What Size Wire for a 100 Amp Feeder: Sizing, Tables, and NEC Rules

A 100 amp feeder wire size is the specific American Wire Gauge (AWG) or kcmil cross-sectional area required to safely carry 100 amperes of continuous or non-continuous current from a main service panel to a subpanel or large appliance without exceeding the conductor's thermal limits. Getting this right changes everything downstream: it dictates the physical conduit diameter you must pull through, the exact inch-pound torque required on the panel lugs, and whether your voltage stays within usable bounds at the far end of the run.

The Core Sizing Table for 100A Feeders

The National Electrical Code (NEC) Table 310.16 is the master reference for conductor ampacity. However, you cannot simply look at the highest number in the 90°C column and call it a day. The size you actually terminate depends on the temperature rating of the breaker and panel lugs, which are almost universally rated for 75°C in modern residential and light commercial gear.

Worked Example: Voltage Drop, Derating, and Ground Upsizing

Let us move beyond the basic table and look at a real-world installation. You are running a 240V, 100A feeder to a detached garage subpanel located 150 feet away from the main house panel. You plan to use copper THHN in PVC conduit.

Step 1: Baseline Voltage Drop
Using 3 AWG copper (resistance ~0.245 ohms per 1,000 ft), the single-phase voltage drop formula is:

Voltage Drop = (2 × Current × Resistance × Length) / 1000
V_drop = (2 × 100A × 0.245 × 150) / 1000 = 7.35V

Dividing 7.35V by 240V gives a 3.06% drop. NEC Informational Note 210.19(A) recommends keeping feeder voltage drop under 3%. At 3.06%, your heavy loads (like an EV charger or air compressor) will experience noticeable voltage sag. We must upsize to 2 AWG copper (0.194 ohms/kft), which drops the loss to 5.82V (2.42%).

Step 2: The Hidden Trap - Ground Upsizing
Most DIYers stop at upsizing the hot and neutral wires, but NEC Article 250.122(B) mandates that if you upsize ungrounded conductors for voltage drop, you must proportionally upsize the equipment grounding conductor (EGC).
A standard 100A breaker requires a 6 AWG copper ground (26,240 circular mils). The ratio of our upsized 2 AWG (66,360 cmil) to the baseline 3 AWG (52,620 cmil) is 1.26. Multiplying our 6 AWG ground by 1.26 yields 33,062 circular mils. Since 4 AWG is 41,740 circular mils, you are legally required to pull a 4 AWG ground instead of a 6 AWG ground.

Step 3: Conduit Fill Consequences
Pulling three 3 AWG hots and one 6 AWG ground fits comfortably in a 1-inch Schedule 40 PVC conduit (40% fill limit is 0.346 sq in; our wires total ~0.305 sq in). But our upsized bundle—three 2 AWG hots and one 4 AWG ground—totals roughly 0.430 sq in. You must now jump to 1.25-inch PVC conduit. This is how a simple voltage drop calculation cascades into physical infrastructure changes.

Where You Meet 100A Feeders in Practice

You will typically spec a 100 amp feeder in three specific residential and light-commercial scenarios:

• Detached Garage Subpanels: A 100A subpanel is the modern sweet spot for a detached garage. It provides enough headroom for standard lighting, 120V receptacles, a 50A RV plug, and a 40A Level 2 EV charger simultaneously without tripping the main feeder breaker.
• Workshop Machinery: Large home workshops running 5HP+ air compressors, CNC mills, or heavy MIG/TIG welders often require a dedicated 100A disconnect and feeder to handle the high inrush currents of inductive motor loads.
• Whole-Home Generator Transfer Switches: When installing a 20kW to 24kW standby generator, the transfer switch or service-rated automatic switch is frequently rated for 100A or 200A, requiring heavy-gauge feeders between the generator and the main distribution panel.

Common Confusions: Feeder vs. Branch and Temperature Columns

Even experienced hobbyists trip over a few specific definitions and code rules when sizing wire. Here is what people commonly confuse:

1. Feeder vs. Branch Circuit
A feeder carries power from the service equipment (or a higher-level panel) to a subpanel or a single large disconnect. A branch circuit carries power from the final overcurrent device (the breaker) directly to the outlets, lights, or appliances. Feeders require different grounding rules (like a separate neutral and ground bar in the subpanel) and often utilize the 75°C column, whereas 14/12/10 AWG branch circuits are strictly limited to the 60°C column by NEC 240.4(D).

2. The 90°C Column Trap
THHN wire is rated for 90°C, leading many to assume they can use the 90°C column in Table 310.16 to get a higher ampacity. Think of the 90°C insulation rating like a wide, high-speed highway, but the 75°C breaker terminal is a narrow toll booth. No matter how much heat the wire insulation can handle on the highway, the toll booth dictates the maximum throughput. You may use the 90°C column to apply derating factors for high ambient temperatures or conduit fill, but your final adjusted ampacity cannot exceed the 75°C column value for termination.

3. Aluminum Oxidation and Torque
If you choose 1 AWG aluminum to save money on a long 100A run, you cannot treat it exactly like copper. Aluminum creeps and oxidizes. You must apply a listed anti-oxidant compound (like Noalox) to the stripped conductor before landing it in the lug, and you must use a calibrated torque screwdriver to hit the exact inch-pound specification printed on the panel label. Under-torqued aluminum lugs will loosen over thermal cycles, leading to high-resistance arcing and melted panel busbars.

References: Sizing and derating calculations based on NFPA 70 National Electrical Code (NEC) Articles 310, 210, and 250. Voltage drop mathematics verified against standard Southwire voltage drop calculation methodologies.