To wire a 100 amp subpanel, use 3 AWG copper or 1 AWG aluminum wire for the two hot conductors and the neutral, paired with a 100A double-pole breaker in the main panel. You must also run an 8 AWG copper equipment grounding conductor.
- Material: Copper (primary) or Aluminum (secondary). Never mix them without rated bimetallic lugs.
- Insulation: THHN/THWN-2 (standard for conduit).
- Temperature Column: 75°C (mandated by NEC 110.14(C) for standard residential terminations).
- Ambient Temperature: 30°C (86°F) or lower.
- Conduit Fill: Maximum 3 current-carrying conductors in the raceway (2 hots + 1 neutral). Ground does not count.
Note: Sizing guidance follows NEC-style rules; your local Authority Having Jurisdiction (AHJ) has final legal authority.
The Core Sizing Decision: Why 3 AWG Copper or 1 AWG Aluminum?
Many DIYers look at the 90°C column on a wire ampacity chart, see that 4 AWG copper is rated for 95 amps, and assume they can use the "next size up" rule to protect it with a 100A breaker. This is a dangerous misinterpretation of the National Electrical Code (NEC).
Under NEC 110.14(C), you must size your wire based on the temperature rating of the terminations (the lugs inside your breaker and subpanel), not just the wire insulation. Standard residential breakers and panels (like Square D Homeline or Eaton BR) are rated for 75°C terminations. Therefore, you must use the 75°C column to determine your baseline ampacity.
| AWG Size | Material | 60°C Column | 75°C Column (Terminations) | 90°C Column (Wire/Derating) |
|---|---|---|---|---|
| 4 AWG | Copper | 70A | 85A | 95A |
| 3 AWG | Copper | 85A | 100A | 110A |
| 2 AWG | Copper | 95A | 115A | 130A |
| 1 AWG | Aluminum | 75A | 100A | 115A |
As the table shows, 4 AWG copper maxes out at 85A for terminations. You cannot use the "next standard size up" breaker rule (NEC 240.4(B)) if your calculated load actually approaches 100A, and using an 85A wire on a 100A breaker leaves an unsafe margin. 3 AWG copper hits exactly 100A at 75°C, making it the minimum legal and safe size. If you are using aluminum (which is cheaper and highly recommended for long feeder runs), you must step up to 1 AWG.
What Changes the Answer: Distance, Bundling, and Environment
The 3 AWG copper / 1 AWG aluminum baseline assumes a short run in a cool environment. Real-world jobsite conditions force you to upsize your wire to prevent voltage drop or thermal derating. Use this decision tree to determine if your specific installation requires larger conductors.
| Condition | Trigger Threshold | Required Action |
|---|---|---|
| Voltage Drop (Distance) | Run exceeds 150 feet (assuming 80A continuous load on a 240V circuit). | Upsize to 2 AWG Copper or 1/0 AWG Aluminum to keep drop under 3%. |
| Conduit Bundling | More than 3 current-carrying conductors in the same conduit (e.g., running two separate subpanel feeders in one pipe). | Apply NEC 310.15(C)(1) derating factors. For 4-6 conductors, multiply 90°C ampacity by 80%. |
| High Ambient Heat | Conduit runs through an attic or roof space where temperatures exceed 30°C (86°F). | Apply temperature correction factors from NEC Table 310.15(B)(1). Upsize wire accordingly. |
The Voltage Drop Math (Why 150 Feet is the Cutoff)
The NEC recommends a maximum 3% voltage drop on feeders for reasonable efficiency. Let's calculate the drop for 3 AWG Copper running 100 feet to a subpanel carrying a heavy 80A continuous load (like an EV charger plus shop tools):
- Formula: VD = (2 × K × I × D) / Circular Mils
- K (Copper resistance) = 12.9
- I (Current) = 80A
- D (Distance) = 100 ft
- CM (3 AWG Circular Mils) = 52,620
- Result: (2 × 12.9 × 80 × 100) / 52,620 = 3.92 Volts
On a 240V circuit, 3.92V is a 1.63% drop. At 150 feet, the drop hits 5.88V (2.45%), which is still acceptable. But at 200 feet, the drop reaches 7.84V (3.26%), crossing the 3% threshold. If your run is over 175 feet, skip 3 AWG and pull 2 AWG copper or 1/0 aluminum from the start.
