The correct 100 amp wire size THHN is 3 AWG for copper and 1/0 AWG for aluminum, dictated by the 75°C ampacity column of NEC Table 310.16 due to standard termination temperature limits. Getting this gauge right changes the physical reality of your installation: it prevents thermal degradation of breaker lugs, stops heat-induced nuisance tripping, and ensures your feeder passes municipal inspection. The most common mistake DIYers and junior apprentices make is confusing the wire’s 90°C insulation rating with the 75°C rating of the equipment lugs, or mistakenly applying NM-B (Romex) 60°C sizing rules to a THHN conduit run.
The 75°C Termination Rule: Why Insulation Rating Isn't the Final Word
THHN (Thermoplastic High Heat-resistant Nylon-coated) wire is manufactured and tested to withstand temperatures up to 90°C in dry locations. If you look at the 90°C column of NEC Table 310.16, you might think you can push more current through a smaller wire. However, NEC Article 110.14(C) explicitly states that you must size your conductors based on the temperature rating of the terminations, not just the wire insulation.
Virtually all residential and commercial 100-amp breakers, panelboard lugs, and disconnect switches are rated for 75°C. Therefore, even though your THHN wire can handle 90°C, the breaker lug cannot. You are legally and physically bound to the 75°C ampacity column for your final sizing.
Worked Example: Sizing a 100A Subpanel Feeder
Let’s walk through a real-world scenario to see how these numbers interact with voltage drop and conduit derating.
The Scenario: You are running a 100A feeder from a 200A main service panel to a detached garage subpanel. The run is 60 feet long, routed through buried Schedule 40 PVC conduit. You are using copper THHN conductors.
- Base Ampacity Check: Looking at the 75°C column for copper, 3 AWG is rated for exactly 100A. This matches our 100A breaker perfectly.
- Voltage Drop Calculation: While the NEC recommends keeping voltage drop under 3% for feeders, it is not strictly mandatory unless specified by local amendments. Let’s calculate the worst-case drop at a full 80A continuous load (a realistic max for a 100A panel).
- Formula: VD = (2 × K × I × D) / CM
- K (Copper) = 12.9
- I (Current) = 80A
- D (Distance) = 60 ft
- CM (Circular Mils for 3 AWG) = 52,620
- VD = (2 × 12.9 × 80 × 60) / 52,620 = 2.35 Volts
- Percentage Drop: 2.35V / 240V = 0.98%. This is well under the 3% recommendation.
Where You Meet This in Practice
You will typically pull 3 AWG copper or 1/0 AWG aluminum THHN in the following specific installations:
- Detached Structure Subpanels: Feeding a 100A main breaker in a garage, workshop, or barn panel.
- High-Capacity EV Chargers: Hardwired Level 2 commercial or high-end residential EV chargers that draw 80A continuous require a 100A breaker and this exact wire size.
- Large Spa and Hot Tub Disconnects: 4-to-6 person spas with multiple heaters and pumps frequently require a 100A GFCI breaker at the outdoor disconnect.
- Commercial HVAC Units: Rooftop package units or large commercial mini-split condensers often specify a 100A Maximum Overcurrent Protective Device (MOPD).
THHN vs. NM-B and XHHW-2 for 100A Circuits
Choosing the right insulation type dictates not just the wire size, but the physical labor required to pull it. NM-B (Romex) at 100A is notoriously difficult to work with due to its stiffness.
| Wire Type | Copper Size for 100A | Temp Column Used | Best Application | Physical Handling |
|---|---|---|---|---|
| THHN / THWN-2 | 3 AWG | 75°C (Termination) | Conduit runs, subpanel feeders | Slippery nylon jacket; pulls easily through PVC/EMT |
| NM-B (Romex) | 1 AWG | 60°C (NEC 334.80) | Indoor wall cavities (rare for 100A) | Extremely stiff; very difficult to bend into panel lugs |
| XHHW-2 | 3 AWG | 75°C (Termination) | Wet locations, underground conduit | Thinner insulation profile than THHN; excellent for tight conduit bends |
Frequently Asked Questions
Can I use 4 AWG THHN wire for a 100 amp breaker?
No. In the 75°C column, 4 AWG copper is only rated for 85 amps. Even if you look at the 90°C column, 4 AWG copper is only rated for 95 amps, which still falls short of the 100A requirement. You must use a minimum of 3 AWG copper to safely and legally terminate on a 100A breaker.
What size aluminum THHN wire do I need for 100 amps?
You need 1/0 AWG (pronounced "one-aught") aluminum wire. In the 75°C column, 1/0 AWG aluminum is rated for exactly 100 amps. Aluminum is significantly cheaper than copper and highly favored for long feeder runs to detached structures, but you must apply an anti-oxidant compound (like Noalox) to the stripped ends before torquing them into the lugs to prevent galvanic corrosion and high-resistance heating over time.
Does the ground wire need to be the same size as the 100 amp THHN conductors?
No. According to NEC Table 250.122, the minimum equipment grounding conductor (EGC) for a 100A overcurrent device is 8 AWG copper or 6 AWG aluminum. You do not need to pull a massive 3 AWG ground wire. However, if you upsized your current-carrying conductors to compensate for voltage drop on a very long run, you are required to upsize the ground wire proportionally.
How many 3 AWG THHN wires can fit in a 1-inch PVC conduit?
For a standard 100A single-phase feeder, you need two 3 AWG hots, one 3 AWG (or smaller, per local code) neutral, and one 8 AWG ground. This equals four wires. According to NEC Chapter 9, Table 5, the cross-sectional area of four such wires easily fits within the 40% fill capacity of a 1-inch Schedule 40 PVC conduit. However, from a practical jobsite perspective, pulling three 3 AWG wires through 1-inch PVC with multiple sweeps is physically grueling. Most seasoned electricians will step up to 1.25-inch or 1.5-inch PVC to save time, prevent insulation scoring, and avoid rope burn.
