For standard 100 amp sub panel wiring, use 3 AWG copper or 1 AWG aluminum wire with a 100-amp double-pole breaker in the main panel. This assumes THHN/THWN-2 insulation in a wet/dry raceway, terminating on 75°C rated lugs, with no more than three current-carrying conductors in the conduit.
- Material: Copper (THHN/THWN-2) or Aluminum (XHHW-2)
- Termination Rating: 75°C (Standard for modern breakers and panel lugs)
- Ambient Temperature: 30°C (86°F) baseline
- Conduit Type: PVC Schedule 80 or EMT, containing exactly 3 current-carrying conductors (2 hots, 1 neutral) + 1 equipment grounding conductor (EGC)
- System Voltage: 240V / 120V single-phase split-phase
The Core Sizing Matrix: Copper vs. Aluminum Feeders
Choosing between copper and aluminum for a 100 amp sub panel wiring run is primarily a decision of budget versus physical space. Copper is denser, more conductive, and easier to terminate in tight spaces. Aluminum is significantly cheaper but requires larger conduit, anti-oxidant paste, and specific torque settings to prevent thermal expansion failures at the lugs.
| Conductor Material | AWG Size | 75°C Ampacity | 90°C Ampacity | Max Run for <3% Drop @ 80A | Req. EGC Size |
|---|---|---|---|---|---|
| Copper (THHN/THWN-2) | 3 AWG | 100A | 115A | 110 ft | 8 AWG Cu |
| Aluminum (XHHW-2) | 1 AWG | 100A | 115A | 85 ft | 6 AWG Al |
| Copper (UF-B Direct Burial) | 1 AWG | 130A* | N/A | 180 ft | 8 AWG Cu |
| Aluminum (URD Direct Burial) | 1/0 AWG | 120A* | N/A | 130 ft | 6 AWG Al |
*Note: Direct burial cables like UF-B and URD often utilize 60°C ampacity columns for smaller sizes, but 1 AWG and larger transition to the 75°C column per NEC Table 310.16. Always verify the specific cable jacket rating.
Why 3 AWG Copper and Not 4 AWG? (The Termination Trap)
A common mistake among DIYers is looking at the 90°C column of the ampacity table, seeing that 4 AWG THHN copper is rated for 95 amps, and assuming it can be protected by a 100A breaker using the NEC 240.4(B) "next standard size up" rule. This is incorrect and violates code.
According to NEC 110.14(C) termination temperature limits, the ampacity of a conductor must be determined based on the lowest temperature rating of any connected device, lug, or terminal. Almost all modern residential breakers and subpanel lugs are rated for 75°C.
- 4 AWG Copper in the 75°C column is rated for 85 amps.
- Your panel lugs are rated for 75°C, capping the wire's legal ampacity at 85A.
- You cannot protect an 85A termination with a 100A breaker, because if the wire pulls 95A, the breaker won't trip, but the 75°C lug will overheat and degrade.
- Therefore, you must step up to 3 AWG Copper, which is rated exactly 100A in the 75°C column.
For aluminum, the same rule applies. 2 AWG aluminum is only 90A at 75°C. You must step up to 1 AWG aluminum to hit the 100A mark at the termination point.
Voltage Drop and Distance: When to Upsize
Ampacity dictates the minimum wire size to prevent a fire, but voltage drop dictates the maximum wire size to ensure your tools and appliances actually work. While the NEC treats voltage drop as an informational note (NEC 210.19(A)) rather than a strict mandate for most residential feeders, best practice dictates keeping feeder drop under 2% to allow for 3% drop on the branch circuits.
Let's run a voltage drop check for our baseline 3 AWG Copper feeder. We will assume a realistic continuous load of 80 amps (subpanels rarely pull a full 100A continuously).
- Formula: VD = (2 × K × I × D) / Circular Mils
- K (Copper): 12.9 ohms
- I (Current): 80 Amps
- CM (3 AWG): 52,620 circular mils
Scenario A: 100-foot run
VD = (2 × 12.9 × 80 × 100) / 52,620 = 3.92 Volts.
At 240V, this is a 1.6% drop. 3 AWG is perfectly adequate.
Scenario B: 200-foot run (e.g., detached garage or workshop)
VD = (2 × 12.9 × 80 × 200) / 52,620 = 7.84 Volts.
At 240V, this is a 3.26% drop. This exceeds the recommended 2% feeder limit. If you are running a 200-foot feed, you must upsize to 2 AWG Copper or 1/0 AWG Aluminum to maintain optimal voltage at the subpanel busbars.
Derating Factors: What Changes the Answer
The baseline sizes above assume a perfect installation. Real-world jobsites introduce thermal bottlenecks that require derating the conductor's ampacity. If any of the following conditions apply, your 3 AWG copper or 1 AWG aluminum wire is no longer sufficient.
1. Conductor Bundling (NEC Table 310.15(C)(1))
If you pull two separate 100A subpanel feeds through the exact same conduit, you now have four current-carrying conductors (the neutral in a standard 240/120V split-phase feeder does not count as a CCC unless it carries unbalanced harmonic loads, but the two hots from each feeder do). Four CCCs require an 80% derating factor.
The Math: 3 AWG THHN (115A at 90°C) × 0.80 = 92 Amps. This is below your 100A breaker size. You must upsize to 1 AWG Copper if bundling four or more current-carrying conductors.
2. High Ambient Temperatures
If your PVC conduit is strapped to the side of a sun-baked stucco wall in Arizona, or run through an unventilated attic in Texas, the ambient temperature easily exceeds the 30°C (86°F) baseline. At 46-50°C (115-122°F), you must apply a 0.82 correction factor to the 90°C column. Always calculate derating from the 90°C column, then verify the final derated ampacity meets the 75°C termination requirement.
You must pull a permit and have your local Authority Having Jurisdiction (AHJ) or a licensed electrical engineer review your 100 amp sub panel wiring plans if:
- You are paralleling conductors (NEC 310.10(H) requires 1/0 AWG or larger, meaning you cannot parallel smaller wires to achieve 100A).
- The subpanel is being installed as a Service Entrance (requires specific grounding electrode systems and main bonding jumper configurations that differ from a standard feeder subpanel).
- Your local municipality has strict amendments, such as Chicago's requirement for EMT conduit instead of NM-B/Romex, or California's Title 24 solar-ready panel mandates.
NEC-style guidance provided here is for educational purposes; your local AHJ always has final authority on code compliance.
Properly sizing your 100 amp sub panel wiring ensures your breakers trip when they should, your lugs don't melt under load, and your 240V welders or EV chargers get the voltage they need to operate efficiently. Stick to 3 AWG copper or 1 AWG aluminum for standard runs, verify your voltage drop for distances over 100 feet, and always torque your lugs to the manufacturer's exact inch-pound specifications.
