For standard 100 amp subpanel wiring, use 3 AWG copper or 1 AWG aluminum conductors, protected by a 100-amp double-pole breaker. This assumes THHN/THWN-2 in conduit, 75°C terminal ratings, 30°C ambient, and max three current-carrying conductors.
The Baseline Assumptions
Wire sizing is never a one-size-fits-all number; it is the result of a specific set of environmental and material variables. Before pulling any wire, you must verify that your installation matches the baseline assumptions used to derive the 3 AWG copper / 1 AWG aluminum recommendation. If your jobsite deviates from these parameters, the required wire size will change.
- Conductor Material: Copper or Aluminum (never mix them directly without rated bimetallic lugs or antioxidant compound).
- Insulation Type: THHN/THWN-2 (the standard dual-rated wire found in most conduit runs).
- Temperature Column: 75°C. While THHN is rated for 90°C, NEC 110.14(C) requires you to size the wire based on the lowest temperature rating of any connected termination, device, or conductor. Almost all standard breakers and subpanel lugs are rated for 75°C.
- Ambient Temperature: 30°C (86°F). If your conduit runs through a hot attic or near a boiler, you must apply ambient temperature correction factors.
- Conduit Fill: Maximum of three current-carrying conductors (two hots and one neutral; the equipment grounding conductor does not count toward this limit per NEC 310.15(B)(3)(a)).
Ampacity and Voltage Drop: The Real Sizing Math
Why 3 AWG copper and not 4 AWG? According to NEC Table 310.16, 4 AWG copper at the 75°C column is only rated for 85 amps. If you protect 4 AWG wire with a 100A breaker, a sustained 95A load will overheat the conductor before the breaker trips, creating a severe fire hazard. 3 AWG copper is rated exactly 100A at 75°C, making it the absolute minimum code-compliant size.
| Conductor Material | Minimum AWG Size | Ampacity (75°C Column) | Breaker Size |
|---|---|---|---|
| Copper (THHN/THWN-2) | 3 AWG | 100 Amps | 100A 2-Pole |
| Aluminum (XHHW-2) | 1 AWG | 100 Amps | 100A 2-Pole |
The Voltage Drop Check
Ampacity tells you the wire won't melt; voltage drop tells you your tools and appliances will actually run correctly. The NEC recommends a maximum 3% voltage drop on feeders. Let's run the math for a 100-foot one-way run using 3 AWG copper at a full 100A load on a 240V system.
- Formula: VD = (2 × K × I × D) / Circular Mils
- K (Copper): 12.9
- I (Current): 100A
- D (Distance): 100 ft
- Circular Mils (3 AWG): 52,620
- Result: (2 × 12.9 × 100 × 100) / 52,620 = 4.9 Volts
A 4.9V drop on a 240V system is a 2.04% drop. This is well within the 3% limit. However, if your subpanel is 160 feet away, the drop hits 3.2%, and you must bump up to 2 AWG copper to maintain optimal performance.
When the Baseline Fails: Derating and Material Swaps
The most common mistake DIYers make is ignoring conduit fill derating. If you pull more than three current-carrying conductors through a single conduit (for example, feeding two subpanels from one main panel using the same pipe), the wires trap heat. You must apply an adjustment factor per NEC Table 310.15(C)(1).
Crucial Pro-Tip: When calculating derating, you are allowed to use the 90°C column of Table 310.16 as your starting base, even though your terminations are limited to 75°C. The final derated ampacity must simply equal or exceed the 100A requirement.
| Installation Scenario | Effect on Sizing | Required Action |
|---|---|---|
| 4 to 6 current-carrying conductors in one conduit | 80% derating factor. 3 AWG Cu (90°C base = 115A × 0.8 = 92A). Fails 100A requirement. | Bump to 2 AWG Copper (130A × 0.8 = 104A. Passes.) |
| Conduit run through an attic at 46°C (115°F) | 0.82 ambient correction factor. 3 AWG Cu (115A × 0.82 = 94.3A). Fails. | Bump to 2 AWG Copper or reroute conduit to conditioned space. |
| Switching from Copper to Aluminum to save cost | Aluminum has lower conductivity and higher expansion rates. | Use 1 AWG Aluminum, apply antioxidant paste, and torque lugs to manufacturer specs. |
| Feeder distance exceeds 145 feet | Voltage drop exceeds 3% at full 100A load. | Bump to 2 AWG Copper or 1/0 AWG Aluminum. |
Code Compliance and AHJ Authority
Wire sizing is only half the battle; proper termination and grounding dictate whether your installation passes inspection. According to OSHA and NEC electrical safety guidelines, improper bonding in subpanels is a leading cause of shock hazards.
- Isolate Neutral and Ground: Per NEC 408.40, the equipment grounding busbar must be bonded to the enclosure, but the neutral busbar must remain completely isolated from the enclosure in a subpanel. Do not install the green bonding screw or strap that came with the panel.
- Equipment Grounding Conductor (EGC): Per NEC 250.122, a 100A feeder requires a minimum 8 AWG copper or 6 AWG aluminum ground wire. This must be pulled in the same conduit as your feeder conductors.
- Detached Structures: If your 100 amp subpanel feeds a detached garage or workshop, NEC 250.32 requires a local grounding electrode system (typically two 5/8-inch copper ground rods driven 6 feet apart and spaced at least 6 feet from each other), bonded to the subpanel's ground bus with a minimum 6 AWG bare copper wire.
Frequently Asked Questions
Can I use 2 AWG aluminum instead of 1 AWG for a 100 amp subpanel?
No. According to NEC Table 310.16, 2 AWG aluminum is only rated for 90 amps in the 75°C column. If you protect 2 AWG aluminum with a 100A breaker, you are violating NEC 240.4, which requires conductors to be protected at their ampacity. You must use a minimum of 1 AWG aluminum (rated 100A) or 1/0 AWG aluminum (rated 120A) for a 100A feeder.
Does a 100 amp subpanel in an attached addition need its own ground rod?
No. If the subpanel is located in an addition that shares a continuous foundation and structure with the main house, the equipment grounding conductor (8 AWG copper) pulled through the conduit serves as the sole fault-current path back to the main panel. Ground rods are only mandated by NEC 250.32 for detached structures separated by a physical gap or dirt.
What size conduit do I need for 100 amp subpanel wiring using 3 AWG copper?
For three 3 AWG THHN conductors and one 8 AWG ground wire, Chapter 9 Table 1 limits conduit fill to 40% for three or more wires. Based on Table 4, a 1-inch Schedule 40 PVC or 1-inch EMT conduit is the absolute minimum. However, most electricians step up to 1-1/4 inch conduit to make the physical pull easier and to prevent damaging the wire insulation around tight sweeps.
When must a licensed electrical engineer or the AHJ confirm my wire sizing?
You must defer to an engineer or your local inspector if your installation involves continuous loads exceeding 80A (requiring a 125% multiplier per NEC 210.20), if the conduit runs through extreme ambient heat exceeding 50°C, or if you are tapping into a service entrance rather than a standard feeder breaker. Local amendments can also override baseline NEC tables, particularly in municipalities with strict energy conservation or fire-prevention codes.
