UF Cable Ampacity

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Dennis Alwon

Dennis Alwon

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To de-rate NM we start from the 90C ampacity. Where do we start from when we de-rate UF.................................
 
One what assume 90° based on the wording of 340.112 although that does say 90° when"used as a substitute of NM"
 
Here is my confusion-- from the 2020 but also the same in the 2017

This is from tables in Chapter 3

The 2017 I believe is 310.104 tables

1670461740233.png
 
Certainly does confuse the issue..... your opportunity for a Code change :)
 
Dennis Alwon said:
Here is my confusion-- from the 2020 but also the same in the 2017
Click to expand...
The text on the left says that only applies to single conductor UF cable. Typical multiconductor UF is governed by Article 340. So ignore that table.

340.80 doesn't have all the language that 334.80 (NM cable) does, so I infer you need to start derating from the 60C ampacity.

Cheers, Wayne
 
UL 493 for UF cable recognizes two types of multiconductor UF: type UF which may have 60C conductors, and type UF-B which is required to have 90C conductors (just like NM-B). But NEC 340.80 is missing the language to allow type UF-B to use the 90C base ampacity for derating.

Cheers, Wayne
 
UNDERGROUND FEEDER AND BRANCH CIRCUIT CABLE (YDUX)

Some multi-conductor cable is surface marked with the suffix ‘‘B’’ immediately following the type letters to indicate the usage of conductors employing 90°C rated insulation.

Such cable may also be installed as Nonmetallic-sheathed Cable, per Section 340.10(4) of the NEC. The ampacities of Type UF cable, with or without the suffix ‘‘B,’’ are those of 60°C rated conductors as specified in the latest edition of the NEC.

wwhitney said:
NEC 340.80 is missing the language to allow type UF-B to use the 90C base ampacity for derating.
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340.10 Informational Note points us to 310.15(A)(3) for temperature limitations

Informational Note: See 310.15(A)(3) for temperature limitation of conductors.

Most UF feeder cable is UF-B. May not be able to get plain UF cable if you tried.
 
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ramsy said:
340.10 Informational Note points us to 310.15(A)(3) for temperature limitations

Informational Note: See 310.15(A)(3) for temperature limitation of conductors.
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Following that train of references doesn't seem to lead to anything that would overcome the plain language of 340.80, and the fact that 340.80 is missing the allowances found in 334.80.

Cheers, Wayne
 
wwhitney said:
Following that train of references doesn't seem to lead to anything that would overcome the plain language of 340.80, and the fact that 340.80 is missing the allowances found in 334.80.

Cheers, Wayne
Click to expand...
Yes, another circular NEC reference from 340 -> 310.15(A)(3) -> Table 310.104 -> back full circle to 340 with no answer.

NEC doing its job to break academic attempts to decipher code without specialized knowledge of material listings & retail inventory.

Most UF feeder cable is UF-B. May not be able to get plain UF cable if you tried.
 
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Yes but 340.112 states that UF is rated 90C when used as nm. How strange is it that using the wire outside or in the earth changes the ampacity to 60C. Normally ratings in wire go from 90C for dry and 75C for wet
 
@wwhitney I am not sure that the Table is for single conductor. I read it as single conductor UF is to use art. 340.

Underground feeder
and branch-circuit
cable — single
conductor (for
Type UF cable
employing more
than one
conductor, see
Article 340).
Click to expand...
 
Dennis Alwon said:
Yes but 340.112 states that UF is rated 90C when used as nm. How strange is it that using the wire outside or in the earth changes the ampacity to 60C. Normally ratings in wire go from 90C for dry and 75C for wet
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If NM & UF-B are both 60C max ampacity, with 90C conductors, then de-rating can start from 90C column of T310.16, as long as adjusted load remains below 60C column.
 
ramsy said:
If NM & UF-B are both 60C max ampacity, with 90C conductors, then de-rating can start from 90C column of T310.16, as long as adjusted load remains below 60C column.
Click to expand...

Not everyone sees it that way. That is how I assumed it was but it is rare to de-rate UF
 
Dennis Alwon said:
Yes but 340.112 states that UF is rated 90C when used as nm.
Click to expand...
No, it says that if you want to use UF as NM, then you need to use UF-B (the kind with 90C conductors), not "UF". Just like all NM you use has to be NM-B. It doesn't say anything about ampacity.

Dennis Alwon said:
I read it as single conductor UF is to use art. 340.
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That's backwards. The text is (to paraphase) "UF, single conductor (not multi-conductor, for that see Article 340)" The whole table is about single conductor wires, there are no entries for multi-conductor cables.

Cheers, Wayne
 
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ramsy said:
If NM & UF-B are both 60C max ampacity, with 90C conductors, then de-rating can start from 90C column of T310.16, as long as adjusted load remains below 60C column.
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Yes, on NM, because that is what 334.80 tells us. No on UF, because 340.80 says the ampacity is based on 60C conductors, full stop. [Unless, of course, UF-B cable is dual rated as NM-B cable already, not sure if that might be true. But then you'd still only be able to derate from the 90C ampacity where the cable could have been NM-B.]

Looks like a good opportunity for a PI for 2026 to change 340.80 to match 334.80. [Or if the intention to limit derating based on the 90C ampacity to dry conditions, 340.112 could be amended to reference 334.80.]

Cheers, Wayne
 
ramsy said:
If NM & UF-B are both 60C max ampacity, with 90C conductors, then de-rating can start from 90C column of T310.16, as long as adjusted load remains below 60C column.
Click to expand...
That is only when being uses as a subtitute for NM cable. For UF applications it's 60° C for everything, including derating.
 
infinity said:
That is only when being uses as a substitute for NM cable.
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While that would be logical, I don't see how the allowance in 334.80 ever applies to UF cable. So you'd be stuck with 60C for everything even when using UF as a substitute for NM.

Cheers, Wayne
 
infinity said:
For UF applications it's 60° C for everything, including derating.
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Seems to me 340.80 clarifies ampacity of cable assembly, not derating adjustments of dry conductors in accordance with 310.15

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