For wiring a 100 amp sub panel using copper conductors in a standard residential run under 100 feet, use 3 AWG THHN/THWN-2 copper wire for the two hots and the neutral, plus 8 AWG bare copper for the equipment ground, all protected by a 100A 2-pole breaker at the main panel.
- Conductor Material: Copper
- Insulation Type: THHN/THWN-2
- Termination Temperature Rating: 75°C (Standard for modern breakers and panel lugs)
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Fill: 3 or fewer current-carrying conductors in a single raceway (no bundling derating required)
Note: This guidance follows NEC-style best practices. Your local Authority Having Jurisdiction (AHJ) always has final say on code compliance.
The Core Sizing Table: Copper Ampacity vs. Terminal Limits
When pulling wire for a subpanel feeder, you cannot simply look at the wire's maximum thermal rating. You must cross-reference the wire's insulation rating with the physical temperature limit of the lugs it terminates in. The table below maps standard copper THHN sizes against the three primary temperature columns found in NEC Table 310.16.
| AWG Size | 60°C Column (Amps) | 75°C Column (Amps) | 90°C Column (Amps) |
|---|---|---|---|
| 4 AWG | 70A | 85A | 95A |
| 3 AWG | 85A | 100A (Target) | 115A |
| 2 AWG | 95A | 115A | 130A |
| 1 AWG | 110A | 130A | 145A |
As highlighted in the table, 3 AWG copper in the 75°C column yields exactly 100 amps. This is your baseline minimum size for the two ungrounded (hot) conductors and the grounded (neutral) conductor. The equipment grounding conductor requires a minimum of 8 AWG copper per NEC Table 250.122 for a 100A overcurrent device.
Why 3 AWG and Not 4 AWG? (The 75°C Terminal Rule)
A common mistake on the jobsite is pulling 4 AWG THHN wire because it physically fits into the breaker lug, or looking at the 90°C column and assuming 4 AWG (95A) is 'close enough.' Both approaches violate NEC 110.14(C) and create a fire hazard.
Here is the mechanical reality: while THHN wire is manufactured with 90°C insulation, the brass or aluminum lugs inside standard residential breakers (like a Square D QO or Eaton BR) and subpanel main lugs are tested and rated for a maximum of 75°C. If you push 100A through a 4 AWG wire, the wire itself won't melt (it's rated for 95A at 90°C), but the heat generated at the termination point will exceed the 75°C limit of the breaker lug. Over time, this thermal cycling degrades the metal, increases resistance, and leads to a melted terminal or an arc fault.
Voltage Drop and Distance: When to Upsize the Feeder
Ampacity dictates the minimum wire size to prevent a fire. Voltage drop dictates the wire size required to ensure your tools and appliances actually work at the subpanel. The NEC recommends a maximum 3% voltage drop on feeders (NEC 310.15(B) informational note). For a 240V system, a 3% drop equals a maximum allowable loss of 7.2 volts.
Using the standard voltage drop formula VD = (2 × K × I × D) / CM (where K=12.9 for copper, I=100A, and CM=52,620 for 3 AWG), here is how distance forces you to upsize your copper conductors:
| One-Way Distance | Voltage Drop (3 AWG) | Percentage Drop | Required Action |
|---|---|---|---|
| 50 Feet | 2.45V | 1.02% | Use 3 AWG |
| 100 Feet | 4.90V | 2.04% | Use 3 AWG |
| 150 Feet | 7.35V | 3.06% | Upsize to 2 AWG |
| 200 Feet | 9.80V | 4.08% | Upsize to 1 AWG |
If your trench or conduit run exceeds 130 feet, 3 AWG copper will technically violate the 3% best-practice guideline at a full 100A load. You must step up to 2 AWG copper (which drops the loss to roughly 2.4% at 150 feet). For precise calculations on your specific route, use the Southwire Voltage Drop Calculator to account for exact conduit routing and actual expected loads.
What Changes the Answer: Aluminum, Bundling, and Heat
The 3 AWG copper rule assumes perfect conditions. Real-world installations frequently introduce variables that force a change in material or size.
Switching to Aluminum Conductors
Copper is expensive. For longer runs, many electricians switch to aluminum (specifically XHHW-2 or THWN-2 aluminum) to save money. Aluminum has a lower conductivity per volume than copper. If you use aluminum, you must use 1 AWG aluminum for the hots and neutral, and 6 AWG aluminum for the ground. Never mix copper and aluminum directly in the same lug without proper anti-oxidant paste (like Noalox) and lugs specifically rated for AL/CU, or you will galvanically corrode the connection.
Conduit Bundling and Derating
If you are pulling multiple circuits through the same conduit, the heat trapped inside the pipe reduces the wire's ability to shed thermal energy. Per NEC 310.15(C)(1), if you have 4 to 6 current-carrying conductors in a single raceway, you must derate the ampacity to 80%. This is where the 90°C column saves you: 3 AWG at 90°C is 115A. Multiplied by 0.80, you get 92A—which is below your 100A breaker size. If you must bundle 4 current-carrying conductors, you must upsize to 2 AWG copper (130A × 0.80 = 104A).
High Ambient Temperatures
If your conduit runs across an unventilated attic in a southern climate where ambient temperatures regularly exceed 110°F (43°C), you must apply an ambient temperature correction factor. At 110°F, the derating factor for 90°C wire is 0.82. Again, 3 AWG (115A × 0.82 = 94.3A) fails the 100A requirement, forcing an upsize to 2 AWG.
Grounding, Bonding, and When to Call the AHJ
Wiring the conductors is only half the job; configuring the subpanel correctly is where DIYers frequently fail inspections.
The Bonding Screw: In a main panel, the neutral bar and the ground bar are bonded together. In a subpanel, they must be isolated. You must physically remove the green bonding screw or bonding strap that ships installed in the subpanel's neutral bar. If you leave this in place, normal neutral return current will travel back to the main panel via both the neutral wire and the equipment ground wire, energizing the grounding system and creating a severe shock hazard.
When to involve an Engineer or the AHJ: The sizing above assumes a standard residential load profile. You must consult a licensed professional engineer or your local electrical inspector if:
- Continuous Loads: If the subpanel will serve continuous loads (defined by the NEC as operating for 3 hours or more, like EV chargers or server racks) that exceed 80A. Per NEC 210.20(A), continuous loads require the overcurrent device to be rated at 125% of the load. An 85A continuous load requires a 106.25A breaker, pushing you to a 125A panel and 1 AWG copper wire.
- Utility Transformer Limits: If your main service is only 100A or 150A, adding a 100A subpanel feeder might exceed the utility transformer's capacity. An NEC Article 220 load calculation on the entire dwelling is required to prove the main service can handle the additional subpanel.
For comprehensive code context and updates on terminal temperature limitations, refer to the National Fire Protection Association (NFPA) NEC portal. Always pull a permit for subpanel installations; the fee is minor compared to the cost of replacing melted lugs or failing a home insurance inspection.
