For a standard 100A subpanel, use #3 AWG copper wire (or #1 AWG aluminum) for the hot and neutral conductors, protected by a 100A 2-pole breaker. This assumes THHN/THWN-2 insulation in conduit, 75°C terminations, and an ambient temperature of 30°C (86°F).

Baseline Assumptions for This Guide:
  • Material: Copper (primary) and Aluminum (secondary).
  • Insulation: THHN/THWN-2 rated for 90°C, but terminated at 75°C.
  • Ambient Temperature: 30°C (86°F) or lower.
  • Installation Method: Individual conductors in PVC, EMT conduit, or flexible raceway.
  • Load Type: Non-continuous, standard residential/commercial mixed loads.

The Core Sizing Decision Tree

Wire sizing is not a one-size-fits-all answer; it depends entirely on the cable type and installation method. Use this decision matrix to find your exact required conductor size.

If Your Installation Is... Required Hot/Neutral AWG (Copper) Required Hot/Neutral AWG (Aluminum) NEC Rule Driving the Decision
THHN/THWN-2 in conduit (Standard) #3 AWG #1 AWG Table 310.16 (75°C Column)
NM-B (Romex) Cable #1 AWG N/A (NM-B is Cu only) 334.80 (60°C Column Limit)
SER Cable (Service Entrance) #1 AWG (typically) #1 AWG 310.12 / 310.16 (75°C Column)
Run distance exceeds 150 feet #2 AWG 1/0 AWG Voltage Drop (Informational Note)

Why #3 AWG Copper and Not #4 AWG?

A common mistake on the jobsite is looking at the 90°C column of NEC Table 310.16 and seeing that #4 AWG THHN is rated for 95A, or assuming that because modern breakers are robust, you can round up. You cannot.

Per NEC 110.14(C), the ampacity of a conductor is limited by the temperature rating of the terminations (lugs) it connects to. Almost all standard 100A breakers and subpanel lugs are rated for 75°C. Therefore, you must use the 75°C column to determine your baseline ampacity.

  • #4 AWG Copper in the 75°C column: 85 Amps. (Fails the 100A requirement).
  • #3 AWG Copper in the 75°C column: 100 Amps. (Exact match).
The NM-B (Romex) Trap: If you are running NM-B cable instead of individual THHN wires in conduit, NEC 334.80 strictly limits the ampacity to the 60°C column, regardless of the fact that the wire's physical insulation might be rated higher. In the 60°C column, #3 AWG is only rated for 85A, and #2 AWG is rated for 95A. To feed a 100A subpanel with NM-B, you are forced to use #1 AWG copper (rated 110A at 60°C). This is why professional electricians almost exclusively pull individual THHN/THWN-2 conductors through conduit for subpanel feeders.

Voltage Drop and Distance Limits

Ampacity tells you the wire won't melt; voltage drop tells you your equipment will actually function correctly at the other end. The NEC recommends a maximum 3% voltage drop on a branch circuit or feeder, and 5% total from the utility transformer to the furthest outlet.

Let's run the math for a 240V, 100A load using #3 AWG copper. The resistance of #3 copper is approximately 0.245 ohms per 1,000 feet.

Scenario A: 100-Foot Run

Using the single-phase voltage drop formula: VD = (2 × Length × Current × Resistance) / 1000

  • VD = (2 × 100 × 100 × 0.245) / 1000
  • VD = 4.9 Volts
  • Percentage = 4.9V / 240V = 2.04%

Verdict: #3 AWG is perfectly acceptable at 100 feet.

Scenario B: 150-Foot Run

  • VD = (2 × 150 × 100 × 0.245) / 1000
  • VD = 7.35 Volts
  • Percentage = 7.35V / 240V = 3.06%

Verdict: You have crossed the 3% threshold. At 150 feet, you must upsize your hot and neutral conductors to #2 AWG copper (Resistance = 0.194 ohms/kft, resulting in a 2.42% drop). According to the Copper Development Association, maintaining voltage stability is critical for the longevity of motors and compressors in outbuildings.

