Table 310.15 75/90 Degree rating/derating

[sssss]

I will have to provide a 4000A indoor Switchgear/Switchboard (depends on the manufacture) being fed by outdoor transformer via underground ductbank. My A/E told me that the derating factor has to apply to the 75 degree column value instead of 90 degree column, because typical manufactures (EATON, Square D, Siemens, etc.) only provides 75 degree rating terminal. The result is 19 sets of 750 MCM being provided per A/E's calculation method.
I doubt that's correct, so please give me some hints on this issue.

Thanks

 

[JFletcher]

Welcome. You derate from the 90* column.

here's a few other topics on this that go into more detail:

http://forums.mikeholt.com/showthread.php?t=93389
http://forums.mikeholt.com/showthread.php?t=175913

 

[charlie b]

You left out a few key details. Why are you talking about derating in the first place? That term generally applies when you have to change the value of ampacity that is given in the table because the ambient temperature is higher than 30 C or because you have more than 3 current carrying conductors in the same conduit. You don't derate because you are going underground. Rather, you need to go though a process to determine what the ampacity will be under those conditions of use. That process might involve a calculation using the Neher McGrath equation. Is that what your A/E used? I tend to doubt it, because that equation does not make use of any ampacity tables, and doesn't use a 90C or 75C column. But if so, what will the installation look like, in terms of how the conductors go into the conduits and how the conduits are spaced apart?

 

[Carultch]

I will have to provide a 4000A indoor Switchgear/Switchboard (depends on the manufacture) being fed by outdoor transformer via underground ductbank. My A/E told me that the derating factor has to apply to the 75 degree column value instead of 90 degree column, because typical manufactures (EATON, Square D, Siemens, etc.) only provides 75 degree rating terminal. The result is 19 sets of 750 MCM being provided per A/E's calculation method.
I doubt that's correct, so please give me some hints on this issue.

Thanks

4000A at 75C = 9 sets of 750 kcmil Cu or 11 sets of 750 kcmil AL. If you keep one set in each conduit, you'll usually just have a basic derate on the 90C ampacity values of your site's ambient temperature alone, assuming the neutral doesn't count as a CCC when using table 310.15.(B)(3)(a). And this isn't usually enough to cause an increase in size above what you already need to meet 75C termination ampacities (which don't require any ampacity adjustments or temperature corrections).

 

[ron]

Why are you talking about derating in the first place? That term generally applies when you have to change the value of ampacity that is given in the table because the ambient temperature is higher than 30 C or because you have more than 3 current carrying conductors in the same conduit. You don't derate because you are going underground.

The end result is there is more wire than typically expected for the ampacity, so lots of folks not having to do the actual Neher McGrath calcs, just call it derating. Granted it is not a multiplier from a table, but lots of folks just call it that.

Depending on depth, spacing and material surrounding the ducts, I often get 1.75 or 2 times the conductors.

 

[Strathead]

4000A at 75C = 9 sets of 750 kcmil Cu or 11 sets of 750 kcmil AL. If you keep one set in each conduit, you'll usually just have a basic derate on the 90C ampacity values of your site's ambient temperature alone, assuming the neutral doesn't count as a CCC when using table 310.15.(B)(3)(a). And this isn't usually enough to cause an increase in size above what you already need to meet 75C termination ampacities (which don't require any ampacity adjustments or temperature corrections).

If you are using dual rated THHN/THWN the you need to use the 75C table, because you are using THWN outside. So there are more factors involved.

 

[Strathead]

I will have to provide a 4000A indoor Switchgear/Switchboard (depends on the manufacture) being fed by outdoor transformer via underground ductbank. My A/E told me that the derating factor has to apply to the 75 degree column value instead of 90 degree column, because typical manufactures (EATON, Square D, Siemens, etc.) only provides 75 degree rating terminal. The result is 19 sets of 750 MCM being provided per A/E's calculation method.
I doubt that's correct, so please give me some hints on this issue.

Thanks

I already mentioned the issue with THWN being rated at 75C anyway. For you, the reason your A/E is wrong or perhaps you are conveying the conversation wrongs is because the wire is not going to carry the amperage listed in the 90C column, so the lugs at the gear are not going to see the heat generated by that. Derating prevents overheating in situations where the temperature will potentially get too high such as higher ambient temperature or being crowded in a conduit.

 

[Dennis Alwon]

Thwn vs thwn-2 is the difference. I must assume you are using conductors rated 90C when installed outdoors. If so then de-rating starts at 90C. If the conductors are simply Thwn and not Thwn-2 then the thwn is rated 75C in wet location and thus the de-rating starts from there. I would be surprised if you are not using Thwn-2....or something such as that--XHHw-2

Anything with a "-2" will be rated 90C outdoors.

 

[Carultch]

If you are using dual rated THHN/THWN the you need to use the 75C table, because you are using THWN outside. So there are more factors involved.

