De-rating UF

[mbrooke]

When de-rating UF (and other insulation types) do you still use the 90*C column or the 60*C column?

 

[bphgravity]

Section 340.10(4) directs you back to Article 334, including 334.80 when Type UF is used as NM cable. Otherwise, it's 60 degree C per 340.80.

 

[jumper]

60C, but then I never use UF inside, except for a small portion of a circuit.

340.80 Ampacity. The ampacity of Type UF cable shall be
that of 60°C (140°F) conductors in accordance with 310.15.

Some feel that this:

340.112 Insulation. The conductors of Type UF shall be
one of the moisture-resistant types listed in Table
310.104(A) that is suitable for branch-circuit wiring or one
that is identified for such use. Where installed as a substitute
wiring method for NM cable, the conductor insulation
shall be rated 90°C (194°F).

means that you can use 90C in derating calcs for UF when used as NM, but I do not see it directly.

NM is more specific:

334.80 Ampacity. The ampacity of Types NM, NMC, and
NMS cable shall be determined in accordance with 310.15.
The allowable ampacity shall not exceed that of a 60°C
(140°F) rated conductor. The 90°C (194°F) rating shall be
permitted to be used for ampacity adjustment and correction
calculations, provided the final derated ampacity does not exceed
that of a 60°C (140°F) rated conductor. The ampacity of
Types NM, NMC, and NMS cable installed in cable tray shall
be determined in accordance with 392.80(A).

 

[mbrooke]

Ok, so my Im getting this:

If UF is used as NM it is to be de-rated off the 90*C column, but if used outside it is to be de-rated off the 60*C column?

 

[jumper]

Using the 340.10 reference that Bryan cited, yes. Inside could be 90C.

Outside is definitely 60C.

 

[mbrooke]

Using the 340.10 reference that Bryan cited, yes. Inside could be 90C.

Outside is definitely 60C.

Odd way to do it imo.

Just to confirm, using table 310.15(B)(2)(a), if I had #6 UF, Id pick 55amps from table 310.15, and with an ambient between 31-35*C I multiply 55 by 0.91= 50amps, so my load is restricted to 50amps.

 

[jumper]

Odd way to do it imo.

Just to confirm, using table 310.15(B)(2)(a), if I had #6 UF, Id pick 55amps from table 310.15, and with an ambient between 31-35*C I multiply 55 by 0.91= 50amps, so my load is restricted to 50amps.

Looks good.

 

[mbrooke]

Got it, Thanks!