For a standard 100 amp panel feeder, use #3 AWG copper THHN/THWN-2 wire in conduit with a 100A double-pole breaker. If using NM-B (Romex) cable, you must step up to #2 AWG copper. This assumes 75°C terminations, 30°C ambient temperature, and no more than three current-carrying conductors in the raceway.
- Material: Copper (Aluminum addressed in decision tree)
- Temperature Column: 75°C for THHN in conduit; 60°C for NM-B cable
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Type: Schedule 40 PVC or EMT, max 3 current-carrying conductors
- System Voltage: 120/240V single-phase, 3-wire plus ground
The Direct Answer: Wire and Breaker Sizing for a 100A Panel
When feeding a 100A subpanel or replacing a 100A main service disconnect, the breaker and wire must be matched precisely to the National Electrical Code (NEC) ampacity tables. The breaker protects the wire; therefore, the wire's ampacity must equal or exceed the breaker's rating.
For a 100A double-pole breaker, your minimum copper wire size is #3 AWG, provided you are using individual conductors (THHN/THWN-2) pulled through a conduit and terminating on 75°C rated lugs. You will need four conductors: two ungrounded (hots), one grounded (neutral), and one equipment grounding conductor (EGC).
| Conductor Role | Wire Size & Type | Insulation Color | NEC Reference |
|---|---|---|---|
| Hot Leg 1 (L1) | #3 AWG THHN/THWN-2 | Black | 310.16 (75°C Col) |
| Hot Leg 2 (L2) | #3 AWG THHN/THWN-2 | Red | 310.16 (75°C Col) |
| Neutral (N) | #3 AWG THHN/THWN-2 | White or Gray | 200.6 / 310.16 |
| Ground (EGC) | #8 AWG THHN or Bare | Green or Bare | 250.122 |
| Overcurrent Protection | 100A 2-Pole Breaker | N/A | 240.4(B) |
NEC Ampacity Rules: Why #3 AWG Copper is the Minimum
The sizing rules are dictated by NFPA 70 (NEC) Article 310.16. When you look at the 75°C column for copper conductors, #4 AWG is rated for 85 amps, and #3 AWG is rated for exactly 100 amps.
Why can't you use #4 AWG and rely on the 'next size up' breaker rule? NEC 240.4(B) allows you to round up to the next standard breaker size only when the conductor's ampacity does not correspond to a standard overcurrent device rating and the ampacity is less than 800A. Since 100A is a standard breaker size (standard sizes are defined in 240.6 as 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, etc.), you cannot use an 85A wire on a 100A breaker. The wire must meet or exceed 100A. Thus, #3 AWG is the hard minimum for copper in conduit.
Furthermore, modern panelboards and breakers (like Square D QO or Siemens QP) feature terminations rated for 75°C. If you are working on a legacy panel from the 1970s with 60°C rated lugs, you would be forced to use the 60°C column, bumping your conduit wire up to #2 AWG. Always check the panel's wiring diagram label for the termination temperature rating.
Decision Tree: Adjusting for Distance, Material, and Bundling
The #3 AWG baseline assumes a short run in normal temperatures. Real-world jobsites rarely stay perfectly within baseline assumptions. Use this decision path to finalize your wire pick.
| Condition / Variable | Impact on Ampacity | Required Action & Final Wire Pick |
|---|---|---|
| Baseline: Under 80 ft, Copper, Conduit | None. #3 AWG handles 100A safely. | Pick: #3 AWG Copper THHN. |
| Distance: Run is 100 feet | Voltage drop becomes a factor. Calculating VD for 100A at 100 ft on 240V using #3 AWG yields ~4.9V (2.04%). | 2.04% is under the 3% NEC recommendation. Pick: #3 AWG Copper THHN is still acceptable. |
| Distance: Run is 150 feet | Voltage drop hits ~7.35V (3.06%), exceeding the 3% feeder recommendation. | Step up one size to mitigate drop. Pick: #2 AWG Copper THHN. |
| Material: Using Aluminum (XHHW-2) | Aluminum has lower conductivity. #3 Al is only 75A. #2 Al is 90A. | Must use 75°C column for Al. Pick: #1 AWG Aluminum XHHW-2 (rated 100A). |
| Bundling: 4 to 6 current-carrying conductors in one conduit | NEC 310.15(C)(1) requires an 80% derating factor. | 100A / 0.80 = 125A required baseline. Pick: #1 AWG Copper THHN (130A at 90°C before derating). |
| Ambient Temp: Attic runs at 110°F (43°C) | Requires temperature correction factor (0.82 for 90°C wire). | Step up to compensate for heat. Pick: #2 AWG Copper THHN. |
Aluminum vs. Copper Note: Aluminum wire (#1 AWG XHHW-2) is significantly cheaper and lighter than copper, making it the default choice for utility service entrances. However, for interior subpanel feeders, copper (#3 AWG THHN) is preferred because it is easier to bend in tight conduit sweeps, requires smaller conduit diameters, and is less prone to creep and oxidation at interior lug terminations. Never mix aluminum and copper directly without rated bi-metallic lugs or antioxidant compound.
