The correct 100 amps wire size is the minimum conductor cross-sectional area required to safely carry a 100-ampere load without exceeding the thermal limits of the wire's insulation and terminal lugs. For a standard residential or commercial 100-amp circuit using copper wire in a typical ambient temperature (30°C/86°F), you need 3 AWG copper or 1 AWG aluminum when terminating on 75°C rated equipment, according to NEC Table 310.16.

Mains Voltage Warning: Working inside a panel with 100A feeders involves lethal mains voltage. Always de-energize the upstream breaker, lock/tag it out, and verify the bus bars are dead with a tested CAT III or CAT IV multimeter before touching any conductors. Local code may require a licensed electrician for subpanel installations.

The Baseline: NEC Ampacity and the 75°C Column

Choosing a wire size changes three critical variables in a real installation: heat dissipation at the termination points, voltage drop over distance, and the physical bend radius inside the enclosure. While the wire's insulation might be rated for 90°C (like THHN/THWN-2), the weakest link in the circuit is almost always the terminal lug on the breaker or busbar.

Under NEC 110.14(C), you must size the wire based on the temperature rating of the termination, which is typically 75°C for modern residential and commercial breakers. This means even if you buy 90°C wire, you must use the 75°C column in NEC Table 310.16 to determine your baseline ampacity.

Worked Numeric Example: Voltage Drop on a 100A Feeder

Ampacity tells you the wire won't melt, but it doesn't guarantee your equipment will run properly. Let's calculate the voltage drop for a 100A load on a 240V circuit using 3 AWG copper over a 100-foot run.

  • Formula: VD = (2 × K × I × L) / CM
  • K (Copper constant): 12.9
  • I (Current): 100 Amps
  • L (One-way length): 100 feet
  • CM (Circular Mils for 3 AWG): 52,620

Calculation: (2 × 12.9 × 100 × 100) / 52,620 = 4.9 Volts dropped.

On a 240V circuit, a 4.9V drop is roughly 2.04%, which is well within the NEC's recommended 3% maximum for feeders. However, if this were a 120V circuit, that same 4.9V drop represents 4.08%, pushing into unacceptable territory and requiring an upsizing to 2 AWG copper.

Where You Meet 100 Amps in Practice

You won't typically find a 100-amp branch circuit powering a single standard appliance. In the field, a 100A wire size requirement almost always points to one of three specific installations:

  1. Detached Structure Subpanels: Feeding a 100A main-lug subpanel in a garage, barn, or workshop to support multiple 20A branch circuits, lighting, and receptacles.
  2. EV Charging Hubs: While a single Level 2 EV charger usually draws 32A to 48A, installing a 100A feeder to a garage subpanel allows for simultaneous charging of two EVs or future expansion without pulling a second trench.
  3. Large Workshop Equipment: Hardwired CNC plasma tables, large MIG/TIG welders, and commercial-grade air compressors often require dedicated 80A to 100A breakers, necessitating this wire tier.

Real-World Scenario Walkthrough: The Subpanel Feed That Melted

To understand why the 75°C rule matters, let's look at a documented field failure involving a DIY subpanel installation.

The Setup

A homeowner ran a 100A subpanel to a detached garage 120 feet away to power a welder. They purchased 4 AWG THHN copper wire because the jacket was stamped "90°C" and they wanted to save money compared to 3 AWG. They terminated the wires directly into a standard 100A double-pole breaker in the main panel and the main lugs of the subpanel.

The Numbers

Looking at the 90°C column of NEC Table 310.16, 4 AWG copper is rated for 95 Amps. The homeowner assumed this was close enough to 100A and protected it with a 100A breaker. However, the terminal lugs on both the main breaker and the subpanel were only rated for 75°C. In the 75°C column, 4 AWG copper is only rated for 85 Amps.

The Outcome

For the first six months, the system worked fine under light loads. But when the homeowner began using the welder and a space heater simultaneously, the draw sustained around 90 Amps. The wire insulation inside the conduit stayed intact, but the terminal lugs overheated. The heat caused the copper to oxidize, increasing resistance, which generated more heat. Eventually, the plastic housing around the main breaker lug melted, creating a high-resistance arc fault that tripped the upstream 200A main service breaker.

What Went Wrong

The homeowner violated NEC 110.14(C) by using the 90°C ampacity column for terminations. The breaker protected the wire from catching fire in the conduit, but it could not protect the 75°C-rated lugs from thermal degradation under an 85A+ load. The correct fix was using 3 AWG copper (100A at 75°C) or upsizing to 2 AWG to account for the 120-foot voltage drop.

Common Confusions: Breaker Size vs. Wire Ampacity

The most frequent mistake makers and DIYers make is conflating the breaker's trip curve with the wire's continuous capacity. Here is what people commonly confuse when sizing for 100 amps:

The "Next Size Up" Rule (NEC 240.4(B)): If your calculated load is 95 Amps, and your wire ampacity is exactly 95 Amps, but there is no standard 95A breaker, the NEC allows you to use the next standard size up (a 100A breaker). However, this only applies if the load is non-continuous (runs for less than 3 hours).

If your 100A load is continuous (like an EV charger or a server room HVAC unit running for 3 hours or more), NEC 210.20(A) requires the breaker and the wire to be sized at 125% of the continuous load. Therefore, a continuous 100A load actually requires wire rated for 125 Amps (which pushes you to 1 AWG copper) and a 125A breaker. A standard 100A breaker and 3 AWG wire are only legally compliant for non-continuous 100A loads.

FAQ: Edge Cases and Derating

What if I have four current-carrying conductors in one conduit?

If you are pulling two hots, a neutral, and a ground for a 120/240V multi-wire branch circuit or subpanel feed, you have three current-carrying conductors (the ground does not count). If you add a second circuit to that same conduit, bringing the total to four current-carrying conductors, NEC Table 310.15(C)(1) requires an 80% derating factor. You must multiply your wire's 90°C ampacity by 0.80. If that derated number falls below 100A, you must upsize the wire.

Is aluminum wire safe for a 100A subpanel feed?

Yes, provided you use the correct size and anti-oxidant compound. For 100 Amps at 75°C, you need 1 AWG aluminum (like XHHW-2 or SER cable). Aluminum is significantly cheaper and lighter than copper, making it the industry standard for service entrance and feeder cables. Always apply a Noalox or similar antioxidant paste to the stripped aluminum strands before torquing the lugs to prevent galvanic corrosion.

Does the type of insulation (THHN vs. XHHW) change the wire size?

Not for the baseline 75°C termination rule, but it matters for conduit fill and wet locations. THHN is slightly thinner and easier to pull in tight conduits, but XHHW-2 has a thicker, more robust cross-linked polyethylene insulation that handles wet locations better and resists physical damage during pulls. Both will yield the same required AWG for a 100A termination.

Getting the 100 amps wire size right is about more than just matching a number on a breaker handle. By respecting the 75°C termination limits, calculating voltage drop for longer runs, and distinguishing between continuous and non-continuous loads, you ensure your installation remains safe, efficient, and fully compliant with industry standards for decades.