The Baseline: 100A Sub Panel Wire Sizing

For a 100 amp sub panel, the minimum code-compliant wire size is 3 AWG copper or 1 AWG aluminum, protected by a 100A double-pole breaker. This assumes 75°C terminations, 30°C ambient temperature, and no more than three current-carrying conductors in the raceway.

Baseline Assumptions for This Guide:
  • Material: Copper (primary recommendation), Aluminum (budget/long-run alternative)
  • Temperature Rating: 75°C column (standard for modern breakers and panel lugs)
  • Ambient Temperature: 30°C (86°F) or lower
  • Conduit/Raceway: PVC, EMT, or SER cable with a maximum of 3 current-carrying conductors (2 hots, 1 neutral; the equipment grounding conductor does not count toward derating)

Ampacity Breakdown: Why 3 AWG Copper and 1 AWG Aluminum?

When determining what gauge wire for a 100 amp sub panel, you must look at the National Electrical Code (NEC) Table 310.16. A common mistake among DIYers is looking at the 90°C column because wire insulation like THHN is rated for 90°C. However, NEC 110.14(C) dictates that unless the equipment is explicitly marked otherwise, termination ampacity is limited to the 75°C column because the lugs inside standard residential panels are only rated for 75°C.

NEC Table 310.16 Ampacity (75°C Column, 30°C Ambient)
Wire Size (AWG/kcmil) Copper (THWN/THHN) Aluminum (XHHW/THWN) 100A Breaker Compliant?
4 AWG 85A 65A No (Undersized)
3 AWG 100A 75A Copper: Yes | Al: No
2 AWG 115A 90A Copper: Yes | Al: No
1 AWG 130A 115A Both Yes
1/0 AWG 150A 120A Both Yes (Oversized)

Why this size and not one smaller?

You cannot use 4 AWG copper (85A) on a 100A breaker because the breaker would not protect the wire from overheating before tripping. While 3 AWG copper is exactly 100A, matching the breaker perfectly, many electricians default to 2 AWG copper or 1/0 AWG aluminum in practice. Why? Because 1 AWG and 3 AWG are sometimes difficult to source in pre-assembled 4-wire SER (Service Entrance Rating) cable at local big-box stores, making 2 AWG Cu or 1/0 Al the standard off-the-shelf feeder picks. However, strictly by the book, 3 AWG Copper is your minimum legal baseline.

Voltage Drop: When Distance Forces a Larger Wire

Ampacity tells you what the wire can handle without melting; voltage drop tells you if your tools and appliances will actually run correctly at the end of the line. The NEC recommends (via Informational Notes in Article 310) keeping voltage drop under 3% for branch circuits and feeders.

Let's run the math for a 100-foot run to a subpanel, assuming an 80A continuous load (80% of the 100A breaker capacity, which is standard practice for sizing subpanel feeders).

Voltage Drop Check at 100 Feet (240V System):
3 AWG Copper: Calculates to a 3.92V drop (1.63%). Passes easily.
1 AWG Aluminum: Calculates to a 4.05V drop (1.68%). Passes easily.

But what if your detached garage is 200 feet away? The voltage drop doubles. For 3 AWG copper at 200 feet, the drop becomes 7.84V, which is 3.26%. This exceeds the 3% recommended limit. At 200 feet, you must step up to 2 AWG copper (which drops the loss to 2.59%) or 1/0 AWG aluminum to maintain power quality and prevent motors from burning out due to undervoltage.

Decision Tree: Finalizing Your Feeder Wire Pick

Use this decision path to lock in your exact material and gauge. Do not mix and match; pick the row that matches your physical installation.

Installation Scenario Copper Pick Aluminum Pick Breaker Size
Run is under 100 feet; standard 30°C ambient; 3 conductors in conduit. 3 AWG 1 AWG 100A
Run is 100 to 150 feet; standard ambient; wanting to minimize voltage drop. 2 AWG 1/0 AWG 100A
Run is over 150 feet (e.g., detached workshop at back of property). 1 AWG 2/0 AWG 100A
Ambient temp in attic/conduit exceeds 30°C (86°F) but is under 40°C (104°F). 2 AWG 1/0 AWG 100A
Pulling 4 or more current-carrying conductors in the same conduit (bundling). 1 AWG 2/0 AWG 100A

Default Recommendation: If you are pulling individual THWN/THHN wires through PVC conduit for a standard backyard shed under 100 feet away, buy 3 AWG Copper for the hots and neutral, and an 8 AWG Copper equipment ground. If you are buying pre-packaged SER cable for a quick indoor run, buy 2-2-2-4 Aluminum SER (which is 2 AWG Aluminum, rated 90A, meaning you must use the next-size-up rule or just step to 1-1-1-3 SER to be fully compliant at 100A without math exceptions).

Variables That Change the Math (and When to Call the AHJ)

Wire sizing is not just about matching a breaker to a spool of wire. Environmental and physical factors force you to derate the wire's capacity.

What changes the answer?

  • Bundling (NEC 310.15(C)(1)): If you run two separate 240V circuits in the same PVC conduit, you now have four current-carrying conductors. You must apply an 80% derating factor. 3 AWG copper (100A) derates to 80A, which is no longer sufficient for a 100A breaker. You must jump to 1 AWG copper.
  • Temperature Derating: If your conduit runs across a hot roof or through an unventilated attic where ambient temperatures hit 40°C (104°F), you must multiply the base ampacity by 0.88. Again, this forces you to bump up one wire size.
  • Aluminum Termination Prep: If you choose aluminum to save money (Aluminum is roughly 40% cheaper than copper per foot in 2026), you must use an anti-oxidant paste like Noalox on the stripped wire ends before torquing the lugs. Aluminum oxidizes rapidly, creating a high-resistance connection that will melt the panel lug over time.
  • Torque Requirements: Modern NEC (110.14(D)) requires terminations to be torqued to the manufacturer's specifications. For a 100A lug, this is typically between 35 and 45 in-lbs. Use a calibrated torque screwdriver; guessing by 'feel' is a leading cause of subpanel fires.

When an Engineer or AHJ Must Confirm

While this guide covers standard residential 100A subpanels, you must pull a permit and have your local Authority Having Jurisdiction (AHJ) or a licensed electrical engineer review your plans if:

  • You are routing the feeder through commercial spaces, hazardous locations, or embedded in concrete.
  • Your local municipality has strict amendments (e.g., Chicago requires all wiring in EMT conduit, which alters fill capacity and derating math).
  • The subpanel is feeding a structure with a calculated continuous load that exceeds 80A, pushing the absolute limits of the 100A breaker.

For the vast majority of home workshops, EV charger additions, and detached garages, 3 AWG Copper or 1 AWG Aluminum on a 100A breaker is your definitive, code-compliant starting point. Verify your distance, check your ambient temperature, and torque those lugs to spec.