The correct THHN wire size for 100 amps is #3 AWG copper or #1 AWG aluminum when terminating on standard 75°C rated lugs, which dictates the maximum safe current-carrying capacity (ampacity) before the insulation degrades or the breaker trips prematurely.
The Core Sizing Rules and Temperature Columns
THHN (Thermoplastic High Heat-resistant Nylon-coated) is the standard individual conductor used in conduit for residential and commercial branch circuits and feeders. However, the nylon jacket and the 90°C rating printed on the wire do not solely determine your wire size. The National Electrical Code (NEC) mandates that wire sizing must be based on the lowest temperature rating of any connected component, which is almost always the termination lug on your breaker or panel.
What this changes in a real circuit is the thermal bottleneck at the termination point. If you size a wire using the 90°C column but land it on a 75°C lug, the lug becomes the weakest link. The heat generated by the current will dissipate into the lug, oxidizing the metal, increasing resistance, and eventually causing a thermal failure or nuisance tripping. According to NFPA 70 (NEC) Article 110.14(C), you must use the 75°C column of Table 310.16 for terminations rated 100A and below, unless the equipment is explicitly marked otherwise.
| Wire Size (AWG/kcmil) | Copper (75°C Column) | Copper (90°C Column) | Aluminum (75°C Column) | Aluminum (90°C Column) |
|---|---|---|---|---|
| #4 AWG | 85 Amps | 95 Amps | 65 Amps | 75 Amps |
| #3 AWG | 100 Amps | 115 Amps | 75 Amps | 85 Amps |
| #2 AWG | 115 Amps | 130 Amps | 90 Amps | 100 Amps |
| #1 AWG | 130 Amps | 145 Amps | 100 Amps | 115 Amps |
Worked Numeric Example: Sizing a 100A Subpanel Feeder
Let's walk through a real-world installation: running a 120/240V single-phase feeder to a 100-amp subpanel in a detached garage, located 60 feet from the main panel. The load includes a 60A continuous EV charger and standard lighting.
Step 1: Conductor Sizing
Because the EV charger is a continuous load (operating for 3+ hours), NEC Article 210.20 requires the branch circuit rating to be 125% of the continuous load. However, for the 100A feeder itself, we size based on the 100A breaker. Using the 75°C column, we select #3 AWG Copper THHN for the two hot legs and the neutral.
Step 2: Voltage Drop Calculation
While the NEC doesn't strictly mandate a specific voltage drop for feeders (it recommends 3% max for feeders, 5% total), good practice dictates checking it. Using the formula VD = (2 × K × I × D) / CM:
- K (Copper constant) = 12.9
- I (Current) = 80A (assuming 80A actual continuous draw on the 100A breaker)
- D (Distance) = 60 feet
- CM (Circular Mils for #3 AWG) = 53,490
VD = (2 × 12.9 × 80 × 60) / 53,490 = 2.31 Volts.
On a 240V circuit, a 2.31V drop is roughly 0.96%, which is well under the 3% recommendation. #3 AWG is electrically sound.
Step 3: Conduit Fill
You are pulling four conductors (2 hots, 1 neutral, 1 ground) through Schedule 40 PVC. According to NEC Chapter 9, Table 1, conduit fill for over two wires is limited to 40%. Three #3 AWG THHN wires plus one #8 AWG ground easily fit inside a 1-inch PVC conduit (which has a 40% fill capacity of roughly 0.33 square inches, while the wires take up about 0.15 square inches).
Where You Meet This in Practice
You will typically encounter the requirement for 100A THHN wire sizing in three specific residential and light-commercial scenarios:
- Subpanel Feeders: Supplying a 100A detached garage, workshop, or basement subpanel. This is the most common use case, requiring individual THHN/THWN-2 conductors pulled through PVC or EMT conduit rather than NM-B (Romex), as NM-B is limited to the 60°C column and would require #1 AWG copper for 100A.
- Hardwired EV Chargers: High-amperage Level 2 chargers. While a standard 48A charger requires a 60A breaker and #6 AWG wire, larger 80A continuous chargers require a 100A breaker and consequently #3 AWG THHN feeders.
- Large Swim Spas and Hot Tubs: Standard hot tubs usually run on 50A or 60A GFCI-protected circuits. However, large multi-pump swim spas with inline heaters frequently require a dedicated 100A GFCI breaker and appropriately sized THHN conductors routed through non-metallic conduit.
Common Confusions: THHN vs. THWN-2 and Ampacity Derating
What people commonly confuse with basic THHN sizing is the distinction between dry and wet location ratings, and how ampacity derating interacts with the 90°C column.
The second major confusion is ampacity derating. When you run four or more current-carrying conductors in a single raceway (like a multi-wire branch circuit or a three-phase feeder), the NEC requires you to reduce the wire's ampacity because the wires heat each other up. This is where the 90°C column actually becomes useful. You are allowed to use the 90°C column only for calculating the derating penalty, provided the final derated ampacity does not exceed the 75°C column limit for the termination.
For example, if you have four current-carrying #3 AWG copper conductors in a conduit, you apply an 80% adjustment factor. Using the 90°C column (115A × 0.80 = 92A), the derated capacity drops below 100A. In this specific scenario, you would be forced to upsize to #2 AWG copper to maintain a 100A rating after derating.
Frequently Asked Questions
Can I use #4 AWG copper THHN for a 100 amp breaker?
No. In the 75°C column, #4 AWG copper is only rated for 85 amps. If you land #4 AWG on a 100-amp breaker, you are violating NEC 110.14(C) and creating a severe fire hazard at the termination lug. You must use a minimum of #3 AWG copper or #1 AWG aluminum for a 100-amp circuit.
Does the 90°C column allow me to downsize my 100 amp wire?
No, not for standard terminations. Even though #4 AWG copper shows 95 amps in the 90°C column (which is still under 100A), and #3 AWG shows 115 amps, standard breakers and panel lugs are rated for 75°C. The heat will transfer from the wire into the lug, potentially melting the breaker's internal thermal-magnetic calibration mechanism. You must always cap your final ampacity at the 75°C column value.
What size ground wire do I need for a 100 amp THHN feeder?
According to NEC Table 250.122, the minimum equipment grounding conductor (EGC) for a 100-amp overcurrent device is #8 AWG copper or #6 AWG aluminum. This ground wire must be pulled in the same conduit as your current-carrying conductors to ensure the magnetic fields cancel out and to provide a low-impedance fault path.
How many #3 AWG THHN wires fit in a 1-inch PVC conduit?
Based on NEC Chapter 9, Table 1 (40% fill rule for over two wires) and Table 4 (Schedule 40 PVC dimensions), a 1-inch PVC conduit has a usable fill area of 0.333 square inches. A single #3 AWG THHN wire has an approximate area of 0.045 square inches. You can comfortably fit up to seven #3 AWG THHN conductors in a 1-inch PVC conduit, though pulling more than four current-carrying conductors will trigger ampacity derating penalties.