Grounding and Neutral Conductor Rules

Sizing the hot legs is only half the battle. The neutral and equipment grounding conductor (EGC) have their own strict rules.

The Neutral

For a standard 120/240V single-phase subpanel, the neutral carries the unbalanced current between the two hot legs. While purely 240V loads (like a water heater) don't use the neutral, a subpanel will almost certainly supply 120V lighting and receptacles. Standard practice—and the safest default—is to size the neutral exactly the same as the hot conductors: #3 AWG copper (or #1 AWG aluminum).

The Equipment Grounding Conductor (EGC)

The ground wire does not carry current under normal operation; it only exists to clear a fault. Therefore, it is sized based on the breaker rating, not the load, per NEC Table 250.122.

Breaker Size Copper Ground AWG Aluminum Ground AWG
100 Amps #8 AWG #6 AWG
Proportional Upsizing Rule (NEC 250.122(B)): If you upsized your hot wires from #3 to #2 AWG to mitigate voltage drop over a long distance, you must proportionally upsize your ground wire. Moving from #3 to #2 is a one-step increase in cross-sectional area. Therefore, your copper ground must move one step up from #8 AWG to #6 AWG.

Derating for Heat and Bundling

The baseline assumption of 30°C (86°F) ambient temperature and three current-carrying conductors (two hots, one neutral) in a single raceway is standard. But physical conditions on the jobsite frequently force derating.

Ambient Temperature: If your conduit runs through an unventilated attic in a hot climate where ambient temperatures regularly exceed 30°C, you must apply a correction factor. Fortunately, THHN is rated for 90°C, so we use the 90°C column for derating calculations before applying the 75°C termination limit. At 40°C (104°F), the correction factor is 0.91. The 90°C ampacity of #3 AWG is 115A. Multiplying 115A by 0.91 yields 104.6A. Since 104.6A is still greater than 100A, #3 AWG survives this derating. However, at 50°C (122°F), the factor drops to 0.82 (115 × 0.82 = 94.3A), forcing an upsize to #2 AWG.

Conductor Bundling: If you pull more than three current-carrying conductors through the same conduit (for example, if you are sharing the conduit with a multi-wire branch circuit or a second feeder), the heat generated by the bundled wires requires derating. Four to six conductors require an 80% derating factor. 115A × 0.80 = 92A. In this scenario, #3 AWG fails, and you must pull #2 AWG.

When to Defer to the AHJ or an Engineer

While the calculations above cover 95% of residential and light commercial subpanel installations, specific edge cases require professional stamping or Authority Having Jurisdiction (AHJ) approval.

  1. Continuous Loads: If the subpanel is dedicated to continuous loads (defined as operating for 3 hours or more at maximum capacity, such as a commercial server room, EV charging station, or grow lights), NEC 210.20 requires the breaker to be sized at 125% of the continuous load. A 100A continuous load requires a 125A breaker, which completely changes the wire sizing to #1 AWG copper minimum.
  2. Utility Specific Requirements: Some local utilities mandate that any feeder acting as a service drop to a detached structure must meet service entrance grounding standards (NEC 250.32), requiring a grounding electrode system (ground rods) at the subpanel in addition to the equipment grounding conductor.
  3. Aluminum Terminations: If you choose #1 AWG aluminum to save on material costs, ensure your main panel lugs and subpanel lugs are explicitly rated for aluminum (ALR or AL/CU). Many older or budget breakers are copper-rated only. Using aluminum in a copper-only lug will result in galvanic corrosion, high resistance, and eventually a melted lug or fire.

By sticking to #3 AWG copper THHN in conduit for runs under 150 feet, and properly sizing your neutral and ground, you ensure a safe, code-compliant, and highly efficient 100A subpanel installation.