In practice, do you ever find such a wire in sizes large enough for paralleling, that isn't also rated for THWN-2?

 

[sssss]

Thanks for everyone quick reply.

To make it clear, below are more information.
1. The switchboard will be located indoor assuming the ambient temperature at 30 degree C
2. The underground ductbank is 75'-100'
3. Assuming the wire is THHN/THWN-2 which is good for 90 degree C, and the neutral is non current carrying
4. The reason of derating is because the conductors are installed underground, and the A/E believe the conductor needs to be derated for thermal reason
5. The A/E did it by hand calculation, and they used the derating factor from IEEE brown book Table 13.8.
6. The A/E said 90 degree C column cannot be used, because if the conductor have a temperature of 90 degree C, the 75 degree C rated terminal maybe damaged when connecting to the conductor.

I'm sure that the A/E didn't think through it, because I know that the derating factor also affect by the soil RHO, depth, conduit separation/arrangement, soil temperature, etc. But in this post, I just want to clear out if 90 degree C column can be used or not.

Thanks again.

 

[charlie b]

Let me completely change the question, briefly. Let's talk about an installation indoors, with a single conduit containing three single phase (two-wire) branch circuits. You have a total of 6 current-carrying conductors in the same conduit. You need to apply the "adjustment factor" (the formal term for the derating factor) of Table 310.15(B)(3)(a). If you are using conductors with an insulation system rated for 90C, then you can start with the ampacity from the 90C column, apply the 80% factor, and obtain a new ampacity. The value you use is either the result of that calculation, or the value in the 75C column (with no derating factor applied), whichever is lower.

That is the essential process of derating. I don't know of any circumstance in which that process would be applied to an underground installation. I am not familiar with the IEEE Brown Book's tables. They have been superseded, and I don't have a copy of the new standard. So I can't speak to how the A/E performed the calculation. I will say, however, that they are flat out wrong about saying you need to derate for underground installations. What you do, as I said before, is to determine what the ampacity is, under the conditions of use. That does not involve the table, coupled with a derating factor.

 

[sssss]

Let me completely change the question, briefly. Let's talk about an installation indoors, with a single conduit containing three single phase (two-wire) branch circuits. You have a total of 6 current-carrying conductors in the same conduit. You need to apply the "adjustment factor" (the formal term for the derating factor) of Table 310.15(B)(3)(a). If you are using conductors with an insulation system rated for 90C, then you can start with the ampacity from the 90C column, apply the 80% factor, and obtain a new ampacity. The value you use is either the result of that calculation, or the value in the 75C column (with no derating factor applied), whichever is lower.

That is the essential process of derating. I don't know of any circumstance in which that process would be applied to an underground installation. I am not familiar with the IEEE Brown Book's tables. They have been superseded, and I don't have a copy of the new standard. So I can't speak to how the A/E performed the calculation. I will say, however, that they are flat out wrong about saying you need to derate for underground installations. What you do, as I said before, is to determine what the ampacity is, under the conditions of use. That does not involve the table, coupled with a derating factor.

I read the NEC again, so, there is no derating factor table for underground installation.
310.15(C) indicates that thermal calculation has to be under engineering supervision, and it advise that the conductor temperature not to exceed the insulator rating.

In Annex B, Table B.310.15(B)(2)(7) provides the allowed ampere for 2x3 or 3x2 underground ductbank configuration. This doesn't apply to my case because I have 4000A service.

So, the A/E is right to set the temperature limit at 75C (even it's THHN/THWN-2), either hand or computer calculation, is that correct??

 

[JFletcher]

Does 4000A gear have the space/lugs to land 19 sets of 750MCM? and even using AL at the 75* table, that's 385A x 19= 7315A, which would be derated by 45% to have an ampacity of 4000A. No idea how you could have such massive derating since you cant get more than 5 750's in a 4" conduit... your underground conduits going thru a hot spring?

 

[Dennis Alwon]

So, the A/E is right to set the temperature limit at 75C (even it's THHN/THWN-2), either hand or computer calculation, is that correct??

Yes the temperature limit is 75C but you can still de-rate from 90C column if the wire is rated 90C

 

[sssss]

Does 4000A gear have the space/lugs to land 19 sets of 750MCM? and even using AL at the 75* table, that's 385A x 19= 7315A, which would be derated by 45% to have an ampacity of 4000A. No idea how you could have such massive derating since you cant get more than 5 750's in a 4" conduit... your underground conduits going thru a hot spring?

I didn't come up with the factor but the A/E did! The A/E is base on the IEEE Std 399 (Brown book) table 13.8. For 4 x 5 ductbank, 750MCM 3/C have the factor as 0.413.

Yes the temperature limit is 75C but you can still de-rate from 90C column if the wire is rated 90C

How to de-rate it if NEC doesn't have de-rate factor table??