Step-by-Step Feeder Routing and Termination
Sizing the wire is only half the battle. Improper termination causes arcing, melted lugs, and fires. Follow these physical installation rules:
- Pull 4 Wires: Modern NEC requires a 4-wire feeder for subpanels (2 Hots, 1 Neutral, 1 Ground). You must use a separate ground bar in the subpanel.
- Remove the Bonding Screw: In the main panel, neutral and ground are bonded. In a subpanel, they must remain isolated. Remove the green bonding screw or strap from the subpanel's neutral bar before wiring. The Electrical Safety Foundation International (ESFI) frequently cites improper subpanel bonding as a primary cause of shock hazards in garages and outbuildings.
- Strip and Seat Fully: Strip the THHN insulation exactly to the length indicated by the lug's wire stop. Do not leave bare copper exposed outside the lug, and ensure no insulation is trapped under the pressure plate.
- Torque to Spec: Do not just "crank it down" with a standard screwdriver. Look at the panel label for the required torque (typically 40 to 50 inch-pounds for 100A lugs). Use a calibrated torque screwdriver (like the Klein Tools 690) to tighten the set screws. Loose connections heat up and fail under heavy load.
When to Pull in a Licensed Electrician or Engineer
While wiring a subpanel is a standard DIY task for competent hobbyists, certain scenarios legally and practically require a licensed professional:
If your main panel is a 200A service and you are adding a 100A subpanel, you must verify you aren't violating the NEC 120% busbar rating rule (NEC 220.87 / 705.12), especially if you have solar panels installed. If the sum of your main breaker and your solar backfeed exceeds 120% of the panel's busbar rating (e.g., 240A on a 200A busbar), you cannot legally add the subpanel without a service upgrade or a line-side tap. An engineer or licensed electrician must calculate this.
Additionally, if your local AHJ requires a permit and inspection for feeder work (most do), or if you are trenching direct-burial cable (like USE-2 or URD) under a driveway where depth and warning ribbon codes strictly apply, hire a pro.
Frequently Asked Questions
How to wire a 100 amp subpanel for a detached garage?
Wiring a detached garage requires the same 4-wire feeder (3 AWG Cu / 1 AWG Al) as an attached subpanel, but with one critical addition: a Grounding Electrode System. Per NEC 250.32, because the garage is a separate structure, you must drive two 5/8-inch copper ground rods at least 6 feet apart into the earth outside the garage. Connect these rods to the subpanel's isolated ground bar using a continuous 6 AWG bare copper grounding electrode conductor. Do not bond the neutral to this ground bar.
What size ground wire is required for a 100 amp subpanel?
According to NEC Table 250.122, the minimum equipment grounding conductor (EGC) for a 100A breaker is 8 AWG copper or 6 AWG aluminum. However, if you had to upsize your hot conductors for voltage drop (e.g., you used 1 AWG copper instead of 3 AWG for a 200-foot run), NEC 250.122(B) requires you to increase the ground wire proportionally. In that specific case, you would need to step the ground up to 6 AWG copper.
Can I use a smaller breaker if my actual calculated load is under 60 amps?
Yes. The "100 amp" rating stamped on the subpanel door is simply the maximum current the panel's busbars can safely handle. It does not dictate the size of the feeder breaker. If your calculated load for a shed is only 45 amps, you can legally feed that 100A-rated subpanel using a 50A double-pole breaker and 6 AWG copper wire. The breaker protects the wire, and the panel rating just acts as a ceiling. This is a highly cost-effective way to install a panel that allows for future expansion without overspending on thick feeder wire today.
