For a standard 100-amp subpanel feeder, use #3 AWG copper or #1 AWG aluminum wire with a 100-amp double-pole breaker. This assumes 75°C rated terminals, 30°C ambient temperature, and a run under 73 feet. For the equipment ground, use #8 AWG copper or #6 AWG aluminum.
The Baseline Assumptions (Read Before Buying)
Wire sizing is not a guessing game; it is a strict calculation based on the National Electrical Code (NEC). Before you cut any wire, verify that your installation matches these baseline assumptions. If your jobsite conditions differ, the required wire size will change.
- Conductor Material: Copper (THHN/THWN-2) or Aluminum (XHHW-2).
- Termination Temperature: 75°C. (Per NEC 110.14(C), you may only use the 75°C column if both the breaker and the subpanel lugs are explicitly marked for 75°C. Most modern Square D, Eaton, and Siemens load centers are, but older panels may default to 60°C).
- Ambient Temperature: 30°C (86°F) or lower. Higher ambient temperatures in attics or hot garages require derating.
- Conduit Fill: Maximum of 3 current-carrying conductors in the raceway (2 hots + 1 neutral for a standard 120/240V split-phase feeder).
- System Voltage: 240V nominal single-phase.
Ampacity Table & Why We Pick #3 AWG Copper
Many DIYers mistakenly buy #4 AWG copper because it looks thick and feels substantial. However, #4 AWG is rated for only 85 amps in the 75°C column. Feeding a 100-amp breaker with 85-amp wire is a direct NEC violation and a fire hazard. We must use Cerrowire's standard ampacity charts (aligned with NEC Table 310.16) to find the exact threshold.
| Wire Size (AWG/kcmil) | Copper (THHN/THWN-2) | Aluminum (XHHW-2) | Passes 100A Breaker? |
|---|---|---|---|
| #4 AWG | 85A | 65A | No (Undersized) |
| #3 AWG | 100A | 75A | Yes (Copper Pick) |
| #2 AWG | 115A | 90A | No (Al Undersized) |
| #1 AWG | 130A | 100A | Yes (Aluminum Pick) |
Notice that aluminum requires two full sizes larger than copper to carry the same current. Never substitute aluminum sizing logic for copper, and never mix the two materials on the same lug without using anti-oxidant paste and lugs explicitly rated 'AL/CU'.
Voltage Drop: When Distance Forces an Upsize
The NEC doesn't strictly enforce voltage drop for branch circuits, but NFPA 70 (NEC) Informational Notes strongly recommend a maximum 3% voltage drop on feeders to ensure equipment operates efficiently. For a 240V system, 3% equals a maximum drop of 7.2 volts.
Let's run the math for #3 AWG Copper at a full 100A load over a 75-foot run:
- Formula: VD = (2 × K × I × D) / CM
- K (Copper): 12.9
- I (Current): 100A
- D (Distance): 75 feet
- CM (#3 AWG Circular Mils): 26,240
- Calculation: (2 × 12.9 × 100 × 75) / 26,240 = 7.37 Volts
Decision Tree: Finalizing Your Wire and Breaker Pick
Use this decision matrix to lock in your exact materials list. Follow the path that matches your specific jobsite conditions. Do not deviate to a smaller wire size to save money on copper prices.
| Condition / Constraint | Hot Wires (2x) | Neutral (1x) | Ground (EGC) | Breaker Size |
|---|---|---|---|---|
| Default: Copper, <73 ft, 75°C lugs | #3 AWG Cu | #3 AWG Cu | #8 AWG Cu | 100A 2-Pole |
| Long Run: Copper, 74 ft to 110 ft | #2 AWG Cu | #2 AWG Cu | #8 AWG Cu | 100A 2-Pole |
| Extra Long: Copper, 111 ft to 165 ft | #1 AWG Cu | #1 AWG Cu | #6 AWG Cu* | 100A 2-Pole |
| Aluminum Default: <73 ft, 75°C lugs | #1 AWG Al | #1 AWG Al | #6 AWG Al | 100A 2-Pole |
| Older Panel: Lugs marked 60°C only | #1 AWG Cu | #1 AWG Cu | #8 AWG Cu | 100A 2-Pole |
*Note: Per NEC Table 250.122, if you upsize ungrounded conductors for voltage drop, you must proportionally upsize the equipment grounding conductor. Moving from #3 to #1 Cu requires bumping the ground from #8 to #6 AWG.
What Changes the Answer (Derating & Bundling)
The sizes above assume your feeder wires are alone in their conduit. The moment you start bundling, the NEC demands derating because trapped heat reduces a wire's ability to shed thermal energy.
The Bundling Trap (NEC 310.15(C)(1)):
If you pull two separate subpanel feeders through the same piece of PVC conduit (e.g., one to a garage, one to a detached workshop), you now have four current-carrying conductors in the pipe. This triggers an 80% derating factor. You must calculate derating using the 90°C column of Table 310.16:
- #3 AWG Cu in the 90°C column is rated 110A.
- 110A × 0.80 (derating factor) = 88A.
- 88A is less than your 100A breaker. #3 AWG fails.
If you are bundling two feeders, you must jump to #1 AWG Copper (130A × 0.80 = 104A) to legally protect the circuit with a 100A breaker.
When to Call an Engineer or the AHJ
While a 100-amp subpanel is a standard residential and light-commercial job, certain edge cases require a licensed professional engineer (PE) or a pre-inspection meeting with your local Authority Having Jurisdiction (AHJ):
- Parallel Conductors: If you are attempting to parallel multiple smaller wires to achieve 100A (rare for this amperage, but sometimes done by DIYers trying to use leftover scrap wire). NEC 310.10(G) generally prohibits paralleling for anything under 1/0 AWG. Do not do this.
- Extreme Voltage Drop Runs: If your subpanel is over 300 feet away (e.g., powering a distant well pump or agricultural outbuilding), the wire size required to mitigate voltage drop will become physically too large to terminate on standard 100A breaker lugs. An engineer must design a step-up/step-down transformer solution or specify custom lug pigtails.
- High Fault Current Zones: If your main service has been upgraded to a high-capacity utility transformer (yielding >10,000 AIC available fault current), standard 10kAIC residential breakers may be inadequate. The AHJ will require you to verify the AIC (Ampere Interrupting Capacity) rating of your 100A breaker matches the utility's calculated fault current at the subpanel bus.
Stick to the #3 AWG Copper (or #1 AWG Aluminum) baseline for standard runs, verify your lug temperature ratings, and do the voltage drop math before you pull the wire. Sizing correctly the first time prevents melted lugs, nuisance tripping, and failed inspections.
