Insulation Types

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Alwayslearningelec

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This is pretty senseless spec. I've never seen this. It usually tell you to use either or in specific situations.
It's basically telling you choose between THHN or XHHW, no?
2.02 CONDUCTORS AND CABLES
A. Conductors and cable for power, lighting, control and signal circuits shall have copper conductors of
not less than 98 percent conductivity and shall be insulated to 600V. All conductors shall be
stranded except where specifically noted otherwise, see 2.06 below.
B. Type of conductor and cable for the various applications shall be as follows:
1. Type THWN-2 or XHHW-2 (75 degrees C) shall be used for branch circuits, panel and
equipment feeders in wet and dry locations, including exterior site wiring.
2. Type THHN-2 or XHHW-2 (90 degrees C) shall be used for branch circuits, panel and
equipment feeders in dry locations only and used where lighting branch circuit conductors are
routed through fluorescent fixture channels


2.04 CONDUCTOR AND CABLE APPLICATIONS
A. Concealed Dry Interior Locations: Use only building wire in raceway, building wire with Type THHN-2/XHHW-2
insulation in raceway, or metal clad cable.
B. Exposed Dry Interior Locations: Use only building wire in raceway, building wire with Type THNN-2/XHHW-2
insulation in raceway.
C. Wet or Damp Interior Locations: Use only building wire with Type THWN-2/XHHW-2 insulation in
raceway.
D. Exterior Locations: Use only building wire with Type THWN-2/XHHW-2 insulation in raceway.
 
Seems pretty boilerplate. For the most part you can just use your regular wire types. It does allow MC cable in dry concealed locations.
 
THWN-2 and XHHW-2 are nearly interchangeable in applications where you can use them. Generally, if THWN-2 can be used, XHHW-2 can be substituted, but not necessarily vice-versa.

Applications where XHHW-2 has the advantage:
Some XHHW-2 has a 600V/1000V dual rating, while THWN-2 does not.
XHHW-2 is sunlight resistant in smaller sizes, where THWN-2 does not achieve this rating until size #2 and larger from what I've seen on most datasheets.
XHHW-2 has an exception to the rooftop adder to ambient temperature.
 
Both are 90 C rated (that’s the -2 rating) so the 75 C line is pointless. You only derate to 75 C because of terminals.

It is also restricting conductors so you can’t use aluminum which these days just drives costs up since aluminum didn’t jump like copper. And it restricts use of all kinds of tray cable, flexible cords where appropriate, and of course USE and NM.
 
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