400 MCM XHHW-2 Stranded Copper Building Wire

XHHW-2 is single-conductor building wire with cross-linked polyethylene (XLPE) insulation, which is where the "X" in its name comes from. "HHW" stands for high heat and water resistance, and "-2" means it's rated 90°C in wet locations as well as dry. 400 MCM copper XHHW-2 is installed in conduit or other NEC-recognized raceways at up to 600V and is rated 335A at 75°C. It's used for 400A home services in a single run and 600A and 1000A services in parallel sets.

Both use cross-linked polyethylene (XLPE) insulation and are rated 600V. The difference is the wet rating. XHHW is rated 90°C in dry locations but only 75°C in wet locations. XHHW-2 is rated 90°C in both, and it passes stricter UL 44 moisture testing. XHHW-2 can be used anywhere XHHW is allowed.

Both THHN/THWN-2 and XHHW-2 are 600V building wires rated 90°C in wet and dry locations, so the real difference is the insulation. THHN/THWN-2 has thin PVC insulation with a nylon jacket. XHHW-2 has a single, thicker layer of cross-linked polyethylene (XLPE), a thermoset material that doesn't melt. XHHW-2 holds up better to abrasion, moisture, and cold. THHN/THWN-2 has a smaller diameter, so more conductors fit in the same conduit, and it usually costs less.

Not directly. XHHW-2 is not rated for direct burial, so it can't go in a trench without conduit. It can be installed underground inside conduit, because underground raceways are wet locations and XHHW-2 is rated 90°C wet. For burial without conduit, use a direct burial cable such as USE-2 or URD.

Yes, in standard building installations. XHHW-2 is a single-conductor building wire meant for conduit or other recognized raceways. It can't be run exposed outdoors or fished through walls like NM cable, so inside a wall it has to be in conduit, such as EMT or flexible metal conduit. The NEC allows a few exceptions, such as cable tray for 1/0 AWG and larger conductors with a CT marking, or open wiring on insulators, but these are uncommon.

Yes. NNC's XHHW-2 is marked sunlight resistant, so it holds up in conduit runs exposed to the sun and where the wire leaves the conduit, such as at a weatherhead. Sunlight resistance doesn't replace the conduit requirement.

Yes. XLPE insulation stays flexible in the cold much better than PVC-insulated THHN, which makes XHHW-2 easier to pull in winter. It's commonly rated for installation down to -40°C (-40°F). Check the spec sheet for the exact minimum.

Yes. Three parallel sets of 400 MCM copper XHHW-2 carry the input or output of a 600 kVA, 480V UPS (902A at 125%). It's also used for feeders between main switchgear and UPS input boards.