A 100 amp THHN wire size refers to the specific American Wire Gauge (AWG) cross-section of Thermoplastic High Heat-resistant Nylon-coated conductor required to safely carry 100 amperes of current without exceeding the thermal limits of the insulation or the terminating equipment. For a standard 100A circuit, the baseline answer is 3 AWG copper or 1 AWG aluminum when terminating at 75°C-rated equipment. However, if your breakers or lugs are only rated for 60°C, you must upgrade to 1 AWG copper or 1/0 AWG aluminum.

Choosing the right gauge isn't just about matching the breaker; it requires navigating the National Electrical Code (NEC) rules on termination temperatures, conduit fill, and ambient heat. Below is the definitive guide to sizing, derating, and installing 100A THHN conductors.

The THHN Ampacity Matrix for 100A Circuits

Before pulling any wire, you need to understand how the NEC National Electrical Code (NFPA 70) categorizes ampacity. THHN wire is manufactured with a 90°C insulation rating, but you rarely get to use that full capacity. The table below maps the exact ampacities for common wire sizes surrounding the 100A threshold across the three critical temperature columns.

Wire Size (AWG/kcmil) Material 60°C Column (Amps) 75°C Column (Amps) 90°C Column (Amps) Use for 100A Load?
4 AWG Copper 70A 85A 95A No (Too small)
3 AWG Copper 85A 100A 115A Yes (If 75°C terminations)
2 AWG Copper 95A 115A 130A Yes (Provides derating buffer)
1 AWG Copper 110A 130A 145A Yes (Required for 60°C terminations)
1 AWG Aluminum 75A 100A 115A Yes (If 75°C terminations)
1/0 AWG Aluminum 100A 120A 135A Yes (Required for 60°C terminations)
How to read this table: Always size your wire based on the lowest temperature rating in the entire circuit. If your THHN wire is 90°C, but the breaker lug is rated for 75°C, you must use the 75°C column to determine your baseline ampacity. The 90°C column is strictly reserved for derating calculations.

What This Changes in a Real Installation

Understanding the 100 amp THHN wire size changes three critical factors in a real circuit: termination compliance, voltage drop management, and conduit derating. If you simply grab 3 AWG copper because it says "100A" on the 75°C chart, you might still fail inspection or create a fire hazard if the installation environment demands adjustments.

The most common installer trap is conduit derating. When you bundle multiple current-carrying conductors inside a single raceway, they trap heat. Think of it like cars idling in a multi-lane tunnel; the more cars (wires) you pack in, the hotter the ambient environment gets, and the less heat each individual engine (conductor) can shed.

Worked Numeric Example: The Derating Trap

Imagine you are running a 100A subpanel feeder through 1.5-inch EMT conduit. You need to pull two hot wires, one neutral, and one ground. Because the neutral carries unbalanced current, it counts as a current-carrying conductor (CCC). You have 3 CCCs. No derating is required yet.

Now, imagine you are running two 100A circuits in the same conduit (4 hots, 2 neutrals = 6 CCCs). According to NEC Table 310.15(C)(1), 4 to 6 CCCs require an 80% derating factor. Here is how the math dictates your wire size:

  • Attempt 1 (3 AWG Copper THHN): The 90°C column rating is 115A. Apply the 80% derating factor: 115A × 0.80 = 92A. Because 92A is less than your 100A load, 3 AWG fails and will overheat.
  • Attempt 2 (2 AWG Copper THHN): The 90°C column rating is 130A. Apply the 80% factor: 130A × 0.80 = 104A. Because 104A is greater than 100A, and 104A also exceeds the 75°C termination limit (115A), 2 AWG is the correct, code-compliant size for this specific conduit run.
Rule of Thumb: For every 3 additional current-carrying conductors added to a conduit past the baseline of 3, expect to bump your wire size up by at least one AWG step to compensate for thermal derating.

Where You Meet This in Practice

You will typically encounter the requirement for a 100 amp THHN wire size in high-load residential and light commercial applications. Here are the most common scenarios where getting this exact sizing right is non-negotiable:

  • Residential Subpanels: Feeding a 100A detached garage or workshop subpanel. While many DIYers attempt to use NM-B (Romex) cable, THHN in conduit is vastly superior here because it allows for future upgrades, offers physical protection, and runs cooler.
  • Hardwired EV Chargers: Level 2 commercial or high-end residential EV chargers often pull 80A continuous. Under NEC Article 210.20(A), continuous loads must be multiplied by 125%. An 80A continuous load requires wire and breaker sizing for 100A. Therefore, you are back to the 3 AWG copper (75°C) baseline.
  • Tankless Electric Water Heaters: Whole-home electric tankless units frequently require dual 40A or 50A breakers, but larger commercial units can demand a dedicated 100A feed.
  • Hot Tubs and Spas: Large spa heaters and multi-pump setups often utilize a 100A GFCI-protected disconnect. Note: If running THHN underground in wet locations, you must use THWN-2 or XHHW-2 rated wire, as standard THHN is only rated for dry locations.

Common Confusions and Code Traps

When discussing wire sizing on forums or with junior apprentices, a few persistent myths always surface. Clearing these up prevents failed inspections and melted lugs.

The 90°C Termination Myth

The most common confusion is assuming that because THHN wire is printed with "90°C" on the jacket, you can use the 90°C ampacity column to size your breaker. EC&M and NEC experts constantly reiterate NEC 110.14(C): you must size the conductor based on the temperature rating of the terminations. Most standard 100A breakers and panel lugs are rated for 75°C. Therefore, even if your wire can handle 90°C, the brass lug it screws into cannot. You must use the 75°C column for your baseline sizing.

Continuous vs. Non-Continuous Loads

People frequently confuse the breaker size with the load size. If you have a 100A breaker protecting a circuit that will run at maximum capacity for 3 hours or more (a continuous load, like commercial lighting or EV charging), the NEC requires the wire to be sized at 125% of the load. If the actual continuous load is 80A, 80 × 1.25 = 100A. You need wire rated for 100A. But if the load itself is a full 100A continuous, you must size the wire for 125A, pushing you into 1 AWG copper territory.

Copper vs. Aluminum Torque Specifications

If you opt for 1 AWG aluminum to save money on a 100A feeder, you must account for oxidation and creep. Aluminum requires specific anti-oxidant compound (like Noalox) and exact torque settings. A 100A aluminum lug typically requires between 40 and 50 inch-pounds of torque, but you must verify the specific manufacturer's spec sheet printed on the breaker label. Under-torquing aluminum guarantees a high-resistance connection that will eventually melt the lug.

Frequently Asked Questions

Can I use 4 AWG THHN copper for a 100A breaker?
No. 4 AWG copper is only rated for 85A in the 75°C column. While NEC 240.4(B) allows you to round up to the next standard breaker size if the wire ampacity doesn't match a standard breaker (e.g., protecting a 95A wire with a 100A breaker), 4 AWG falls too far short of a 100A load requirement. You must use at least 3 AWG.

Is THHN or XHHW-2 better for a 100A feeder?
XHHW-2 is generally superior for feeders. While both share the 90°C rating, XHHW-2 has a cross-linked polyethylene (XLPE) insulation that is more resistant to moisture, chemicals, and physical abrasion. Furthermore, XHHW-2 is rated for wet locations at 90°C, whereas THHN drops to 75°C (as THWN) in wet conditions.

What size ground wire do I need for a 100A THHN circuit?
According to NEC Table 250.122, a 100A overcurrent protective device requires a minimum 8 AWG copper or 6 AWG aluminum equipment grounding conductor. If you upsize your current-carrying conductors for voltage drop, you must proportionally upsize the ground wire as well.