For installing 100 amp sub panel feeds under standard conditions, use 3 AWG copper or 1 AWG aluminum wire, protected by a 100A 2-pole breaker. This assumes 75°C terminations, 30°C ambient temperature, and no more than three current-carrying conductors in a single raceway.
- Material: Copper (default recommendation) or Aluminum (cost-saving alternative).
- Temperature Column: 75°C (Standard for modern breakers and subpanel lugs; do not use the 90°C column for final sizing unless specific equipment is rated for it).
- Ambient Temperature: 30°C (86°F) or lower.
- Conduit Configuration: Single raceway containing a maximum of 3 current-carrying conductors (2 hots, 1 neutral; the equipment grounding conductor does not count).
- Insulation Type: THHN/THWN-2 or XHHW-2.
NEC Ampacity Breakdown: Why 3 AWG and Not Smaller?
When sizing feeder conductors, the wire's ampacity must meet or exceed the overcurrent protective device (OCPD) rating, which in this case is 100 amps. According to NFPA 70 National Electrical Code (NEC) Table 310.16, we look strictly at the 75°C column because subpanel lugs and breaker terminals are universally rated for 75°C unless explicitly marked otherwise.
| Wire Size (AWG/kcmil) | Material | 75°C Ampacity | Verdict for 100A Feed |
|---|---|---|---|
| 4 AWG | Copper | 85A | Fail: Too small. Next standard breaker is 90A. |
| 3 AWG | Copper | 100A | Pass: Exact match for 100A breaker. |
| 2 AWG | Aluminum | 90A | Fail: Too small for a 100A OCPD. |
| 1 AWG | Aluminum | 100A | Pass: Exact match for 100A breaker. |
You cannot use 4 AWG copper. While NEC 240.4(B) allows you to round up to the next standard breaker size if the wire ampacity doesn't match a standard breaker, the next standard size up from 85A is 90A. A 90A breaker does not provide a 100A subpanel capacity. Therefore, 3 AWG copper (100A) is the hard code minimum. (Note: The NEC 83% sizing rule for service conductors does not apply to subpanel feeders; feeders must be sized at 100% of the OCPD rating).
Voltage Drop: When Distance Forces an Upsize
Ampacity keeps the wire from melting; voltage drop keeps your equipment running properly. The NEC recommends a maximum 3% voltage drop on branch circuits and feeders (NEC 210.19(A) Informational Note). For a 240V feeder, 3% is 7.2 volts.
Using the standard circular mil (CM) values from the Engineering Toolbox AWG reference, we calculate voltage drop using the formula: VD = (2 × K × I × L) / CM. We will assume an 80A continuous load (a safe real-world baseline for a 100A panel) and copper's K constant of 12.9.
- At 100 feet: VD = (2 × 12.9 × 80 × 100) / 52,620 = 3.92V (1.6%). Well within limits.
- At 150 feet: VD = (2 × 12.9 × 80 × 150) / 52,620 = 5.88V (2.4%). Still acceptable.
- At 200 feet: VD = (2 × 12.9 × 80 × 200) / 52,620 = 7.84V (3.2%). Exceeds 3%. You must upsize to 2 AWG copper.
If your run from the main panel to the subpanel exceeds 150 feet, voltage drop becomes your governing constraint, not thermal ampacity.
Decision Tree: Final Wire & Breaker Selection
Use this decision matrix to lock in your exact materials list. Do not mix and match; pick the row that matches your physical installation constraints.
| Installation Condition | Copper Pick | Aluminum Pick | Breaker Size |
|---|---|---|---|
| Run < 150 ft, standard 30°C ambient | 3 AWG | 1 AWG | 100A 2-pole |
| Run 150 ft to 200 ft | 2 AWG | 1/0 AWG | 100A 2-pole |
| Run > 200 ft | 1 AWG | 2/0 AWG | 100A 2-pole |
| 4+ current-carrying conductors in one conduit | 1/0 AWG* | 3/0 AWG* | 100A 2-pole |
*Derating applied per NEC 310.15(C)(1). See section below.
What Changes the Sizing? (Derating & Edge Cases)
The baseline sizes above assume ideal conditions. Three specific jobsite realities will force you to buy larger wire:
- Conductor Bundling (Derating): If you pull more than three current-carrying conductors through a single conduit, the wires heat each other up. Per NEC 310.15(C)(1), 4 to 6 conductors require an 80% derating factor. If you somehow had 4 current-carrying wires, 3 AWG copper (100A × 0.80 = 80A) is no longer sufficient. You must upsize to 1/0 AWG copper (130A × 0.80 = 104A).
- High Ambient Temperatures: If your conduit runs through an unventilated attic where temperatures regularly exceed 113°F (45°C), you must apply temperature correction factors from NEC Table 310.15(B)(1). At 114-122°F, the correction factor for 75°C wire is 0.82. Your 3 AWG wire drops to an effective 82A. Upsizing is mandatory.
- Aluminum vs. Copper: Never treat them interchangeably. Aluminum expands and contracts more than copper under thermal cycling. If using aluminum (1 AWG or larger), you must use anti-oxidant compound (like Noalox) on the stripped wire ends before torquing the lugs, and you must use connectors explicitly rated for AL/CU.
The 4-Wire Feeder Requirement & Conduit Reality
Since the 2008 NEC cycle, a subpanel feeder must consist of four distinct wires: two ungrounded (hots), one grounded (neutral), and one equipment grounding conductor (EGC). The neutral and ground must remain strictly separated at the subpanel.
Conduit Fill Reality Check: Wire size dictates your conduit size. 3 AWG THHN is thick. A single 3 AWG wire takes up 0.0973 square inches of cross-sectional area. Four of them equal 0.3892 sq in. A 1-inch PVC Schedule 40 conduit only allows 0.346 sq in at 40% fill. You must use a minimum of 1.25-inch PVC conduit (which allows 0.581 sq in) to pull four 3 AWG wires legally and without damaging the insulation.
For termination, modern code requires precise torque. Do not just tighten the lugs until they 'feel tight.' Check the breaker and subpanel labels for the required inch-pound (in-lb) torque specification and use a calibrated torque screwdriver or torque wrench. For further reading on termination torque and code updates, Electrical Contractor Magazine's codes section provides excellent field guidance on avoiding loose connections that cause arc faults and fires.
When to Call the AHJ or an Engineer
While this guide covers 95% of residential and light commercial 100A subpanel installations, you must pull a permit and have the local Authority Having Jurisdiction (AHJ) or a licensed professional engineer review your plans if:
- Heavy Continuous Loads: Your subpanel will supply continuous loads (running 3 hours or more) that exceed 80A. The 100A breaker will trip on a standard thermal-magnetic curve if loaded to 100A continuously.
- Parallel Feeds: You are attempting to run two parallel sets of smaller wires to achieve 100A. (Note: NEC 310.10(G) generally prohibits paralleling conductors smaller than 1/0 AWG, making this illegal for a 100A feed anyway).
- Complex Routing: The conduit transitions through multiple temperature zones (e.g., buried underground, then up an exterior wall in direct sun, then into a hot attic), requiring multiple overlapping derating calculations.
Stick to 3 AWG copper or 1 AWG aluminum for standard runs under 150 feet, use 1.25-inch conduit, separate your neutrals and grounds at the sub, and torque the lugs to spec. That is the exact formula for a safe, code-compliant 100A feeder.
