XHHW vs THWN

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wasasparky

Senior Member
Is there a real advantage in using XHHW vs THWN in underground feeders?
XHHW mindset claims better dielectric strength. (marginal?)
Price? and pulling conductors are other issues...

Is one clearly better than the other? or is it 6 of one and....?
 
Some "old time" electrician once told me that XHHW insulation is less susceptible to be eaten by ants....:rolleyes:
 
egr said:
Some "old time" electrician once told me that XHHW insulation is less susceptible to be eaten by ants....:rolleyes:
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If thats true then let me get a few rolls! Will it keep them out of wire nuts too? We have a huge problem with fire ants. Three times in the last two weeks I have had shorted wires from them. If we ever get any rain here it will be worse. It always starts after a good rain, the nests they build in boxes, ETC. get damp, then BUZZ!
 
Clearly better is subjective. For an installation where current leakage is an issue- OR line isolated power supplies come to mind- XHHW is a good choice. Underground maybe if it was a very long run. By and large, nah.
 
Check Table 310.13. One thing that is not immediately obvious is XHHW is a bit thicker than THWN, but still a bit less than THW and some of the more common Rubberized (R--) conductors. Anything over about 30 mils thick for any of these more common insulating materials is actually selected more for mechanical rather than electrical (dielectric) properties. This is not usually an electrical problem except on some high resistance grounded systems (HRGS) applications.

Thermoplastic insulations (T---) are usually PVC based and use plasticizers that may leach out over time. This does not affect their insulation value but is why they may become brittle. Depending on the manufacturer, some plasticizers are chemically very similar to margarine (they are, in fact, edible) and may attract pests.

Thermosets (X--- and R---) use a different chemical process (cross-linking) to achieve flexibility.
 
Thwn is 75C.
Xhhw-2 is 90C.
Which may not matter if you don't have a derate issue.

Now for something you might have not known. And if you are lucky, you might never get to verify:

I've pulled xhhw down to about -20F. Thwn is about like trying to pull a coil spring.

carl
 
wasasparky said:
and pulling conductors are other issues...

Is one clearly better than the other? or is it 6 of one and....?
Click to expand...

Yes!

Southwire has "pre-lubed" wire. The outer coating is super slick so you don't need any wire lube.

I was not convinced until I saw a demo. Very nice labor saver!
 
LarryFine said:
True, but most building wire is now THHN/THWN, which is 90C.
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Isn't it only 90 C for dry locations? An underground run would be a wet location.

Steve
 
One advantage of XHHW is its smoke and burn characteristics. It is a cross-linked polymer and does not produce as much toxic and acidic smoke when burned. Also it burns like rubber (stays put on the conductor) instead of the insulation melting and dripping off the conductor like other plastics.
 
iwire said:
That is 90 dry, 75 wet.

Or if it is THWN-2 it is 90 dry / 90 wet.
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Isn't most equipment rated for 60/75 or 75 cu connections, therefore having to rate in the 75C column anyway? I was high on thwn-2 until finding out this fact in the NEC.
 
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