For a standard 100 amp sub panel wiring diagram, use 3 AWG copper or 1 AWG aluminum wire, protected by a 100A double-pole breaker at the source. This minimum sizing assumes 75°C terminations, 30°C ambient temperature, and a run under 100 feet without voltage drop issues.
- Conductor Material: Copper (THHN/THWN-2) or Aluminum (XHHW-2)
- Termination Rating: 75°C (Standard for residential breakers and lugs per NEC 110.14(C))
- Ambient Temperature: 30°C (86°F) baseline; no extreme attic heat derating applied
- Conduit Type: EMT or PVC, maximum 3 current-carrying conductors (no bundling derating)
- System Voltage: 240V / 120V split-phase residential
Note: NEC-style guidance provided here is for educational planning. Your local Authority Having Jurisdiction (AHJ) has final authority on all installations.
Core Sizing Assumptions and Ampacity Data
When drafting your 100 amp sub panel wiring diagram, the National Electrical Code (NEC) Table 310.16 is your primary reference. A common mistake is sizing wire based on the 90°C column because THHN wire is rated for 90°C. However, unless your breakers and panel lugs are explicitly marked for 90°C (which residential gear almost never is), you must use the 75°C column to determine your baseline ampacity.
| Conductor Size | Material | Insulation Type | 75°C Column Ampacity | 90°C Column Ampacity |
|---|---|---|---|---|
| 4 AWG | Copper | THHN/THWN-2 | 85A | 95A |
| 3 AWG | Copper | THHN/THWN-2 | 100A | 115A |
| 2 AWG | Aluminum | XHHW-2 | 90A | 100A |
| 1 AWG | Aluminum | XHHW-2 | 100A | 115A |
Voltage Drop Check at 100 Feet
Ampacity tells you the wire won't melt; voltage drop tells you your tools and appliances will actually run correctly. The NEC recommends a maximum 3% voltage drop for feeders. Let's run the math for 3 AWG copper at a full 100A load over a 100-foot one-way distance.
Formula: VD = (2 × K × I × D) / CM
- K (Copper constant) = 12.9
- I (Current) = 100A
- D (Distance) = 100 ft
- CM (Circular Mils for 3 AWG) = 52,620
Calculation: (2 × 12.9 × 100 × 100) / 52,620 = 4.90V drop
Percentage: 4.90V / 240V = 2.04% (Passes the <3% rule)
Edge Case: If your run is 150 feet, the drop becomes 7.35V (3.06%). You must upsize to 2 AWG copper to bring the drop back down to 2.42%. Use a dedicated voltage drop calculator to verify your specific run length.
Why 3 AWG Copper? (And Why Not 4 AWG)
It is tempting to use 4 AWG copper to save money, as it is rated for 95A in the 90°C column. However, NEC 110.14(C) strictly limits you to the 75°C column for equipment rated 100A or less. At 75°C, 4 AWG copper is only good for 85A.
You cannot protect an 85A wire with a 100A breaker. While NEC 240.4(B) allows the "next standard size up" rule for overcurrent protection, that rule only applies if the calculated load does not exceed the wire's ampacity. If your sub panel could realistically pull 90A or 100A, an 85A wire will overheat the breaker terminations over time, leading to melted lug insulation or a nuisance trip. 3 AWG copper provides exactly 100A at 75°C, perfectly matching the breaker and the panel's main lugs without relying on code loopholes.
Variables That Change Your Wire Size
The baseline 3 AWG copper / 1 AWG aluminum answer assumes ideal conditions. Real-world jobsite variables will force you to upsize. Use this decision tree to adjust your 100 amp sub panel wiring diagram:
| Condition | Impact on Sizing | Action Required |
|---|---|---|
| Run exceeds 100-120 ft | Voltage drop exceeds 3% at full load | Upsize wire by 1-2 AWG steps (e.g., use 2 AWG or 1 AWG Cu) |
| More than 3 current-carrying conductors in one conduit | Ampacity derating per NEC 310.15(C)(1) | Apply derating factor to the 90°C column; ensure final adjusted ampacity is ≥ 100A |
| Using Aluminum instead of Copper | Higher resistance, larger physical diameter | Use 1 AWG Al minimum; apply anti-oxidant paste (e.g., Noalox) to all terminations |
| Continuous loads (>3 hours at a time) | NEC 210.20(A) requires 125% sizing for continuous loads | If continuous load is 80A+, upsize breaker to 125A and wire to 1 AWG Cu / 1/0 AWG Al |
| Ambient temperature > 30°C (e.g., hot attic) | Ampacity drops as heat rises | Apply ambient temperature correction factors from NEC Table 310.15(B)(1) |
When to Call an Engineer or the AHJ
While a 100A feeder is a standard DIY or journeyman task, certain scenarios require professional stamping or explicit AHJ approval:
- Service Entrance vs. Feeder: If this 100A panel is not a sub panel fed from your main, but rather a new service drop from the utility pole or meter, utility regulations and service entrance rules (NEC Article 230) apply. You cannot DIY service entrance work in most jurisdictions.
- Heavy Continuous Loads: If you are wiring a sub panel specifically for a commercial-style kitchen, a massive EV charger array, or a woodworking shop with 5HP+ dust collectors running continuously, a load calculation by a licensed electrical engineer ensures you don't experience brownouts.
- Local Aluminum Bans: Some local AHJs strictly prohibit aluminum feeder wire inside residential structures due to historical issues with improper terminations. Always check with your local building department before purchasing 1 AWG aluminum.
100 Amp Sub Panel Wiring Diagram FAQ
Do I need a ground rod for my 100 amp sub panel wiring diagram?
If the sub panel is in a detached structure (like a garage or shed), NEC 250.32 requires you to install a grounding electrode system (typically two 8-foot ground rods spaced 6 feet apart) and bond it to the sub panel's ground bar. If the sub panel is in the same building as the main panel (e.g., a basement partition), you do not need ground rods; you simply run an equipment grounding conductor back to the main panel's ground bar.
Can I use a 100A main breaker panel as a sub panel?
Yes. Buying a "main breaker" panel instead of a "main lug" panel is often cheaper and more widely available at big-box stores. When used as a sub panel, the 100A main breaker simply acts as a local disconnect switch. It does not replace the 100A breaker at the source (the main panel). Just ensure you pull the 120V feeder wires from the main panel into the sub panel's main lugs, bypassing the sub panel's main breaker, or feed the main breaker directly if it's configured for backfeeding.
What size ground wire do I need for a 100 amp sub panel?
According to NEC Table 250.122, the minimum equipment grounding conductor for a 100A overcurrent device is 8 AWG copper or 6 AWG aluminum. This ground wire must run in the same conduit or cable assembly as your ungrounded (hot) and grounded (neutral) conductors. Do not rely on the conduit itself as the ground path for a residential feeder; always pull a dedicated wire.
Does the neutral and ground need to be separated in the sub panel?
Yes, this is the most critical safety rule for sub panels. Per NEC 250.142, the neutral (grounded) and ground (equipment grounding) bus bars must be strictly isolated in a sub panel. If your panel came with a green bonding screw or a bonding strap connecting the neutral bar to the metal chassis, you must remove it. Neutral current must only return to the source via the neutral wire, not via the ground wire or the metal conduit, which prevents the chassis from becoming energized during a neutral fault.
