THHN, THWN-2 AND XHHW-2: WHAT THE JACKET ACTUALLY RATES

Two Materials: Thermoplastic and Thermoset

THHN, THWN and THWN-2 are thermoplastic. A PVC insulation is extruded over the conductor and a thin nylon jacket is extruded over the PVC. XHHW and XHHW-2 are thermoset: cross-linked polyethylene, with no nylon jacket at all. The distinction is not marketing language. Thermoplastic softens as it heats and can deform under fault current; thermoset chars and holds its shape.

In practice the thermoplastic family pulls more easily, because the nylon jacket is slick and abrasion resistant, which is why it dominates conduit work and why most building wire in a Kansas shop is THHN rather than XHHW. The thermoset family is chosen where the conductor will sit wet, where deformation under heat matters, or where a specification has named XHHW-2 and means it.

• THHN: 90°C dry and damp, 600 volts, nylon jacketed, not a wet-location conductor on its own.
• THWN: 75°C dry and wet, 600 volts, nylon jacketed.
• THWN-2: 90°C dry and wet, 600 volts, the version most stocked today.
• XHHW: 90°C dry, 75°C wet, 600 volts, cross-linked polyethylene.
• XHHW-2: 90°C dry and wet, 600 volts, also built in 1 kV and 2 kV constructions.
• TFFN: fixture wire, 18 and 16 AWG, 90°C, 600 volts, never a branch-circuit conductor.

Dry, Wet and the Meaning of the Dash Two

The dash two is the wet-location 90 degree rating. A conductor marked THWN-2 carries its 90 degree ampacity in a wet location; one marked THWN carries 75 degree ampacity there; one marked only THHN is not permitted in a wet location at all. Dual and triple markings are common, so the whole legend has to be read.

Most THHN sold today is printed THHN or THWN-2 on one line, which is how the same reel can be accepted on one job and refused on another by an inspector reading only the first three letters. The print governs. If the jacket is faded past reading, the wire is unidentified, and unidentified wire is priced lower.

90 Degrees With Conditions

A 90 degree rating is a ceiling, not a permit. Conductors may be sized from the 90 degree column only where the terminations, the equipment and the conductor are all rated 90 degrees. Breakers and lugs are usually 75 degree, so the finished circuit lands on the 75 degree column. The 90 degree ampacity remains useful as the starting point for ambient correction and conduit-fill derating.

This is where most counter arguments begin. A 1/0 copper THWN-2 carries 170 amperes in the 90 degree column and 150 in the 75 degree column, and behind a 75 degree breaker the second figure is the one that counts. Small conductors are capped again at 15, 20 and 30 amperes for 14, 12 and 10 AWG.

Oil, Gasoline, Sunlight and Where Each Type Belongs

The nylon jacket is what gives THHN and THWN-2 their gasoline and oil resistance, shown on the legend as an oil-resistant or gasoline and oil resistant marking. XHHW-2 relies on the cross-linked polyethylene instead, and whether a given reel carries an oil-resistant marking depends on the manufacturer. Read the print rather than assuming it.

Sunlight resistance is a separate marking and a separate question. A conductor installed exposed outdoors needs a sunlight-resistant legend, which most modern THWN-2 and XHHW-2 reels carry and older stock often does not. Direct burial needs a direct-burial rating, which single-conductor building wire generally does not have.

• THHN and THWN-2 for conduit indoors, and for wet conduit where the dash two is printed.
• XHHW-2 for feeders, wet locations, exterior runs and anything a specification names.
• MTW for machine tools, control panels and cabinet interiors, at 90°C dry and 60°C wet.
• TFFN inside luminaires and fixture whips, where Article 402 applies rather than Article 310.
• Green THHN or XHHW as an equipment grounding conductor, sized from the overcurrent device.

What to Photograph, and What We Do Not Buy

A photograph of the legend is worth more than a photograph of the box. The box says 12 AWG THHN. The legend says THHN or THWN-2, 600 volts, 90 degrees, sunlight resistant, and that is the sentence a quote is built from.

We buy wire and cable and nothing that switches it. Transformers, switchgear, breakers, motor control centers, generators, chillers and machinery are outside the offer. If the wire is armored or bundled for a tray rather than loose in a conduit, MC, AC and tray cable is the closer page, and the quote form takes either.

• The full legend along one conductor, unrolled flat for 20 inches or more.
• The spool face and the manufacturer label, with footage if it is printed there.
• The spool or reel size, since a 500-foot spool and a 2,500-foot reel are different lots.
• The conductor end, showing solid or stranded, and the strand count on 8 AWG and larger.
• The storage conditions, and whether the wire has been pulled and returned.