Step-by-Step Feeder Installation & Verification
Sizing the wire correctly is only half the job. A 100A feeder carries enough energy to start a fire if terminations are loose. Follow this bench-tested procedure:
- De-energize and Verify: Shut off the upstream breaker. Use a non-contact voltage tester, then verify dead with a calibrated multimeter (CAT III or IV rated) across L1-L2, L1-N, L2-N, and L1-G. Lock out the upstream panel if possible.
- Pull the Wires: Use a fiberglass fish tape and wire pulling lubricant (like Polywater J) for #3 AWG. Do not exceed the conduit fill capacity. For four #3 AWG THHN wires plus a #8 ground, the minimum conduit size is 1-inch Schedule 40 PVC or EMT (based on NEC Chapter 9, Table 1, 40% fill rule).
- Strip and Prep: Strip exactly the length specified by the breaker and panel lug manufacturer (usually 5/8' to 3/4'). Do not nick the copper strands. If using stranded wire, do not 'tinning' the ends with solder; lugs are designed to compress bare copper. Solder can cold-flow and cause a loose connection over time.
- Torque to Spec: This is where most DIYers fail. A 100A breaker lug typically requires 40 to 50 inch-pounds of torque. Use a calibrated inch-pound torque screwdriver or torque wrench. Guessing by 'feel' leads to under-torqued lugs that arc and melt, or over-torqued lugs that snap the screw head or crush the strands.
- Separate Neutral and Ground: At the main panel, neutral and ground are bonded. At a subpanel, they must remain strictly isolated. Ensure the subpanel's green bonding screw or strap is removed. Land the white neutral on the isolated neutral bar, and the bare/green ground on the chassis-grounded EGC bar.
- Verify and Energize: Double-check all connections. Ensure no bare copper is exposed outside the lugs. Turn on the upstream breaker and measure 240V across L1-L2 and 120V from each hot to neutral at the new subpanel lugs.
For complex runs or to verify voltage drop on long distances before pulling wire, utilize the Southwire Voltage Drop Calculator to model your exact length, material, and expected continuous load.
When to Call an Engineer or the AHJ
While the NEC provides clear tables for standard residential and light commercial feeders, certain edge cases require professional engineering or explicit Approval from the Authority Having Jurisdiction (AHJ / local electrical inspector).
- Continuous Loads Exceeding 80A: If the calculated continuous load (operating for 3 hours or more) on the subpanel exceeds 80A, NEC 210.20(A) requires the overcurrent device to be rated at 125% of the continuous load. An 80A continuous load requires a 100A breaker, but the wire must be sized for 125% of 80A (100A), which lands you back at #3 AWG. However, if the continuous load is 85A, you need a 110A breaker (round up to 125A) and wire sized accordingly. This requires a formal load calculation.
- High Ambient Environments: If the conduit runs through a boiler room, a commercial kitchen ceiling, or an unventilated attic in a desert climate where ambient temperatures routinely exceed 104°F (40°C), the derating factors will severely impact ampacity. An engineer should calculate the exact thermal profile.
- Parallel Conductors: If you attempt to parallel two sets of #6 AWG wires to achieve 100A (which is generally not permitted for sizes smaller than 1/0 AWG per NEC 310.10(G)), the AHJ will immediately flag it. Stick to single-conductor runs for 100A.
- Service Entrance Upgrades: If this 100A panel is a new main service disconnect replacing an older 60A service, you are altering the service entrance. This requires utility coordination, a new meter base, and a permitted inspection. Never attempt service entrance work without a licensed electrician and utility sign-off.
By default, for a standard 100A interior subpanel feeder under 100 feet, pull four strands of #3 AWG Copper THHN/THWN-2 and one #8 AWG ground through a 1-inch conduit, torque the lugs to 40 in-lbs, and keep the neutral bar isolated. This provides a safe, code-compliant, and low-voltage-drop installation that will pass inspection on the first try.
