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THWN vs THHN comparison

Electrical and mechanical properties

Section titled “Electrical and mechanical properties”
Designation Conductor material Rated voltage Gauge (AWG/kcmil) Conductor diameter Cross-section Max. electrical resistance Cu at 20 °C Typical current rating Cu (60 °C)
THHN Soft copper 600 V 14 AWG to 1000 kcmil 1–29 mm / 0.04–1.14 in (depending on gauge) 2,08–507 mm² / 0.0032–0.786 in² Varies with gauge; e.g., 12 AWG: 5,2 Ω/km / 1,6 Ω/kft 15–500 A (depends on gauge)
THWN-2 (dual) Soft copper 600 V 14 AWG to 1000 kcmil 1–29 mm / 0.04–1.14 in 2,08–507 mm² / 0.0032–0.786 in² Same as THHN Same as THHN in dry, 75 °C in wet

Electrical resistance and ampacity vary with gauge; full values are shown in the following tables.

Resistance and ampacity per AWG gauge (copper conductor)

AWG Diameter (mm / in) Cross-section (mm²) Resistance Cu (Ω/km) / (Ω/kft) Max. ampacity 60 °C (A)
14 1,63 mm / 0.064 in 2,08 8,2 Ω/km / 2,5 Ω/kft 24
12 2,05 mm / 0.081 in 3,31 5,2 Ω/km / 1,6 Ω/kft 34
10 2,59 mm / 0.102 in 5,26 3,3 Ω/km / 1,0 Ω/kft 52
8 3,26 mm / 0.128 in 8,37 2,1 Ω/km / 0,64 Ω/kft 75
6 4,11 mm / 0.162 in 13,3 1,3 Ω/km / 0,40 Ω/kft 95
4 5,19 mm / 0.204 in 21,2 0,81 Ω/km / 0,25 Ω/kft 120
2 6,54 mm / 0.258 in 33,6 0,51 Ω/km / 0,16 Ω/kft 170
1/0 8,25 mm / 0.325 in 53,5 0,32 Ω/km / 0,098 Ω/kft 200

Resistance per kft is obtained as R(kft) = R(km) × 0,3048

The data correspond to bare copper conductors per AWG; ampacities are for a single conductor in free air with PVC insulation, ambient temperature 30 °C.

Thermal properties

Section titled “Thermal properties”
Characteristic THHN THWN-2 (dual)
Insulation material PVC with nylon jacket PVC with nylon jacket
Maximum temperature in dry 90 °C / 194 °F 90 °C / 194 °F
Maximum temperature in wet environment Not specified (not suitable for wet locations) 90 °C / 194 °F

Classic THHN cable is limited to 90 °C in dry and has no rating for wet environments. The THWN (or THWN-2) designation adds the ability to operate in the presence of moisture. The THWN-2 version withstands 90 °C in both dry and wet, so most modern commercial products are supplied as dual THHN/THWN-2.

Resistance to moisture and chemical agents

Section titled “Resistance to moisture and chemical agents”
  • THHN: Resistant to gasoline and oil, but not rated for installation in conduits that may accumulate water or in outdoor areas exposed to rain.
  • THWN/THWN-2: Resistant to gasoline, oil, moisture, and water. Suitable for use in outdoor conduit, direct burial (when specified), and wet locations per NEC.

The nylon coating over PVC provides additional mechanical and chemical protection in both variants.

Typical applications

Section titled “Typical applications”
Application THHN THWN/THWN-2
Building wiring in dry conduit Yes Yes
Interior feeders (dry locations) Yes Yes
Installation outdoors or in wet locations No Yes
Connections for pumps, irrigation systems No Yes (with adequate protection)
Underground service entrances No Yes (THWN-2)

Both types are used in control, lighting, and power circuits up to 600 V. The choice depends on the presence of moisture at the site.

Key differences

Section titled “Key differences”
Aspect THHN THWN/THWN-2
Meaning of the acronym Thermoplastic High Heat-resistant Nylon-coated Thermoplastic Heat and Water-resistant Nylon-coated
Maximum temperature in wet Not specified 75 °C (THWN) or 90 °C (THWN-2)
Water resistance No Yes
Outdoor use Not permitted Permitted
UL reference standard UL 83 UL 83 / UL 1063 (THWN-2)
Duality with THHN – Generally manufactured as THHN/THWN-2, combining both characteristics

In practice, cables marked as THHN/THWN-2 cover all common needs, so pure THHN is rare in new installations.

Frequently Asked Questions (FAQ)

Section titled “Frequently Asked Questions (FAQ)”

What is the maximum ampacity of a THWN cable 10 AWG in conduit?

Section titled “What is the maximum ampacity of a THWN cable 10 AWG in conduit?”

A THWN copper conductor 10 AWG can carry up to 52 A under ambient temperature conditions of 30 °C, although correction factors for grouping and temperature are applied in actual installations.

At what temperature can the insulation of a THWN-2 cable operate in a wet environment without degrading?

Section titled “At what temperature can the insulation of a THWN-2 cable operate in a wet environment without degrading?”

The insulation of THWN-2 is designed to withstand 90 °C continuously in a wet environment, equivalent to 194 °F.

What is the electrical resistance of 1000 feet of THHN copper conductor 12 AWG?

Section titled “What is the electrical resistance of 1000 feet of THHN copper conductor 12 AWG?”

A 12 AWG copper conductor has a resistance of approximately 1,6 Ω per 1000 feet (5,2 Ω/km), measured at 20 °C.

How much voltage can a THWN-2 cable withstand between phase and ground?

Section titled “How much voltage can a THWN-2 cable withstand between phase and ground?”

The maximum rated voltage is 600 V, for both THHN and THWN-2, according to UL 83 and NEC standards.

What gauge of THHN is needed to carry 200 A in a single-phase main feeder at 75 °C?

Section titled “What gauge of THHN is needed to carry 200 A in a single-phase main feeder at 75 °C?”

A 2/0 AWG copper conductor is required, which at 75 °C supports 225 A, although the exact ampacity depends on the insulation type and ambient temperature.

What is the approximate outer diameter of a THHN/THWN-2 cable 4 AWG with insulation?

Section titled “What is the approximate outer diameter of a THHN/THWN-2 cable 4 AWG with insulation?”

The total diameter is around 8–9 mm (0,31–0,35 in), adding the copper conductor (5,19 mm / 0.204 in) plus the thickness of PVC and the nylon jacket.

References

Section titled “References”