Conductor Temp. Pro-rating Factor

[vheinz]

In Annex B of the handbook, Tables B.310.5, .6 & .7 are based on a Conductor Temperature of 75 degree C.
I need these ampacites for 90 degree C cable. Is there a prorating factor for the 75 degree C cable ampacities, to come up with the ampacities for a 90 degree C cable in the same application?

 

[pierre]

Re: Conductor Temp. Pro-rating Factor

vheinz
I am not 100% sure, but I do not believe you are allowed to use the 90 degree rating for you calculations.
Maybe someone here knows more?

Pierre

 

[dereckbc]

Re: Conductor Temp. Pro-rating Factor

You are allowed to use the 90-degree rating in 310.16 for de-rating. It is a simple straightforward process.

1 Select a conductor from 90-degree column.
2. Apply temperature de-rating at bottom of 310.16 if applicable.
3. De-rate per 310.15.b (not sure that is the right table, code book is not available) for number of current carrying conductors in raceway.
4. Compare de-rated cable amperage to either the 60 or 75-degree column, whichever applies to termination temperature. If it is less or equal, you are good to go.

 

[charlie b]

Re: Conductor Temp. Pro-rating Factor

The Tables in Annex B function differently from Table 310.16, in several ways. First, they are for underground ducts, in one of three configurations. The more conduits you install in close proximity, the lower the ampacity. Secondly, they allow selection of a value for ?RHO,? the thermal resistivity of the surrounding fill. For example, dry sand does not conduct heat away as well as damp dirt, so the ampacities go down for higher values of RHO. Finally, they include an obscure variable known only as ?LF.? IMHO (and I think many AHJs would agree), you need to have ?Engineering Supervision? (i.e., someone with a PE seal) to take advantage of the LF50 columns. But that?s another topic. Now, to directly answer the question:

Originally posted by vheinz: Tables B.310.5, .6 & .7 are based on a Conductor Temperature of 75 degree C.

The 75C here means that if you install the cable type shown in the configuration shown and run the amps shown, the result is that the temperature of the cable (considering ambient plus I**2 R losses) will not exceed 75C. This is important because the terminations are likely to be rated no higher than 75C.

Is there a prorating factor for the 75 degree C cable ampacities, to come up with the ampacities for a 90 degree C cable in the same application?

If you look closely at the cable types shown in these tables, you will see that they are also listed in the 90C column of Table 310.16. In other words, Tables B.310.5 ? 7 already account for the use of 90C rated conductors. There is no other pro-rating to be had. Sorry.

 

[vheinz]

Re: Conductor Temp. Pro-rating Factor

Charlie B

If you look closely at the cable types shown in these tables, you will see that they are also listed in the 90C column of Table 310.16. In other words, Tables B.310.5 – 7 already account for the use of 90C rated conductors.

Actually the only two cable types that are listed in the 90C column of 310.16 (and Table B.310.5,6&7) are THHW & XHHW and those are only qualified at 90C in Dry Locations per Table 310.13. Correct?

Maybe I should be a little more specific, we will be using RHW-2 cable (90C cable) and the terminations will be rated for 90C.

So going back to my original post, is there a pro-rating factor for the Tables B.310.5 , .6 & .7 for 90C Cables, or not?

 

[charlie b]

Re: Conductor Temp. Pro-rating Factor

You are right about the cable types. I read the wrong column. My mistake.

My answer is the same one I gave you this morning on your other question. I?m afraid there is no ?pro-rating factor.? If the configuration of your interest does not correspond to any NEC Table, and if you need to adjust the ampacities from the tabulated values by any means other than those described in the NEC (e.g., for ambient temperature or for more than 3 current-carrying conductors), then I believe you are in the realm of ?engineering supervision.?

If I were hard-pressed for a fast answer, I might be able to justify a 4%, or if needed a 9% pro-rating factor. This would require my putting together an argument based on the concept that ?adding 15 degrees at the high end? is no worse than ?subtracting 5 (or even 10) degrees at the low end.? Tables B310.5-7 have a correction factor for ambient temperatures lower than 20C.

The easier argument would compare two cases: (1) Starting at an ambient of 15C instead of 20C (worth a 4% pro-rating) and allowing a 60C temperature rise, for a total temperature of 75C, as opposed to (2) Starting at an ambient of 20C and allowing the same 60C temperature rise, for a total temperature of 80C (which your cable and your terminations can safely handle).

The harder argument would compare two analogous cases: (1) Starting at an ambient of 10C instead of 20C (worth a 9% pro-rating) and allowing a 65C temperature rise, for a total temperature of 75C, as opposed to (2) Starting at an ambient of 20C and allowing the same 65C temperature rise, for a total temperature of 85C (which your cable and your terminations can safely handle).

The problem with both arguments is that there are other physical factors that must also be taken into account. One is the fact that a higher temperature gives a higher resistance, and therefore a higher rate of generating heat (this factor runs contrary to pro-rating). Another is the fact that a higher temperature at the source forces heat to pass through the surrounding environment faster (this factor is favorable to pro-rating). Which of the various factors outweighs the others? I?m not sure I could tell for certain, without performing a formal Neher-McGrath calculation. That brings us back to ?engineering supervision? again.