (125A). | Upsize to 1 AWG copper (130A at 75°C). |
| Using Aluminum Wire | Aluminum has higher resistance and lower ampacity per AWG than copper. | Use 1 AWG aluminum (rated 100A at 75°C). Ensure lugs are rated AL/CU. |
When to Defer to an Engineer or AHJ
While the guidelines above cover 95% of residential and light commercial 100-amp subpanel feeders, certain scenarios require stamped engineering drawings or explicit approval from your local Authority Having Jurisdiction (AHJ). Never guess on the following:
- Service Entrance Conductors: If this 100-amp feed is the main service drop from the utility meter to your main disconnect panel, utility company specifications and local service rules supersede standard NEC feeder rules. The utility may require specific insulation types (like USE-2) or minimum sizes regardless of calculated load.
- High-Fault Current Environments: In areas with massive available fault current (common in dense urban commercial grids), the let-through current and AIC (Ampere Interrupting Capacity) rating of your breaker and wire bracing must be verified by an electrical engineer.
- Mixed Material Terminations: If you are transitioning from copper to aluminum (e.g., using a bimetallic lug or an AL/CU rated breaker), the torque specifications and anti-oxidant compound (like Noalox) requirements must strictly follow the manufacturer's datasheet and AHJ inspection standards to prevent galvanic corrosion and subsequent thermal failure.
Frequently Asked Questions
Can I use 4 AWG copper wire for a 100 amp subpanel if my actual load is only 80 amps?
No. The breaker protects the wire, not the load. If you install a 100-amp breaker, the wire must be rated to safely carry 100 amps indefinitely without degrading. Even if your measured load is only 80 amps today, a future homeowner or tenant could plug in additional equipment and push the circuit to 95 amps, which would overheat and melt the insulation on 4 AWG copper (rated for 85A at 75°C). You must size the wire to the breaker rating, or downsize the breaker to 85A (which is not a standard breaker size, making 90A the practical maximum for 4 AWG).
What size aluminum wire do I need for 100 amps instead of copper?
You need 1 AWG aluminum wire for a 100-amp circuit, assuming 75°C terminations and XHHW-2 or THWN-2 insulation. Aluminum is significantly cheaper and lighter than copper, making it popular for long feeder runs. However, you cannot interchange them blindly. Aluminum expands and contracts more than copper under thermal cycling, which can loosen connections over time and cause arcing. You must use a torque screwdriver to tighten lugs to the exact inch-pound specification printed on the breaker label, and apply an approved anti-oxidant paste if required by the lug manufacturer.
Does a 100 amp continuous load change the wire size for an EV charger?
Yes, drastically. If you are wiring a heavy-duty commercial EV charger or industrial equipment that will draw 100 amps continuously for 3 hours or more, NEC Article 210.19(A)(1) mandates that the conductors be sized at 125% of the continuous load. This means your wire must have an ampacity of at least 125 amps (100A × 1.25). In the 75°C column, 1 AWG copper is rated for 130 amps, making 1 AWG copper the minimum legal size for a 100-amp continuous load. Furthermore, you would need a breaker rated for at least 125 amps; since 125A is a standard size, you would use a 125A breaker with 1 AWG copper wire.
