The correct cable size for 100 amp service is the minimum wire gauge that can safely carry 100 amps of continuous or non-continuous current without exceeding the insulation's temperature rating or causing a hazardous voltage drop. Getting this right dictates the thermal safety of your entire installation, ensuring the wire dissipates heat faster than the resistive load generates it, while also guaranteeing the physical dimensions of the conductor actually fit inside the terminal lugs of your breaker or disconnect switch.

The Core Calculation: Sizing for 100 Amps

When selecting a wire gauge, you are balancing three variables: material conductivity, ambient temperature, and the temperature rating of your termination points. According to NEC Article 310.16, the allowable ampacity of a wire changes drastically depending on which temperature column you are legally permitted to use.

Most modern residential breakers and panel lugs are rated for 75°C. However, NEC 110.14(C) requires you to use the 60°C column if the equipment is not explicitly marked otherwise, or if you are using NM-B (Romex) cable, which is strictly limited to 60°C ampacities regardless of its 90°C internal insulation.

Baseline Wire Sizes for a 100A Breaker (75°C Terminations):
• 3 AWG Copper (Rated exactly 100A)
• 1 AWG Aluminum (Rated exactly 100A)

A Worked Numeric Example: The 100A Subpanel Feed

Imagine you are feeding a detached garage subpanel with a 100A main breaker. The calculated non-continuous load is 85 amps, and the one-way distance is 120 feet. You are pulling individual THHN/THWN-2 conductors through PVC conduit.

  1. Ampacity Check: Using the 75°C column for THHN in conduit, 3 AWG copper is rated for 100A. This perfectly matches your 100A breaker.
  2. Voltage Drop Calculation: We use the formula Vd = (2 × K × I × D) / CM. For copper, K is roughly 12.9. The load (I) is 85A. Distance (D) is 120 feet. The circular mils (CM) for 3 AWG is 52,620.
    Vd = (2 × 12.9 × 85 × 120) / 52,620 = 4.99 volts.
  3. Percentage Check: On a 240V feeder, 4.99V is a 2.08% drop. The NEC recommends keeping feeder drops under 3%, so 3 AWG copper is mathematically sound.

If you chose aluminum to save money, you would need 1 AWG aluminum (100A at 75°C). Because aluminum has higher resistance, your voltage drop would increase to roughly 3.3%, pushing you to the absolute limit. In this scenario, upsizing to 1/0 AWG aluminum would bring the drop back down to a safe 2.6%.

Where You Meet This in Practice

You will rarely size a 100A feeder for a single appliance. In modern electrical work, a 100A cable run typically shows up in three specific scenarios:

  • Detached Structure Subpanels: Feeding a garage, workshop, or ADU (Accessory Dwelling Unit) that requires multiple 20A branch circuits and a 240V receptacle for heavy tools.
  • Hardwired EV Level 2 Chargers: Many modern EVSE (Electric Vehicle Supply Equipment) units draw 80 amps continuously. NEC 210.19 requires continuous loads to be derated by 125%. An 80A continuous load requires wire and an overcurrent device rated for 100A (80 × 1.25 = 100).
  • Large HVAC or Heat Pump Systems: Commercial-grade or high-tonnage residential heat pumps with integrated electric resistance strip heaters often demand a 100A dedicated disconnect.

In all these cases, the physical stiffness of the cable becomes a practical factor. Bending 3 AWG copper or 1 AWG aluminum into tight panel gutters requires proper wire bending tools and adherence to NEC 300.34 bend radius limits to avoid damaging the insulation or stressing the breaker lugs.

Common Confusions and Code Traps

What people commonly confuse with wire sizing is the assumption that the breaker size and wire ampacity are interchangeable concepts. They are not. The breaker protects the wire; the wire carries the load. Here are the most frequent traps DIYers and junior electricians fall into:

The NM-B (Romex) 60°C Trap

This is the most common code violation in residential 100A feeds. Many builders pull 3 AWG NM-B cable and terminate it on a 100A breaker. This is a violation. NEC 334.80 mandates that NM-B ampacity must be derived from the 60°C column. In the 60°C column, 3 AWG copper is only rated for 85 amps. Under NEC 240.4(B), the next standard breaker size up from 85A is 90A. Therefore, 3 AWG NM-B can only be protected by a 90A breaker. To legally use a 100A breaker with NM-B, you would need 1 AWG copper (rated 110A at 60°C), which is virtually impossible to source in NM-B format. The fix: Use THHN in conduit or 75°C-rated SER (Service Entrance) cable.

Continuous vs. Non-Continuous Loads

If your 100A service is for a continuous load (defined by the NEC as operating for 3 hours or more), the wire cannot be sized at exactly 100A. You must multiply the load by 1.25. A 100A continuous load requires conductors rated for 125A, pushing you up to 1 AWG copper or 1/0 AWG aluminum, and necessitating a 125A breaker.

Ignoring the Grounding Conductor

People often size the hot and neutral wires correctly but undersize the equipment grounding conductor (EGC). For a 100A circuit, NEC Table 250.122 strictly requires a minimum 8 AWG copper or 6 AWG aluminum ground wire. Undersizing the ground prevents the breaker from tripping instantaneously during a dead-short fault.

Frequently Asked Questions

What size wire do I need for a 100 amp subpanel 100 feet away?

For a 100-foot run, 3 AWG copper or 1 AWG aluminum (using the 75°C column) will handle the 100A ampacity requirement. At 100 feet, the voltage drop for 3 AWG copper at full load is roughly 1.7%, which is well within the NEC's recommended 3% maximum for feeders. If your actual continuous load will frequently exceed 80 amps, consider upsizing to 2 AWG copper to keep voltage drop and thermal buildup to an absolute minimum.

Can I use aluminum wire for a 100 amp service?

Yes, aluminum is perfectly legal and highly cost-effective for 100A feeder wires, provided you use 1 AWG aluminum (rated 100A at 75°C). However, you must use terminations rated for aluminum (marked ALR or CU/AL) and apply an antioxidant compound like Noalox to the stripped conductor before torquing the lug to the manufacturer's specified inch-pound rating to prevent galvanic corrosion and high-resistance heating over time.

What size ground wire is required for a 100 amp feeder?

According to NEC Table 250.122, the minimum equipment grounding conductor for a 100A overcurrent device is 8 AWG copper or 6 AWG aluminum. If you upsized your hot conductors to mitigate voltage drop (for example, using 1 AWG copper instead of 3 AWG), NEC 250.122(B) requires you to proportionally increase the size of your ground wire as well.

Will 2 AWG wire handle 100 amps safely?

Yes, 2 AWG copper is rated for 115 amps in the 75°C column and 95 amps in the 60°C column. If your terminations are rated for 75°C, 2 AWG copper will handle 100 amps safely and provides an excellent buffer for voltage drop on longer runs. If you are forced to use the 60°C column (such as when using NM-B cable), 2 AWG is rated for 95A, meaning you must use the next-size-up rule to protect it with a 100A breaker, which is perfectly code-compliant under NEC 240.4(B).