The direct answer for a standard 100-amp residential feeder or branch circuit is 3 AWG copper or 1 AWG aluminum, assuming your terminations are rated for 75°C. However, while those are the absolute code minimums for ampacity, most working electricians pull 1 AWG copper or 1/0 AWG aluminum for 100A subpanel feeders to mitigate voltage drop over distance and provide thermal headroom.

Sizing wire isn't just about matching a breaker size to a chart; it requires understanding temperature columns, derating factors, and the physical limitations of your run. Below is the complete reference data you need to size your 100A circuit correctly the first time.

The Master AWG Chart for 100 Amp Circuits (NEC Table 310.16)

This data is pulled directly from NFPA 70 (National Electrical Code) Table 310.16, which dictates the allowable ampacities for insulated conductors rated up to 2000 volts.

How to read this table: The rows represent the American Wire Gauge (AWG) or kcmil size. The columns are split by conductor material (Copper vs. Aluminum/Copper-Clad) and then by the temperature rating of the wire insulation (60°C, 75°C, 90°C). To find your required wire size, locate your target amperage (100A) in the column that matches your termination temperature rating, then read left to find the minimum AWG.
Table 310.16 Allowable Ampacities (Ambient Temperature 30°C / 86°F)
Wire Size (AWG/kcmil) Copper 60°C Copper 75°C Copper 90°C Aluminum 60°C Aluminum 75°C Aluminum 90°C
4 AWG 70A 85A 95A 55A 70A 80A
3 AWG 85A 100A 115A 65A 80A 90A
2 AWG 95A 115A 130A 70A 90A 100A
1 AWG 110A 130A 145A 85A 100A 115A
1/0 AWG 125A 150A 170A 100A 120A 135A

Quick-Jump Reference: Common 100A Scenarios

Bookmark these rows for the most frequently wired 100-amp applications:

  • 100A Subpanel Feeder (Short Run < 50ft): 3 AWG Copper (THHN/THWN-2) or 1 AWG Aluminum (XHHW-2). Requires 75°C rated lugs.
  • 100A Subpanel Feeder (Long Run > 100ft): 1 AWG Copper or 1/0 AWG Aluminum. Upsized strictly for voltage drop mitigation.
  • 100A EV Charger (Hardwired): 3 AWG Copper. Must be routed in conduit; NM-B cable cannot be used for high-continuous loads in this configuration without upsizing due to 60°C NM-B limits.
  • 100A Service Entrance (Overhead): 1/0 AWG Aluminum Triplex. Utility standards often dictate 1/0 Al minimum for service drops regardless of NEC ampacity tables.

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers make when looking up the AWG for 100 amps is using the 90°C column because they bought THHN wire, which is stamped '90°C' on the jacket. This is a code violation waiting to happen.

Per NEC Section 110.14(C), the ampacity of a circuit is determined by the lowest temperature rating of any connected component. This is known as the 'weakest link' rule.

  • The 60°C Column: Applies if you are using NM-B (Romex) cable, or if your breaker and panel lugs are older and only rated for 60°C. (Note: NM-B maxes out at 2 AWG anyway, so you cannot use NM-B for a 100A feeder).
  • The 75°C Column: This is the standard for virtually all modern residential and commercial breakers, panelboard lugs, and disconnect switches. Even if your wire is THHN (90°C), you must use the 75°C column to determine your final ampacity because the breaker lug is only rated to 75°C.
  • The 90°C Column: You may only use this column for ampacity if every single termination point (breaker, lug, busbar, and device) is explicitly stamped and rated for 90°C. In residential work, this is exceptionally rare. The 90°C column is primarily used as a starting point for calculating derating factors before applying the 75°C termination limit.
Pro-Tip for Aluminum Wire: If you choose 1 AWG Aluminum to save money on a 100A feeder, you must strip the wire cleanly, apply an anti-oxidant compound (like Noalox) to the bare strands, and torque the lugs to the exact inch-pound specification printed on the breaker label. Aluminum creeps and oxidizes; improper torque is the leading cause of melted lugs on 100A subpanels.

Derating, Voltage Drop, and What the Chart Cannot Tell You

NEC Table 310.16 assumes you are running your wires in an ambient temperature of 30°C (86°F) and that you have no more than three current-carrying conductors in a single raceway (conduit). When real-world conditions deviate from this baseline, the chart's base values no longer apply directly.

How Derating Rows Modify the Base Value

If you pull more than three current-carrying conductors through a single conduit, the wires heat each other up. You must apply a derating multiplier from NEC Table 310.15(C)(1).

  • 4 to 6 conductors: Multiply base ampacity by 80%.
  • 7 to 9 conductors: Multiply base ampacity by 70%.

Example: You are pulling two 100A circuits (4 hot wires + 2 neutrals = 6 current-carrying conductors) in one PVC conduit. If you use 3 AWG Copper THHN, the base 90°C ampacity is 115A. Applying the 80% derating factor (115A x 0.80) yields 92A. Because 92A is less than your 100A breaker, 3 AWG is no longer legal. You must upsize to 2 AWG Copper (130A x 0.80 = 104A) to pass inspection.

What the Table Cannot Tell You: Voltage Drop

Table 310.16 only tells you the wire size required to prevent the insulation from melting and starting a fire. It tells you absolutely nothing about voltage drop. The NEC recommends a maximum 3% voltage drop on branch circuits and 5% on the total feeder+branch combined (NEC Informational Note 310.15(B)).

Let's run the math on a 100A subpanel located 150 feet from the main panel, using the code-minimum 3 AWG Copper:

  • 3 AWG Copper resistance = ~0.245 ohms per 1,000 feet.
  • Total round-trip distance = 300 feet.
  • Total resistance = 0.0735 ohms.
  • Voltage Drop at 100A = 7.35 Volts.
  • Percentage Drop (on a 240V system) = 3.06%.

At exactly 100A, a 3 AWG wire over 150 feet exceeds the recommended 3% drop limit. Your subpanel will receive only ~232V under peak load, which can cause motors to overheat and sensitive electronics to brown out. This is exactly why experienced electricians automatically upsize a 150-foot, 100A feeder to 1 AWG Copper (yielding a much healthier 1.9% drop) or 1/0 AWG Aluminum, even though 3 AWG is technically 'legal' for ampacity.

Always calculate voltage drop for any 100A run exceeding 75 feet. Sizing wire is a balance between satisfying the strict minimums of the NEC ampacity tables and the practical physics of the electrons traveling down your conduit.