The correct size of wire for 100 amp panel feeders is 3 AWG copper or 1 AWG aluminum, protected by a 100-amp breaker. This baseline assumes 75°C rated terminations, 30°C (86°F) ambient temperature, and THHN/THWN-2 insulation routed in standard conduit. Always verify local codes before pulling wire.
- Conductor: Copper (primary calculations), Aluminum (secondary)
- Insulation: THHN/THWN-2 or XHHW-2
- Temperature Column: 75°C (per NEC 110.14(C))
- Ambient Temperature: 30°C (86°F)
- Installation: Raceway (conduit), maximum 3 current-carrying conductors plus an equipment grounding conductor (EGC).
NEC Ampacity Tables and the 75°C Termination Rule
Sizing a feeder is not just about the wire's ability to carry current; it is equally about the temperature rating of the lugs on your breakers and panelboards. Under NEC 110.14(C), equipment rated 100A or less is generally assumed to have 60°C terminations unless marked otherwise. However, virtually all modern 100A main breakers, subpanel lugs, and AL/CU rated connectors are explicitly marked for 75°C. Therefore, we use the 75°C column of NEC Table 310.16 to size the wire for termination limits.
| AWG Size (Copper) | 60°C Column | 75°C Column | 90°C Column | Allowable for 100A Breaker? |
|---|---|---|---|---|
| 4 AWG | 70A | 85A | 95A | NO (Fails 75°C limit) |
| 3 AWG | 85A | 100A | 110A | YES (Minimum baseline) |
| 2 AWG | 95A | 115A | 130A | YES (Required for long runs) |
| 1 AWG | 110A | 130A | 145A | YES (Required for continuous loads) |
Why 3 AWG Copper and Not 4 AWG?
A common mistake on the jobsite is looking at the 90°C column for THHN wire and assuming 4 AWG (rated 95A at 90°C) is close enough to 100A, or attempting to use the 90°C ampacity for termination sizing. This will fail inspection.
The NEC enforces a 'weakest link' principle. Even though the THHN insulation wrapped around the 4 AWG copper can withstand 90°C, the mechanical lug inside the 100A breaker cannot. The breaker lug is rated for 75°C. If you push 95A through a lug rated for 85A (the 75°C rating of 4 AWG), the lug will overheat, potentially melting the breaker housing or causing a high-resistance fault. You are legally required to size the conductor based on the lowest temperature rating of any connected termination, device, or conductor in the circuit. Thus, 3 AWG copper (exactly 100A at 75°C) is the hard minimum.
Voltage Drop: When Length Forces an Upsize
Ampacity tables tell you what size wire will prevent a fire. Voltage drop calculations tell you what size wire will actually run your equipment efficiently. While the NEC treats voltage drop as an Informational Note (NEC 310.15(B) recommends a maximum 3% drop on feeders), most local Authorities Having Jurisdiction (AHJs) enforce it as a strict rule.
Let's run the math for a standard 240V, 100A feeder run to a detached garage subpanel located 150 feet away from the main panel.
K = 12.9 (Copper at 75°C), I = 100 Amps, D = 150 Feet
Scenario A: Using 3 AWG Copper
Circular Mils (CM) for 3 AWG = 52,620.
VD = (2 × 12.9 × 100 × 150) / 52,620 = 7.35 Volts.
Percentage Drop = (7.35 / 240) × 100 = 3.06%.
Result: Fails the 3% recommendation. Your 240V tools will only see 232.6V at the subpanel, causing motors to run hot and draw excess current.
Scenario B: Upsizing to 2 AWG Copper
Circular Mils (CM) for 2 AWG = 66,360.
VD = (2 × 12.9 × 100 × 150) / 66,360 = 5.82 Volts.
Percentage Drop = (5.82 / 240) × 100 = 2.42%.
Result: Passes. If your one-way conduit run exceeds 140 feet, you must upsize to 2 AWG copper to maintain power quality.
Derating Factors: Bundling, Ambient Heat, and Aluminum
The baseline 3 AWG answer evaporates the moment your installation conditions deviate from a single wire in a cool basement. When you bundle wires or run them through hot environments, you must apply derating factors from NEC Table 310.15(C)(1) and 310.15(B)(1). Note that derating is calculated using the 90°C column, but the final derated ampacity must still meet or exceed the 75°C termination requirement.
| Installation Condition | NEC Reference | Multiplier / Action | Required Copper Size |
|---|---|---|---|
| 4 to 6 Current-Carrying Conductors in one conduit | 310.15(C)(1) | 80% of 90°C rating | 2 AWG (130A × 0.8 = 104A) |
| Ambient Temp 41°C - 45°C (e.g., hot attic) | 310.15(B)(1) | 0.82 of 90°C rating | 2 AWG (130A × 0.82 = 106.6A) |
| Using Aluminum Conductors (SER or THWN) | 310.16 / 110.14 | Use Al 75°C Column | 1 AWG Aluminum (100A at 75°C) |
A Note on Aluminum: Aluminum is significantly cheaper and lighter than copper, making 1 AWG aluminum SER (Service Entrance Rated) cable a popular choice for indoor subpanel feeders. However, aluminum expands and contracts more than copper under thermal load. You must apply an anti-oxidant compound (like Noalox) to the stripped aluminum conductor before torquing it into the panel lugs to prevent galvanic corrosion and high-resistance arcing over time. Never use copper sizing charts for aluminum wire.
Continuous Loads and AHJ Sign-Off Requirements
The final variable that dictates your wire size is the nature of the load. Under NEC Article 100, a continuous load is any load where the maximum current is expected to continue for 3 hours or more. Examples include EV chargers, basement freezers, and hardwired HVAC systems.
NEC 215.3 requires that feeders supplying continuous loads be sized at 125% of the continuous load. If your 100A subpanel will supply an 80A continuous load (like a large EVSE), the wire and breaker must be sized for 100A (80 × 1.25). In this specific edge case, your 100A breaker is maxed out, and you must upsize to a 125A breaker and 1 AWG copper wire (130A at 75°C) to remain code-compliant.
When to involve the AHJ or an Engineer:
While sizing a subpanel feeder is standard DIY or journeyman work, you must pull a permit and involve your local AHJ if this 100A feed is a Service Entrance (the conductors coming directly from the utility meter to your first main disconnect). Service entrance conductors fall under utility jurisdiction and NEC Article 230, which dictates strict rules for physical protection, meter base clearances, and grounding electrode systems. Furthermore, if you are routing feeders through structurally sensitive areas or exceeding 400 feet (requiring complex voltage drop mitigation and potentially parallel runs), a licensed electrical engineer should stamp the plans.
For a comprehensive breakdown of termination provisions and how inspectors evaluate lug temperatures, refer to the EC&M guide on NEC 110.14(C). Always consult the latest edition of NFPA 70 (National Electrical Code) and your local municipal amendments before purchasing wire.
