Question about derating -3/4" liquid tight conduit - Three 12 UF-B cables - less than 2 foot distance

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Curiousv

Member
if conduit distance(liquid tight) is less than 2 feet long - Can we run three 12-2 UF-B cable in 3/4" conduit without worrying about derating?

if I look under non metallic liquid conduit chart - I under 60% but why 2 wire is 31% and one wire is 53%
how can 2 wire be less than 1 wire?

if 3/4" is not big enough 1" will be?

I know UF-B does not need a conduit but this is an external wall - and UB-B cable will terminate inside 200A Eaton BR panel

Also, when three cables - so total 6 current carrying conductors or just 3 ( I mean neural count as current carrying?)

These are 2 kitchen receptacle circuits and1 dishwasher circuit. @Afjes @wwhitney @WorthFlorida

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wwhitney

Well-Known Member
- There is no reason to put 12/2 UF-B in LFNC. If the 200A panel is interior, or if it's on the exterior face of the siding and you enter the panel from the back, then you can use regular NM. If do use a sleeve of LFNC for a cable method, then the issue of securing the cable to the panel enclosure as required is a bit tricky.

- Conduits 24" in length or shorter are exempt from derating. In a 2-wire circuit, both conductors are current carrying.

- The fill ratio for 2 conductors is smaller because the two conductors can jamb side by side.

- If you actually did pull 3 flat cables inside a conduit (almost never a good choice), for each flat cable you'd need to take the largest dimension and treat the cable area as the circle of that diameter. Then check your fill ratio.

Cheers, Wayne

Curiousv

Member
- There is no reason to put 12/2 UF-B in LFNC. If the 200A panel is interior, or if it's on the exterior face of the siding and you enter the panel from the back, then you can use regular NM. If do use a sleeve of LFNC for a cable method, then the issue of securing the cable to the panel enclosure as required is a bit tricky.

- Conduits 24" in length or shorter are exempt from derating. In a 2-wire circuit, both conductors are current carrying.

- The fill ratio for 2 conductors is smaller because the two conductors can jamb side by side.

- If you actually did pull 3 flat cables inside a conduit (almost never a good choice), for each flat cable you'd need to take the largest dimension and treat the cable area as the circle of that diameter. Then check your fill ratio.

Cheers, Wayne
1. Panel is on exteror face of siding but I am planning to enter in to panel either by side knockout of using a hole saw create a hole from the top.
2. If I enter from the top by creating a hole in panel - the distance is less than 2 ft but if I enter from side its like 4-5 feet
- I am thinking of doing but if I enter from bottom it will be like 5-6 ft
3. I have no particular preference form LFNC or UF-B but those are the items I already have so why buy something new instead of using what I have .. I can use myers' hub and liquid tight fitting if I decide to come from top (less than 2 ft conduit size)
4. I calculated fill and you are right 3/4" conduit is not enough because each cable (southwire 12-2 UF-B is 11.76 mm in diameter) so I have to upsize to at least 1 1/4" conduit either pvc 40 or LFNC
5. If I decide to go from side - (it will be more than 2 feet) - so if its more than 2 feet I have to derate (also 3 uf B cables so 6 CCC) -
now confusion is how do I derate ? and honestly I dont understand the concept of derating ..we are already using only 40% of the conduit - still derating? also derating of UF-B cable we have to start with 90°?
6. 340.80 Ampacity The ampacity of Type UF cable shall be that of 60°C (140°F) conductors in accordance with 310.14 - so if individual conductor in cable is rated for 60 than how can we start derating from 90? @Afjes @WorthFlorida @wwhitney

wwhitney

Well-Known Member
1) Entering from the back would be best if at all possible. Next best is the bottom. If you are going to use UF cable, you don't necessarily need any conduit, you could just stable UF to your siding. If that's judged to be subject to damage, you could mount a suitable (e.g. cedar) running board over your siding, staple the UF to that, and build a removeable wood cover to protect the cables and make it look better.

