For a standard residential feeder, a 100 amp sub panel diagram requires 3 AWG copper THHN/THWN-2 wire (or 1 AWG aluminum XHHW-2) protected by a 100A 2-pole breaker at the main panel. This baseline assumes a 240V split-phase system, 75°C terminations, and a run under 100 feet.
The Baseline Assumptions (Crucial for Sizing)
Wire sizing is not a one-size-fits-all lookup. Before pulling any wire through conduit, you must lock in your environmental and material variables. If your jobsite deviates from these baseline assumptions, the required wire gauge will change.
- Material: Copper (THHN/THWN-2) or Aluminum (XHHW-2)
- Termination Rating: 75°C (Standard for residential breakers like Eaton BR or Square D HOM)
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Type: EMT or PVC Schedule 80, containing exactly 3 current-carrying conductors (2 hots, 1 neutral)
- System: 120/240V Single-Phase, 3-wire feeder plus equipment ground
Decoding the Ampacity Table (NEC 310.16)
The most common mistake DIYers make when reading NEC Table 310.16 is looking at the 90°C column because they bought THHN wire (which is rated for 90°C). However, NEC 110.14(C) dictates that you must size your wire based on the lowest temperature rating of any connected component. Since standard 100A breaker lugs and subpanel main lugs are rated for 75°C, you must use the 75°C column.
| Wire Size (AWG) | Copper (75°C Column) | Aluminum (75°C Column) | Max Breaker Size |
|---|---|---|---|
| 4 AWG | 85A | 65A | 90A (Cu only) |
| 3 AWG | 100A | 75A | 100A |
| 2 AWG | 115A | 90A | 125A (Cu) / 100A (Al) |
| 1 AWG | 130A | 100A | 100A (Al) |
Why 3 AWG Copper and Not One Size Smaller?
You might wonder why you can't use 4 AWG copper (85A) and rely on the NEC 240.4(B) "next size up" rule to protect it with a 100A breaker. That rule only applies when your calculated load is less than the wire's ampacity but doesn't match a standard breaker size. If your subpanel load calculation actually demands 100A, an 85A wire will overheat before the 100A breaker trips. Therefore, 3 AWG (exactly 100A at 75°C) is the hard minimum for copper.
Voltage Drop: The Hidden Variable
Ampacity tells you if the wire will melt. Voltage drop tells you if your tools and appliances will actually work at the far end. The NEC recommends a maximum 3% voltage drop for feeders. Let's run the math for a 3 AWG copper feeder carrying an 80A continuous load (80% of the 100A breaker capacity) at 240V.
Formula: VD = (2 × K × I × D) / CM
Where K = 12.9 (Copper), I = 80A, D = One-way distance in feet, CM = 52,620 (Circular Mils for 3 AWG).
- At 50 feet: VD = (2 × 12.9 × 80 × 50) / 52,620 = 1.96V (0.81%). Perfectly safe.
- At 150 feet: VD = (2 × 12.9 × 80 × 150) / 52,620 = 5.88V (2.45%). Still under the 3% limit.
- At 200 feet: VD = (2 × 12.9 × 80 × 200) / 52,620 = 7.84V (3.26%). Exceeds 3%. You must upsize to 2 AWG copper.
Decision Tree: Finalizing Your Feeder Pick
Use this decision matrix to lock in your exact material and gauge. Do not mix copper and aluminum terminations without proper bimetallic connectors and anti-oxidant paste (like Noalox).
| Scenario / Condition | Material Choice | Required Wire Gauge | Main Breaker Size |
|---|---|---|---|
| Run under 100 ft, standard indoor conduit | Copper (THHN) | 3 AWG | 100A 2-Pole |
| Run under 100 ft, budget-conscious / direct burial | Aluminum (XHHW-2) | 1 AWG | 100A 2-Pole |
| Run 100 ft to 175 ft (Voltage drop mitigation) | Copper (THHN) | 2 AWG | 100A 2-Pole |
| Run 100 ft to 175 ft (Voltage drop mitigation) | Aluminum (XHHW-2) | 1/0 AWG | 100A 2-Pole |
| More than 3 current-carrying conductors in pipe | Copper (THHN) | 2 AWG (Derated) | 100A 2-Pole |
Reading the 100 Amp Sub Panel Diagram & Terminations
When wiring the physical 100 amp sub panel diagram, you are dealing with a 4-wire system (NEC 250.140). The feeder must consist of two ungrounded conductors (Line 1 and Line 2), one grounded conductor (Neutral), and one equipment grounding conductor (Ground).
- Line 1 & Line 2 (Hots): Land these on the main breaker lugs of the subpanel (if it has a main disconnect) or the main feeder lugs. Torque to the manufacturer's specification.
- Neutral (White/Gray): Land on the isolated neutral bar. Critical: In a subpanel, the neutral bar must be physically isolated from the metal enclosure. Remove the green bonding screw or strap.
- Ground (Green/Bare): Land on the equipment grounding bar, which must be bonded directly to the metal subpanel enclosure. Add an accessory ground bar (like an Eaton GBKP26) if the panel doesn't have one pre-installed.
When an Engineer or AHJ Must Confirm
While the picks above cover 95% of residential and light-commercial workshop builds, local Authority Having Jurisdiction (AHJ) inspectors or professional engineers must review your plans if you hit any of these edge cases:
- High Ambient Heat: If your conduit runs through an attic or boiler room where temperatures regularly exceed 86°F (30°C), NEC 310.15(B)(1) requires temperature correction factors that will force you to upsize the wire.
- Conduit Bundling: If you are pulling multiple circuits through the same raceway and have more than 3 current-carrying conductors, you must apply the derating percentages in NEC 310.15(C)(1).
- Service Entrance Confusion: If this 100A panel is being fed directly from the utility meter (a service disconnect) rather than from another interior panel (a feeder), the grounding and neutral bonding rules change entirely. Meter-side work almost always requires a licensed electrician and utility coordination.
By sticking to 3 AWG copper or 1 AWG aluminum for runs under 150 feet, verifying your 75°C terminations, and keeping your subpanel neutral floating, your 100A feeder will pass inspection and run safely for decades.
