For a standard 100-amp sub-panel feeder, use 2 AWG copper or 1/0 AWG aluminum wire, protected by a 100-amp double-pole breaker. This assumes THHN/THWN-2 insulation in conduit, a 75°C termination rating, and a run under 100 feet.
- Material: Copper (default) or Aluminum (alternative).
- Temperature Column: 75°C (Standard for modern breakers and panel lugs per NEC 110.14(C)).
- Ambient Temperature: 30°C (86°F) or lower.
- Conduit Fill: EMT or PVC conduit with 3 current-carrying conductors + 1 ground (no bundling derating required).
The Baseline: NEC Ampacity and the 75°C Rule
When sizing feeders, amateur builders often make the mistake of looking at the 90°C column of NEC Table 310.16 because THHN wire is technically rated for 90°C. However, the lugs inside your main breaker and sub-panel are almost universally rated for 75°C. The National Electrical Code (NEC) mandates that you must size the wire based on the lowest temperature rating of any connected component.
Therefore, we strictly use the 75°C column. At 75°C, 3 AWG copper is rated for exactly 100 amps, and 1/0 AWG aluminum is rated for 120 amps. So why do we default to 2 AWG copper instead of 3 AWG? The spec sheet below breaks down the exact physical and thermal realities of a 100-amp feeder.
| Parameter | Copper Default | Aluminum Alternative |
|---|---|---|
| Feeder Wire Size | 2 AWG | 1/0 AWG |
| Ampacity (75°C Column) | 115A | 120A |
| Equipment Ground (EGC) | 8 AWG Copper | 6 AWG Aluminum |
| Conduit Minimum (EMT) | 1 inch | 1.25 inch |
Copper vs. Aluminum: The Decision Tree
Copper and aluminum are not interchangeable; they have different expansion rates, oxidation profiles, and ampacities. Aluminum is significantly cheaper but requires larger gauge sizes and specialized anti-oxidant paste (like Noalox) at terminations to prevent thermal runaway. Use the decision matrix below to lock in your exact material and size.
| Condition / Scenario | Action Required | Concrete Pick (Buy This) |
|---|---|---|
| Run < 100 ft, standard budget, pulling through bends | Use Copper | 2 AWG THHN Cu + 8 AWG Cu Ground |
| Run < 100 ft, tight budget, straight conduit pulls | Use Aluminum | 1/0 AWG XHHW Al + 6 AWG Al Ground |
| Run > 150 ft (240V nominal) | Bump 1 size for Voltage Drop | 1 AWG Cu or 2/0 AWG Al |
| Two 100A feeders in the same conduit | Derate 80% for bundling | 1 AWG Cu minimum (115A x 0.8 = 92A fails on 2AWG) |
Voltage Drop: When Distance Forces a Larger Wire
Ampacity tells you what size wire will prevent a fire. Voltage drop tells you what size wire will actually deliver usable power to your tools and appliances. The NEC recommends a maximum 3% voltage drop on branch circuits and feeders. Let us run the math for a 100-amp load at 240V using our default 2 AWG copper wire.
The formula for single-phase voltage drop is: VD = (2 × K × I × D) / CM
- K (Copper resistivity) = 12.9
- I (Current) = 100A
- D (Distance) = 150 feet
- CM (Circular Mils for 2 AWG) = 66,360
VD = (2 × 12.9 × 100 × 150) / 66,360 = 5.83V
5.83V is exactly 2.42% of 240V. This passes the 3% threshold comfortably.
Why Not One Size Smaller? The Physics of Thermal Headroom
Electrical inspectors frequently reject 3 AWG copper for 100-amp feeders, even though the 75°C column lists it at exactly 100 amps. There are two practical, physics-based reasons why 2 AWG is the mandatory default for a 100-amp breaker.
First, ambient temperature derating. If your conduit runs through an attic or an unconditioned garage where summer temperatures reach 40°C (104°F), NEC Table 310.15(B)(1) forces an 88% correction factor. A 3 AWG wire rated at 100A drops to 88A. Your 100-amp breaker will not trip until 100A, meaning the wire is now operating outside its safe thermal envelope. A 2 AWG wire (115A × 0.88) leaves you with 101.2A of safe capacity.
Second, continuous load calculations. If your sub-panel will supply continuous loads (anything running for 3 hours or more, like EV chargers, HVAC, or server racks), NEC 210.20(A) requires the breaker and wire to be sized at 125% of the continuous load. If your calculated continuous load is 80A, you need a 100A breaker, but the wire must safely carry the thermal equivalent without degradation. 2 AWG provides the necessary thermal headroom that 3 AWG simply lacks in real-world environments. Furthermore, 3 AWG THHN is a non-standard stock size for most electrical suppliers, making 2 AWG both safer and easier to source.
Grounding, Bonding, and AHJ Sign-Off
Sizing the hot and neutral conductors is only half the job. According to NEC Table 250.122, the Equipment Grounding Conductor (EGC) for a 100-amp feeder must be a minimum of 8 AWG copper or 6 AWG aluminum. Do not undersize the ground; it is the only path for fault current to trip the breaker during a short circuit.
When terminating at the sub-panel, you must isolate the neutral bar from the ground bar. Unlike a main service panel where neutral and ground are bonded together, a sub-panel requires a 'floating neutral.' If you bond neutral to ground at the sub-panel, you create a parallel path for neutral current to flow back to the main panel through the grounding system, which is a severe shock hazard and an immediate code violation.
When to call an engineer or defer to the AHJ (Authority Having Jurisdiction):
- If you are trenching for a detached structure and local frost lines or soil conditions dictate unusual burial depths for your PVC conduit.
- If your feeder run exceeds 300 feet, requiring complex voltage drop mitigation or step-up/step-down transformers.
- If you are pulling more than three current-carrying conductors in a single conduit, triggering complex NEC Chapter 9 bundling derations that require stamped engineering calculations.
For 95% of residential and light-commercial garage, shop, or addition sub-panels under 150 feet, pulling 2 AWG Copper THHN with an 8 AWG Copper ground through a 1-inch EMT conduit on a 100-amp breaker is the definitive, code-compliant, and physically robust solution.
