THHN vs XHHW — What's the Difference?
THHN uses PVC + nylon insulation; XHHW uses cross-linked polyethylene. Same NEC Table-5 fill area, different chemistry, slightly different applications.
THHN and XHHW are two of the most common 90°C insulated building wires in the US. They share many properties but differ in insulation chemistry — affecting cost, outdoor durability, and high-temp ageing.
Side-by-side
| Property | THHN/THWN-2 | XHHW/XHHW-2 |
|---|---|---|
| Insulation | PVC | Cross-linked polyethylene (XLPE) |
| Outer jacket | Nylon | None (XLPE alone) |
| Dry temp | 90°C | 90°C |
| Wet temp (THWN-2 / XHHW-2) | 90°C | 90°C |
| Voltage | 600V | 600V |
| NEC Table 5 area | Same | Same |
| Cost | Lower | Higher (10–20%) |
| UV resistance | Moderate (nylon helps) | Good (XLPE excels) |
| Long-term high-temp ageing | Acceptable | Better |
When to choose XHHW over THHN
Use XHHW-2 instead of THHN/THWN-2 if:
- Solar PV (PV system DC wiring): UV exposure in module wiring, long outdoor service life. XHHW-2 (and USE-2) dominate this application.
- High ambient temperature (continuous 50°C+): XLPE ages better than PVC at elevated temperatures.
- Chemical / oil environments: XLPE has better resistance to some industrial chemicals than PVC.
- Outdoor circuits in conduit exposed to direct sun: XHHW-2 lasts longer.
Stick with THHN/THWN-2 if:
- General interior building wire
- Cost-sensitive residential / commercial work
- Wet or dry conditions in conventional raceways
- Easy pulling — nylon jacket reduces friction
Same fill, same chart
Since NEC Table 5 gives identical area values:
| Gauge | THHN | XHHW | THWN-2 | XHHW-2 |
|---|---|---|---|---|
| 14 AWG | 0.0097 | 0.0097 | 0.0097 | 0.0097 |
| 12 AWG | 0.0133 | 0.0133 | 0.0133 | 0.0133 |
| 10 AWG | 0.0211 | 0.0211 | 0.0211 | 0.0211 |
| 8 AWG | 0.0366 | 0.0366 | 0.0366 | 0.0366 |
| 4/0 | 0.3237 | 0.3237 | 0.3237 | 0.3237 |
The THHN wire fill chart and the XHHW wire fill chart show identical max conductor counts. Toggling the insulation dropdown in the conduit fill calculator only matters when you switch to RHH, RHW, or TW — which use different (larger) values.
Pulling notes
THHN's nylon jacket gives it a noticeably slipperier surface than XHHW's bare XLPE. For tight pulls (high fill, multiple bends), THHN pulls easier and with less lubricant. XHHW can require more lube and lower pull tension limits.
Quick reference
- General building wire (cheapest, easiest pull): THHN/THWN-2
- Solar PV array wiring (UV / outdoor): XHHW-2
- High-temp industrial: XHHW-2 for ageing
- Same conduit fill math for both — only price and durability differ