Wire and Breaker Sizing for Installing a 100 Amp Sub Panel

For installing a 100 amp sub panel, use 3 AWG copper or 1 AWG aluminum wire, protected by a 100A double-pole breaker. This baseline assumes 75°C terminations, 30°C ambient temperature, THHN/XHHW-2 insulation, and no bundling derating.

Baseline Assumptions for This Sizing

• Conductor Material: Copper or Aluminum (sized separately below)
• Insulation Type: THHN/THWN-2 or XHHW-2 (90°C rated wire)
• Termination Rating: 75°C column (per NEC 110.14(C))
• Ambient Temperature: 30°C (86°F) or lower
• Installation Method: Up to 3 current-carrying conductors (CCCs) in a single raceway

The Core Sizing Table: Copper vs. Aluminum Feeders

When pulling feeders for a subpanel, you are sizing for the feeder breaker, not the sum of the branch circuits inside the subpanel. The table below maps standard wire gauges to their allowable ampacities under the 75°C and 90°C columns of NEC Table 310.16, alongside the maximum standard breaker size they can terminate on.

Why #3 AWG Copper and Not #4 AWG?

A common mistake on the jobsite is attempting to use #4 AWG copper THHN for a 100A feeder because the wire's jacket is rated for 90°C, and #4 AWG at 90°C is rated for 95A. However, you cannot use the 90°C column for final sizing.

Under NEC 110.14(C), the allowable ampacity of a conductor is limited by the temperature rating of the weakest termination in the circuit. Standard residential and light-commercial breaker lugs and subpanel busbars are rated for 75°C. Therefore, you must size the wire using the 75°C column.

• #4 AWG Copper at 75°C: 85A. The next standard breaker size up (per NEC 240.6) is 90A. Since your calculated load is 100A, a 90A breaker is insufficient. #4 AWG fails.
• #3 AWG Copper at 75°C: 100A. This perfectly matches the 100A double-pole breaker and the 100A load limit.

You only use the 90°C column as a starting point if you need to apply ambient temperature or bundling derating factors. After derating the 90°C ampacity, the final result must still be equal to or greater than the 75°C ampacity required by your termination.

Voltage Drop and Distance Derating

Ampacity dictates whether the wire will melt or trip the breaker; voltage drop dictates whether your tools and appliances will actually run correctly at the subpanel. NEC 215.2 recommends a maximum 3% voltage drop on feeders.

Let us run a voltage drop check for a 100A load at 240V using #3 AWG copper. Using the standard single-phase formula VD = (2 × K × I × L) / CM (where K=12.9 for copper, and Circular Mil area for #3 is 52,620):

If your run from the main panel to the subpanel exceeds 100 feet, you should strongly consider stepping up to #2 AWG copper or #1/0 AWG aluminum to maintain voltage stability, especially if the subpanel will run high-inrush loads like air compressors, welders, or well pumps. You can verify exact figures for your specific installation using the Southwire Voltage Drop Calculator.

What Changes the Answer: Bundling, Aluminum, and the AHJ

The baseline #3 Cu / #1 Al answer holds true for a standard, short, single-feeder installation. Here is what forces you to change that wire size.

1. Conductor Bundling (More than 3 CCCs)

A standard 4-wire subpanel feeder (two ungrounded hots, one grounded neutral, one equipment ground) contains exactly three Current-Carrying Conductors (CCCs). The ground never counts. The neutral counts as a CCC on single-phase systems because it carries the unbalanced 120V return current.

If you pull two separate 100A feeders through the same PVC conduit, you now have 6 CCCs. Per NEC Table 310.15(C)(1), 4 to 6 CCCs require an 80% derating factor applied to the 90°C column.

• #3 AWG Cu (90°C base = 115A): 115A × 0.80 = 92A. This is less than 100A. #3 fails.
• Solution: You must step up to #1 AWG Copper (90°C base = 130A × 0.80 = 104A) to maintain a 100A rating in a bundled conduit.

2. Switching to Aluminum

Aluminum (specifically XHHW-2 or THWN-2) is vastly cheaper than copper—often saving $150 to $300 on a 100A feeder run. However, it requires strict installation discipline:

• Size: You must use #1 AWG aluminum (rated 100A at 75°C).
• Oxidation: Aluminum oxidizes rapidly when stripped. You must wire-brush the conductor and apply an anti-oxidant compound (like Noalox or Deox) immediately before landing it in the lug.
• Torque: NEC 110.14(D) mandates the use of a calibrated torque screwdriver or wrench. Aluminum creeps under pressure; under-torqued lugs will loosen over thermal cycles and arc, while over-torqued lugs will shear the wire strands. Check the panel manufacturer's torque chart (typically 40-60 in-lbs for these lug sizes).
• Mixed Metals: Never land an aluminum wire on a lug rated only for copper. Most modern breakers and subpanel lugs are rated AL/CU, but always verify the stamp on the metal.

3. When an Engineer or the AHJ Must Confirm

Do not rely on standard tables and proceed without a permit and professional review if your installation hits any of these edge cases:

• Continuous Loads: If the subpanel will serve loads that run at maximum current for 3 hours or more (e.g., commercial lighting, continuous HVAC, EV chargers), NEC 210.20(A) requires the breaker and wire to be sized at 125% of the continuous load. A 100A continuous load requires a 125A breaker and #1 AWG copper wire.
• High Ambient Temperatures: If the conduit runs through an attic space in a hot climate where ambient temperatures regularly exceed 30°C (86°F), you must apply ambient temperature correction factors from NEC Table 310.15(B)(1). An attic at 50°C (122°F) requires a 0.82 correction factor on the 90°C column.
• Service Entrance vs. Feeder: This guide covers feeders from a main panel to a subpanel. If you are installing a 100A panel as a separate service disconnect directly from the utility meter, service entrance rules (NEC Article 230) and utility company specifications override standard feeder rules. Utility engineers often mandate specific wire types, conduit depths, and grounding electrode systems that vary wildly by municipality.