To feed a 100 amp sub panel, use 3 AWG copper THHN/THWN-2 conductors (or 1 AWG aluminum XHHW-2) protected by a 100-amp 2-pole breaker at the main panel. This baseline assumes a standard 4-wire feed under 100 feet in length. Getting the sizing right prevents nuisance tripping and terminal overheating.

The Baseline Assumptions and Ampacity Data

Baseline Assumptions Block:
  • Material: Copper (THHN/THWN-2) or Aluminum (XHHW-2)
  • Temperature Column: 75°C (per NEC 110.14(C) for standard 100A lugs)
  • Ambient Temperature: 30°C (86°F)
  • Conduit Type: Single raceway (PVC or EMT) with a maximum of 3 current-carrying conductors

When sizing conductors, we look at NEC Table 310.16. While THHN wire is rated for 90°C, the lugs on standard 100-amp breakers (like the Square D QO2100 or Eaton BR2100) and subpanel bus bars are almost universally rated for 75°C. Therefore, the final ampacity must be based on the 75°C column.

Spec Sheet: NEC Table 310.16 (75°C Column)
Wire Size (AWG/kcmil) Material Ampacity (75°C) Insulation Type
4 AWG Copper 85A THHN/THWN-2
3 AWG Copper 100A THHN/THWN-2
2 AWG Aluminum 90A XHHW-2
1 AWG Aluminum 100A XHHW-2

Decision Tree: Sizing by Material and Run Length

Copper and aluminum are not interchangeable. Aluminum is cheaper but requires a larger gauge to carry the same current, and it requires anti-oxidant compound (like Noalox) at terminations. Use this decision path to lock in your exact wire size.

Decision Tree Table: Final Conductor Pick
IF Material is... AND Run Length is... THEN Buy This Wire Breaker Size
Copper Under 100 ft 3 AWG THHN 100A
Copper 100 ft to 150 ft 2 AWG THHN 100A
Aluminum Under 100 ft 1 AWG XHHW-2 100A
Aluminum 100 ft to 150 ft 1/0 AWG XHHW-2 100A
What changes this answer? If you are bundling more than three current-carrying conductors in a single conduit, NEC 310.15(C)(1) requires ampacity derating. For example, 4 to 6 current-carrying conductors require an 80% derating factor. If your ambient temperature exceeds 30°C (like in a hot attic), you must apply Table 310.15(B)(1) correction factors. In either case, you will need to upsize to the next gauge.

Wiring Diagram Essentials for the 100A Subpanel

When executing the wiring diagram for 100 amp sub panel installations, the physical layout is just as critical as the wire gauge. Since the 2008 NEC cycle, a 4-wire feed is strictly mandatory for all subpanels (NEC 250.32). You must pull two hot conductors (Black and Red), one neutral (White), and one equipment grounding conductor (Green or Bare).

Here are the non-negotiable wiring diagram rules for the subpanel interior:

  • Isolate the Neutral: The neutral bar must be completely isolated from the panel enclosure. Remove the green bonding screw or bonding strap that ships pre-installed on the neutral bar. In a subpanel, neutral current and fault current must never share the same path back to the source.
  • Add a Ground Bar: If the panel doesn't have a dedicated ground bar, buy an accessory ground bar (e.g., Eaton GBKP26 or Square D PK7GTA) and bond it directly to the panel's metal chassis.
  • Torque the Lugs: Do not guess the tightness. A 100A lug typically requires 40 to 50 in-lbs of torque depending on the manufacturer. Use a calibrated inch-pound torque screwdriver or torque wrench. Loose lugs cause high-resistance faults and melted bus bars.
  • The Main Disconnect: If the subpanel is in the same building as the main panel, a main breaker in the subpanel is not strictly required by the NEC, though it is highly recommended for local shutoff. If the subpanel is in a detached structure (like a garage or shed), a main disconnecting means (a 100A main breaker in the subpanel) is absolutely required.

Voltage Drop Calculations and When to Upsize

Ampacity tells us what the wire can handle without melting; voltage drop tells us if the equipment at the end of the run will actually function properly. NEC recommends a maximum 3% voltage drop for feeders.

Let's run the math for 3 AWG Copper carrying a full 100A load at 240V:

  • At 100 feet: The voltage drop is approximately 2.04% (4.9 Volts). This is well within the 3% recommendation. Stick with 3 AWG.
  • At 150 feet: The voltage drop climbs to 3.07% (7.3 Volts). We have crossed the 3% threshold. Upsize to 2 AWG Copper to bring the drop back down to ~1.9%.
Warning: Continuous High-Draw Loads
If your subpanel is being installed specifically to feed a continuous high-draw load like an 80A EV charger or a large welder, remember that continuous loads (running 3 hours or more) must be derated to 80% of the breaker rating (NEC 210.20). An 80A continuous load on a 100A breaker is perfectly legal, but it will generate significant heat in the conduit. Upsizing to 2 AWG copper provides a thermal buffer and reduces voltage sag during heavy draws.

Why Not 4 AWG? The "Next Size Up" Rule Trap

A common mistake on the jobsite is attempting to use 4 AWG copper wire on a 100-amp breaker. The logic usually goes: "4 AWG is rated for 85 Amps, and the NEC allows rounding up to the next standard breaker size."

This is a misapplication of NEC 240.4(B). The "next size up" rule applies when your calculated load falls between standard breaker sizes. It does not allow you to arbitrarily place a 100A breaker on a wire rated for 85A just because the panel bus is rated for 100A. The breaker's primary job is to protect the wire. If you put a 100A breaker on 4 AWG wire, the wire can overheat and melt its insulation before the breaker ever trips. To safely utilize the full 100A capacity of the panel, you must use a wire with a baseline ampacity of at least 100A. That means 3 AWG copper.

When to Pull the Permit and Call the AHJ

While the guidelines above cover 95% of residential and light-commercial subpanel feeds, you must stop and consult a licensed engineer or your local Authority Having Jurisdiction (AHJ) under the following conditions:

  1. Calculated Loads Exceed 80A Continuous: If your subpanel load calculation shows continuous loads exceeding 80A, you must size the conductors and breaker at 125% of that load, pushing you into 125A or 150A feeder territory.
  2. High Ambient Environments: If the conduit runs through an environment where ambient temperatures regularly exceed 40°C (104°F), such as near a boiler, on a sun-baked roof, or in an unventilated metal building in the desert, standard 30°C ampacity tables no longer apply.
  3. Complex Conduit Fill: If you are pulling multiple circuits through the same conduit and exceed 9 current-carrying conductors, the 70% derating factor will drastically reduce your wire's capacity, requiring a massive upsize that may not physically fit in the panel lugs.

For standard runs under 100 feet in normal conditions, buy your 3 AWG copper THHN, a 100A 2-pole breaker, and torque your lugs to spec. Always verify the main breaker is off and test for dead with a non-contact voltage tester and a multimeter before terminating any conductors. For further reading on subpanel grounding and bonding rules, refer to Mike Holt's NEC subpanel wiring archives.