Determine Ampacity

[bassil]

I have the following situation:

5 sets of 3-500 MCM RHW + 1-2/o in duct bank undergound comming from the main transformer to a switchgear with main breaker (1300AF/2000AS)

The main transformer is by PSE&G with a secondary voltage of 480v 3phase.

How can I calculate the ampacity of the wires?

Thanks,
Bassil

 

[don_resqcapt19]

Re: Determine Ampacity

Look at Annex B and Table B.310.6. The ampacity for each conductor can vary based on the types of soil. The table shows ampcities from 299 amps to 414 amps per conductor, based on an installation per Detail 2 in figure B.310.1.
Don

I misread the original post and looked at the table for 3 ducts. The ampacities shown above will not be correct for a 5 duct bank.

[ June 30, 2004, 04:23 PM: Message edited by: don_resqcapt19 ]

 

[charlie b]

Re: Determine Ampacity

This is an extraordinarily loaded question, though that might not have been your intent. The simple answer is to look at the 75C column (for type RHW) in Table 310.16, discover that a 500 MCM is good for 380 amps, multiply by 5, and determine the ampacity to be 1900 amps.

So why do I think the question is loaded? Because there are many opinions on the proper way to determine the ampacity of conductors in a ductbank, for those installations that do not EXACTLY match the ductback layout sketches that appear in the NEC. Part of the problem is that there are no tables or sketches for 480 volt systems within the enforceable portion of the NEC. There are tables and sketches in Appendix B, but they provide information and not requirements. Also, none of the tables or sketches deal with 5 parallel conductors.

My professional opinion on this topic is that it requires a formal ?Neher-McGrath? calculation by a licensed engineer. I am not alone in that opinion, but we who hold to it are in a minority. I have used two separate software packages to perform this type of calculation. Neither is cheap; the less expensive of the two costs over $2000. I will mention that the results of such a calculation have always shown the ampacity to be far lower than that derived from 310.16. This might explain why I find myself holding a minority opinion.

 

[charlie b]

Re: Determine Ampacity

Moderator?s Note: This question has been moved from a separate topic, and should be answered as part of this topic.

Originally posted by bassil: I have this situation:

5 sets of 3-500 MCM RHW + 1-2/o in duct bank coming from a PSE&G transformer to a switch gear with main fuses size 1350Amps.

I determined the the ampacity of the wires as follows:
380A x 3 = 1140 amps.

Can some one tell me why a 1350amp fuse is used?

Thank you very much...
Bassil

 

[charlie b]

Re: Determine Ampacity

Originally posted by bassil: 5 sets of 3-500 MCM?. I determined the ampacity of the wires as follows: 380A x 3 = 1140 amps.

You should have multiplied the 380 times the 5, as I mention in an earlier post. The phrase ?5 sets of 3-500 MCM? means that you have 3 conductors, each of size 500 MCM, representing phases A, B, and C, in a conduit, and that you have five such conduits. So in total, the five phase A conductors will have a combined ampacity of 1900 amps.