Benefits of XHHW conductors versus THHN???

[SparkyJunior]

I know XHHW type conductors provide higher short circuit ratings and have overall better thermal characteristics than THHN conductors. What other advantages are there?

 

[davidbeach]

They are also subject to much less insulation damage during installation. THHN includes a nylon jacket that is often damaged during installation. Many people don't seem to worry too much about that, but if the jacket wasn't necessary, how likely would the manufacturers be to include it?

 

[resqcapt19]

XHHW also has a lower "leakage" level and is often required for "leakage" sensitive
As far as the outer nylon jacket of THHN or THWN, it is just that, a jacket and is not part of the insulation system. When UL tests the insulating properties of these types of conductors the jacket is removed. Damage to the jacket is not a problem but damage to the PVC insulation under the jacket is. The jacket serves two purposes, one, to protect the insulation, and two, to low the friction when the conductors are installed into raceways.
Don

 

[rcw retired EE]

THHN is not rated for wet locations at 90 C conductor temperature. XHHW is. THHN is usually dual rated THHN/THWN meaning it is only rated for 75C in wet locations. I have been told that the nylon jacket gives the THWN its wet rating and if the nylon is torn, the wet PVC will deteriorate faster.

THHN is slicker than the XLPE insulation used in XHHW so it pulls easier.

Note that using the 90C ampacity rating allows derating for multiple conductors in a conduit or high ambient temperatures without having to increse the wire size. With THHN you have to start the derating calculation at the 75C ampacity.

 

[itsmoked]

Can't you run THWN in oily places too?

 

[davidbeach]

rcwilson's statement about wet locations was truer in years gone by than it is now. Much THHN is now available dual labeled THHN/THWN-2. THWN-2 is listed for 90C wet. XHHW is only listed for 75C wet, but XHHW-2 is listed for 90C wet, although I don't recall seeing any that was only XHHW, it all seems to be XHHW-2.

THHN can be derated starting at the 90C ampacity, as can THWN-2 and XHHW-2. THWN and XHHW would have to be derated starting at the 75C ampacity. If the application is dry location only, only the THHN rating applies, the THWN or THWN-2 label is irrelevant.

 

[dpc]

Agree with David Beach's comments. I have seen quite a few instances where the nylon jacket on THHN was badly damaged during fairly routine installations. Even more so when "electrician" tied the conductors to the back of his pickup and pulled them in by driving away.

FWIW, we typically allow THWN/THHN for smaller lighting and receptacles circuits and specify XHHW for sizes larger than #10 AWG.

Just my $0.02