To feed a 100 amp subpanel, use a 100A 2-pole breaker and 3 AWG copper or 1 AWG aluminum wire for the hot and neutral conductors, plus an 8 AWG copper or 6 AWG aluminum equipment grounding conductor. This assumes THHN/THWN-2 insulation in a raceway at 75°C terminations.
- Conductor Material: Copper and Aluminum evaluated separately (never interchangeably).
- Termination Rating: 75°C column (standard for 100A+ panel lugs per NEC 110.14(C)).
- Ambient Temperature: 30°C (86°F) baseline.
- Raceway Type: PVC Schedule 80, EMT, or flexible metal conduit (FMC).
- Load Type: Standard residential non-continuous mixed loads.
The Core Sizing Matrix: Wire, Breaker, and Conduit
Sizing a subpanel feeder is not the same as sizing a main service entrance. A common and dangerous mistake DIYers make is applying the NEC 83% rule (NEC 310.12) to subpanels. The 83% rule allows 4 AWG copper (rated 85A) to be used on a 100A service breaker. However, this rule strictly applies only to main service entrance conductors. For a subpanel feeder, the wire must have an ampacity equal to or greater than the breaker rating. Therefore, 4 AWG copper is a code violation for a 100A subpanel; you must step up to 3 AWG.
| Conductor Material | Hot/Neutral Wire Size | Ampacity at 75°C | Ground Wire Size (Table 250.122) | Min. Conduit Size (EMT) |
|---|---|---|---|---|
| Copper (THHN/THWN-2) | 3 AWG | 100A | 8 AWG Copper | 1 inch |
| Aluminum (XHHW-2) | 1 AWG | 100A | 6 AWG Aluminum | 1.25 inch |
Always purchase wire explicitly marked THHN/THWN-2 or XHHW-2. The dual rating ensures the insulation can handle both the dry heat of a panel enclosure and the potential moisture inside underground conduit sweeps. For the equipment grounding conductor (EGC), NEC Table 250.122 dictates the minimum size based on the breaker rating, not the ungrounded conductor size.
Voltage Drop and Distance Adjustments
Wire ampacity dictates fire safety; voltage drop dictates equipment performance. The NEC recommends a maximum 3% voltage drop on feeders (NEC 310.15(B) Informational Note). What changes the answer from our baseline matrix is distance, bundling, and ambient heat.
Let us run a voltage drop check at a stated distance of 150 feet for a 240V circuit carrying an 80A continuous load using our baseline 3 AWG copper.
- Formula: VD = (2 × K × I × L) / CM
- K (Copper): 12.9
- I (Current): 80A
- L (Length): 150 ft
- CM (Circular Mils for 3 AWG): 52,620
VD = (2 × 12.9 × 80 × 150) / 52,620 = 5.87 Volts.
Percentage Drop = (5.87 / 240) × 100 = 2.44%.
At 150 feet, 3 AWG copper is perfectly acceptable. However, if your trench is 225 feet long, the drop becomes 3.66%, exceeding the 3% recommendation. At that distance, you must upsize to 2 AWG copper or 1/0 AWG aluminum to maintain equipment longevity, especially for motor-driven loads like well pumps or HVAC compressors which suffer reduced torque and excess heat under low voltage.
| Condition | Action Required | Code Reference |
|---|---|---|
| Run exceeds 150 ft (Cu) or 120 ft (Al) | Upsize one gauge to maintain < 3% VD | NEC 310.15(B) Note |
| More than 3 current-carrying conductors in one raceway | Apply 80% derating factor (requires 2 AWG Cu) | NEC Table 310.15(C)(1) |
| Conduit routed through an attic exceeding 30°C ambient | Apply temperature correction factors (Table 310.15(B)(1)) | NEC 310.15(B)(1) |
When to Call the AHJ or an Engineer
While the NFPA NEC guidelines provide the baseline, local Authority Having Jurisdiction (AHJ) amendments and complex load profiles require professional sign-off. You must pull a permit and have an inspector or electrical engineer confirm your design under the following conditions:
- Continuous Loads Exceeding 80A: If your subpanel will supply continuous loads (defined as running for 3 hours or more, like EV chargers or commercial lighting) that total more than 80A, NEC 210.20 requires the breaker to be sized at 125% of the continuous load. A 100A breaker can only handle 80A of continuous load. If your calculated continuous load is 85A, you need a 125A breaker and 1 AWG copper wire.
- High Ambient Temperature Environments: If the feeder conduit runs through a boiler room, a poorly ventilated attic in a southern climate, or along a sun-baked exterior wall, the 30°C baseline assumption fails. At 40°C ambient, the 75°C ampacity of 3 AWG copper derates to 88A, meaning it can no longer be protected by a 100A breaker.
- Utility Transformer Limits: If adding a 100A subpanel pushes your total calculated residential load past your main service rating (e.g., adding a 100A shop panel to a house already pulling 150A on a 200A main), an engineer must perform a formal NEC Article 220 load calculation to ensure the utility transformer and service drop will not overheat.
FAQ: Long-Tail Questions on 100A Subpanels
Can I use a 100A main breaker panel as a subpanel?
Yes, you can use a panel with a 100A main breaker as a subpanel, but the main breaker simply acts as a local disconnect switch. The critical step is removing the green neutral-to-ground bonding screw or strap inside the subpanel. In a subpanel, the neutral and ground must remain strictly isolated. The feeder must be a 4-wire setup (two hots, one neutral, one ground), and the ground bar must be bonded to the panel enclosure while the neutral bar floats.
Do I need a ground rod for a 100 amp subpanel in a detached garage?
Yes. Any subpanel in a separate structure requires a local grounding electrode system, regardless of the 4-wire feeder bringing the equipment ground from the main house. You must install two 5/8-inch copper-clad steel ground rods, spaced at least 6 feet apart, driven into the earth. Connect them to the subpanel's ground bar using a continuous 6 AWG bare copper grounding electrode conductor (GEC) per NEC 250.32. The Department of Energy wiring resources also emphasize ensuring the soil moisture is adequate for low-resistance grounding.
How much does it cost to install a 100 amp subpanel in 2026?
Material costs for a standard 100A 12-space subpanel (like a Square D Homeline or Siemens QP) range from $120 to $160. 3 AWG THHN copper wire costs approximately $3.50 to $4.20 per foot in 2026. For a 50-foot run requiring four conductors, wire costs alone will be around $700 to $840, plus $150 for PVC conduit, fittings, and a 100A 2-pole breaker. Total materials usually land between $1,000 and $1,200. Professional electrician labor for trenching, pulling, and terminating typically adds $800 to $1,500 depending on local rates and site complexity.
Can I run 2 AWG aluminum wire for a 100 amp subpanel?
No. A common misconception is that 2 AWG aluminum is sufficient for 100A because it is often used for 90A ranges or specific service entrance applications. According to the 75°C column of NEC Table 310.16, 2 AWG aluminum is rated for exactly 90A. Because standard breaker sizes do not include a 90A option, and the next standard size up (100A) exceeds the wire's ampacity, 2 AWG aluminum cannot be legally protected by a 100A breaker for a subpanel feeder. You must use 1 AWG aluminum (rated 100A at 75°C) or step up to 1/0 AWG aluminum for additional voltage drop headroom.
