THHN (Thermoplastic High Heat-resistant Nylon-coated) is a single-conductor wire insulation rated for 90°C in dry locations, primarily pulled through conduit for branch circuits and feeders. If you are asking what size THHN for 100 amps, the direct answer is 3 AWG copper or 1 AWG aluminum. This sizing is dictated not by the wire's maximum 90°C heat tolerance, but by the 75°C temperature limit of standard breaker and panel lugs under NEC 110.14(C).
Choosing the correct gauge changes the physical conduit fill requirements, the bending radius at junction boxes, the voltage drop over long runs, and ultimately whether your breaker terminals will overheat under continuous load. Below, we break down the exact ampacity tables, run a real-world voltage drop calculation, and clear up the most common code violations DIYers make when pulling 100-amp feeders.
The Direct Answer: Sizing THHN for 100 Amps
Wire sizing for a 100-amp circuit hinges on understanding the difference between the wire's insulation rating and the termination's rating. Modern THHN wire is actually dual-rated as THWN-2, meaning it can handle 90°C in dry locations and 75°C in wet locations. However, the lugs on a standard 100-amp breaker and the bus bars in a residential load center are almost universally rated for a maximum of 75°C.
For most residential and light commercial installs, you will pull individual THHN conductors through PVC or EMT conduit. You will need three current-carrying conductors (two hots, one neutral) plus an equipment grounding conductor. Minimum Ground: 8 AWG Copper or 6 AWG Aluminum is required for a 100-amp feeder per NEC 250.122.
NEC Ampacity Table: Copper vs. Aluminum THHN
The table below extracts the critical ampacity values from NEC Table 310.16. Notice how the allowable ampacity shifts dramatically depending on which temperature column your equipment supports. For 99% of 100-amp subpanel and breaker installations, the 75°C column is your legal limit.
| Wire Size (AWG) | Copper (60°C Column) | Copper (75°C Column) | Copper (90°C Column) | Aluminum (75°C Column) | Aluminum (90°C Column) |
|---|---|---|---|---|---|
| 4 AWG | 70A | 85A | 95A | 65A | 75A |
| 3 AWG | 85A | 100A | 115A | 75A | 85A |
| 2 AWG | 95A | 115A | 130A | 90A | 100A |
| 1 AWG | 110A | 130A | 145A | 100A | 115A |
| 1/0 AWG | 125A | 150A | 170A | 120A | 135A |
Note: Always verify the specific temperature rating printed on your breaker and panel labels. If a panel explicitly states '60°C Only' (common in very old installations), you would be forced to use 1 AWG copper to achieve 100 amps, though this is exceptionally rare in modern hardware.
Worked Example: 100-Amp Subpanel Feeder with Voltage Drop
Ampacity tells you the wire won't melt, but it doesn't guarantee your equipment will run correctly. The NEC recommends a maximum voltage drop of 3% for branch circuits and 5% for the combined feeder and branch circuit. Let's look at a real-world scenario where 3 AWG copper is legally allowed, but practically insufficient.
The Scenario: You are running a 240V, 100-amp subpanel to a detached workshop. The total one-way wire distance is 150 feet. You plan to pull 3 AWG copper THHN.
The Math:
We use the single-phase voltage drop formula: VD = (2 × K × I × D) / CM
- K (Copper resistivity) = 12.9 ohms-cmil/ft
- I (Current) = 100 Amps (worst-case full load)
- D (Distance) = 150 feet
- CM (Circular Mils for 3 AWG) = 52,620
VD = (2 × 12.9 × 100 × 150) / 52,620
VD = 387,000 / 52,620 = 7.35 Volts
The Result: 7.35V dropped across a 240V system is a 3.06% voltage drop. This slightly exceeds the NEC 3% recommendation for feeders. Furthermore, if your workshop load is continuous (running 3 hours or more), you must derate the breaker to 80% (80A), which lowers the drop, but starting heavy inductive loads like welders or large air compressors will still cause noticeable dimming and motor strain.
Where You Meet This in Practice (and Common Confusions)
You will typically pull 100-amp THHN feeders for detached garage subpanels, hardwired Level 2 EV charging stations (like the ChargePoint Home Flex or Tesla Wall Connector configured for 80A continuous draw), and large workshop machinery like 3-phase rotary converters or heavy welders.
When executing these installs, bench and jobsite experience reveals three major points of confusion that lead to failed inspections or unsafe conditions:
Confusion 1: THHN vs. NM-B (Romex) Ampacity
Many DIYers assume that because 3 AWG THHN is good for 100 amps, 3 AWG NM-B (Romex) is as well. This is false. NM-B cable is strictly limited to the 60°C column per NEC 334.80, regardless of the fact that the individual wires inside the sheath might have 90°C insulation. In the 60°C column, 3 AWG copper is only rated for 85 amps. To run 100 amps using NM-B, you would need 1 AWG copper, which is virtually impossible to find in standard retail NM-B formats and incredibly difficult to bend. Always use THHN in conduit for 100-amp feeders.
Confusion 2: Conduit Fill and Pulling Tension
THHN requires a raceway. For three 3 AWG conductors and one 8 AWG ground, the cross-sectional area dictates a minimum of 1-inch Schedule 40 PVC or 1-inch EMT. However, pulling four wires through 150 feet of 1-inch conduit with multiple sweeps is a recipe for damaged nylon coatings and stuck pulls. Upsizing to 1.25-inch conduit costs pennies more per foot but saves hours of frustration and prevents insulation scoring, which can lead to ground faults inside the pipe.
Confusion 3: Aluminum Terminations and Torque
If you choose 1 AWG aluminum THHN to save on material costs (aluminum is roughly 40-50% cheaper than copper by weight and length), you must treat the terminations differently. Aluminum oxidizes rapidly and expands/contracts at a different rate than copper or brass lugs.
- Apply an anti-oxidant compound (like Noalox) to the stripped aluminum conductor before insertion.
- Use a calibrated torque screwdriver. A standard 100-amp Square D or Siemens breaker lug typically requires 40 to 50 in-lbs of torque. Under-torquing causes arcing and heat; over-torquing strips the lug threads or crushes the softer aluminum strands, increasing resistance.
For exact torque values, always reference the manufacturer's spec sheet printed on the breaker label or available via major wire and breaker manufacturers. Never guess torque on a 100-amp termination; the thermal expansion at full load will loosen an undertorqued lug within months, creating a severe fire hazard.