 

[david luchini]

I read the NEC again, so, there is no derating factor table for underground installation.
310.15(C) indicates that thermal calculation has to be under engineering supervision, and it advise that the conductor temperature not to exceed the insulator rating.

310.15(C) doesn't require that you use Engineering Supervisions, it permits it.

So, the A/E is right to set the temperature limit at 75C (even it's THHN/THWN-2), either hand or computer calculation, is that correct??

Yes, the conductor temperature limit would be 75C.

 

[Strathead]

Bottom line, the person who is telling you this stuff is wrong. Top line. Table 310.15(B)(16) clearly states it is for not more than three current carrying conductors in a conduit or directly buried in earth. Period. Derating from there is on other sections 310.15(B)(2) if the ambient temperature is more than 30ºC. it is EXTREMELY unlikely that a conduit buried 2 feet in the ground is over that, unless it is buried in a volcano. So if he is derating for being buried make him justify an ambient temperature above 30. Next derate, the number of current carrying conductors per 310.15(B)(16) He can claim the neutral is a current carrying conductor, and that one becomes difficult to argue, but in most cases it is not or the code would not even have this exception. In case it is then you are derating to 80% from the 90ºC column of the above table. The heat doesn't extend to the end of the wire it is confined to the areas where it can't dissipate, period.

Anything else is personal preference or misunderstanding of the code. At that point if the customer is willing to foot the bill then so be it.

 

[Richard S]

IEEE 399 (Brown Book) Table 13-8 for those who are interested. Had to brush off the dust. Not sure if I would ever get an underground duct bank job using these adjustment factors. Kind of crazy or perhaps CYA.

 

[MasterTheNEC]

OK....just the fact that your A/E stated the following :

"The A/E said 90 degree C column cannot be used, because if the conductor have a temperature of 90 degree C, the 75 degree C rated terminal maybe damaged when connecting to the conductor."

Tell me he/she doesn't understand derating very well. However, beyond that if he is using a IEEE value then he (and I guess you) will need to explain that to the plans examiner and/or electrical inspector who will be looking in the NEC and not finding that information. As an antidotal statement, most if not all wire and cable manufactures express the ampacity values given in Table 310.15(B)(16) and clearly your A/E is using a reference outside of that. Point being, in my position as head of a wire and cable technical division I would say the wire is good for the values established in the NEC and the use of anything otherwise is on the shoulders of the A/E. We assume no liability in other words but to each his own.

If the drawings are stamped and approved through your local AHJ process....have at it but in most cases don't expect any warranty on said wire. With that said the error in his statement is him saying you can't use the 90C value for adjustments and corrections when it clearly says you can in section 310.15(B). Now, the end result can't exceed that of the terminals ratings which in your case you expressed as 75C. So with that said I question the A/E's logic. Assuming of course the wire has a 90C rated insulation in a wet location..eg. THHN/THWN-2 or XHHW-2 as others have stated.

However, they are also allowed to provide engineering supervision in their judgement as well and be held accountable for it. So if they change a given ampacity on a conductor without the consent of the manufacturer of that product then...well......it's best I not go down that rabbit hole any more than I have.

These are my opinions to which everyone can agree to disagree.

 

[kwired]

OK....just the fact that your A/E stated the following :

"The A/E said 90 degree C column cannot be used, because if the conductor have a temperature of 90 degree C, the 75 degree C rated terminal maybe damaged when connecting to the conductor."

Tell me he/she doesn't understand derating very well. However, beyond that if he is using a IEEE value then he (and I guess you) will need to explain that to the plans examiner and/or electrical inspector who will be looking in the NEC and not finding that information. As an antidotal statement, most if not all wire and cable manufactures express the ampacity values given in Table 310.15(B)(16) and clearly your A/E is using a reference outside of that. Point being, in my position as head of a wire and cable technical division I would say the wire is good for the values established in the NEC and the use of anything otherwise is on the shoulders of the A/E. We assume no liability in other words but to each his own.

If the drawings are stamped and approved through your local AHJ process....have at it but in most cases don't expect any warranty on said wire. With that said the error in his statement is him saying you can't use the 90C value for adjustments and corrections when it clearly says you can in section 310.15(B). Now, the end result can't exceed that of the terminals ratings which in your case you expressed as 75C. So with that said I question the A/E's logic. Assuming of course the wire has a 90C rated insulation in a wet location..eg. THHN/THWN-2 or XHHW-2 as others have stated.

However, they are also allowed to provide engineering supervision in their judgement as well and be held accountable for it. So if they change a given ampacity on a conductor without the consent of the manufacturer of that product then...well......it's best I not go down that rabbit hole any more than I have.

These are my opinions to which everyone can agree to disagree.

Unless I misunderstood something though the A&E is asking for larger conductors then NEC would require as minimum, so any liability on the wire manufacturer part is not effected anyway as the manufacturer already claims it has a higher ampacity then it is being used for.