3) A meyer's hub is only listed for use with RMC/IMC, i.e. tapered threads, not for a box connector fitting. You could use a short length of RMC or IMC, and a changeover fitting purpose designed to go from RMC / IMC to LFNC. But really just avoid the top entry outdoors.

5) If you don't know how to derate, then just avoid bundling. But for 2 conductor #12 or #14 NM or UF cables, you can bundle up to 4 of them without any net consequence after derating, assuming any temperature correction required is short enough that it can be ignored due to the 10' / 10% rule. Because:

6) The final ampacity is limited to the unadjusted 60C value, but if you have to do ampacity adjustment or correction (such as for bundling, or ambient temperature in excess of 30C), you can start at the 90C value, apply the correction/adjustment factors, and then you use the lesser of the unadjusted 60C value, and the corrected/adjusted 90C value.

Cheers, Wayne

Curiousv

Member
1) Entering from the back would be best if at all possible. Next best is the bottom. If you are going to use UF cable, you don't necessarily need any conduit, you could just stable UF to your siding. If that's judged to be subject to damage, you could mount a suitable (e.g. cedar) running board over your siding, staple the UF to that, and build a removeable wood cover to protect the cables and make it look better.

3) A meyer's hub is only listed for use with RMC/IMC, i.e. tapered threads, not for a box connector fitting. You could use a short length of RMC or IMC, and a changeover fitting purpose designed to go from RMC / IMC to LFNC. But really just avoid the top entry outdoors.

5) If you don't know how to derate, then just avoid bundling. But for 2 conductor #12 or #14 NM or UF cables, you can bundle up to 4 of them without any net consequence after derating, assuming any temperature correction required is short enough that it can be ignored due to the 10' / 10% rule. Because:

6) The final ampacity is limited to the unadjusted 60C value, but if you have to do ampacity adjustment or correction (such as for bundling, or ambient temperature in excess of 30C), you can start at the 90C value, apply the correction/adjustment factors, and then you use the lesser of the unadjusted 60C value, and the corrected/adjusted 90C value.

Cheers, Wayne
1. You said for 2 conductor #12 or #14 NM or UF cables - u can bundle up to 4 of them without worrying about derating.
UF-B cable I have - has one black one white and one bare ground so I think I have total 6 current carrying conductors - 3 black(hot) and 3 neutral - total 6 - I think you are saying neutral does not count as CCC ? thru neutral current will go back so its carrying is not it?
2. You said - 'but if you have to do ampacity adjustment or correction (such as for bundling, or ambient temperature in excess of 30C), you can start at the 90c - but why ? and in case if I am adjusting what will be reduced amp ?will it be 90x80%= 25A?

wwhitney

Well-Known Member
1) The terminology 2 conductor cable just counts the white and the black, the green is assumed. [Note that cords are different, the ground is explicitly counted.] On a 2 wire circuit, both conductors are CCCs. So yes, 3 x 2 = 6 CCCs.

For 90C conductors limited to 60C overall, a #12 Cu conductor has a base ampacity for derating of 30A (90C) but is limited to 20A at the end of all computations (60C). For 7-9 CCCs, the derating factor is 70%. So 30A * 70% = 21A; that's greater than the 20A (60C) limit, so the cable is still good for 20A.

Thus for up to (4) 2 conductor NM or UF cables, if there's no temperature correction, you can ignore the ampacity adjustment due to CCCs.

2) Because the conductors inside the cable actually have insulation that is rated 90C, so it's OK for those conductors to get up to 90C. The 60C overall limit is just an extra layer of conservativeness imposed on NM cable, since it's perhaps the least robust wiring method the NEC allows. For #14, #12, and #10 conductors, it also corresponds to the "small conductor OCPD" rule, which limits those conductors to 15A, 20A, and 30A, respectively, matching their 60C base ampacity.

Some applications, like motor circuits, are exempt from that small conductor rule, but because of the 60C NM limit, that exemption doesn't make any difference for NM cable. I.e. if you have a piece of HVAC equipment marked with an MCA of 20A and MOCP of 30A, then you could supply it with #14 MC cable (75C ampacity = 20A) on a 30A breaker, but if you use NM cable, you need #12 NM on a 30A breaker.

If the last paragraph is too confusing, just ignore it.

Cheers, Wayne

Curiousv

Member
1) The terminology 2 conductor cable just counts the white and the black, the green is assumed. [Note that cords are different, the ground is explicitly counted.] On a 2 wire circuit, both conductors are CCCs. So yes, 3 x 2 = 6 CCCs.

For 90C conductors limited to 60C overall, a #12 Cu conductor has a base ampacity for derating of 30A (90C) but is limited to 20A at the end of all computations (60C). For 7-9 CCCs, the derating factor is 70%. So 30A * 70% = 21A; that's greater than the 20A (60C) limit, so the cable is still good for 20A.

Thus for up to (4) 2 conductor NM or UF cables, if there's no temperature correction, you can ignore the ampacity adjustment due to CCCs.

2) Because the conductors inside the cable actually have insulation that is rated 90C, so it's OK for those conductors to get up to 90C. The 60C overall limit is just an extra layer of conservativeness imposed on NM cable, since it's perhaps the least robust wiring method the NEC allows. For #14, #12, and #10 conductors, it also corresponds to the "small conductor OCPD" rule, which limits those conductors to 15A, 20A, and 30A, respectively, matching their 60C base ampacity.

Some applications, like motor circuits, are exempt from that small conductor rule, but because of the 60C NM limit, that exemption doesn't make any difference for NM cable. I.e. if you have a piece of HVAC equipment marked with an MCA of 20A and MOCP of 30A, then you could supply it with #14 MC cable (75C ampacity = 20A) on a 30A breaker, but if you use NM cable, you need #12 NM on a 30A breaker.

If the last paragraph is too confusing, just ignore it.

Cheers, Wayne
Ok now its clear about up to 8 ccc - but can you tell me more about 10' 10% rule

wwhitney

Well-Known Member
Ok now its clear about up to 8 ccc - but can you tell me more about 10' 10% rule

Wayne

Curiousv

Member
I saw a video that says - one can strip UF or NM cable and the insulated conductors would be either THNN or THNW-2 individual conductors and its legal to use them in a conduit - what do you think can I strip jacket and still be code complaint?

wwhitney

Well-Known Member
I saw a video that says - one can strip UF or NM cable and the insulated conductors would be either THNN or THNW-2 individual conductors and its legal to use them in a conduit - what do you think can I strip jacket and still be code complaint?
You could if the individual conductors in the NM cable were labeled as THHN or THWN-2 (and in the latter case you could avoid splicing). This is typically the case for, say, MC cable. However, on NM cable the conductors are unlabeled, so you can't do that.

Cheers, Wayne

Curiousv

Member
You could if the individual conductors in the NM cable were labeled as THHN or THWN-2 (and in the latter case you could avoid splicing). This is typically the case for, say, MC cable. However, on NM cable the conductors are unlabeled, so you can't do that.

Cheers, Wayne
Can we have use pvc sch 40 white on exterior wall - exposed to sunlight - it does not say sunlight resistant and say 73F
I guess not because I just fond they are plumbing pipe and not electrical conduit

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wwhitney

Well-Known Member
Can we have use pvc sch 40 white on exterior wall - exposed to sunlight - it does not say sunlight resistant and say 73F
I guess not because I just fond they are plumbing pipe and not electrical conduit
You can't use plumbing pipe where you need electrical conduit.

But if the electrical conduit is just for physical protection of UF cable, not to run individual wires in, then perhaps the white plumbing pipe would serve that purpose. That would be at the AHJ's discretion.

Or you could just use grey electrical conduit and then paint it white after inspection.

Cheers, Wayne

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