pad mount transformer secondary

[bsh]

I have a 2500 KVA 277/480 volt pad mount transformer (3007 amp secondary current) which will feed a 3500 amp main breaker in a switchboard. An underground cable installation needs to be derated per the NEC and IEEE 390. I will need to use eighteen 500 KCM cables to get the 3500 amps required. The circuit breaker in te switchboard will accept 11 cables.
What experiences have other had with this problem?

 

[charlie b]

An underground cable installation needs to be derated per the NEC and IEEE 390.

I don?t know what IEEE 390 might have to say on this subject, but what NEC article tells you that an underground installation must be derated? I know of no such NEC requirement.

Much depends on your answer to the following question: What is the calculated load? For if you used an article 220 process to calculate the load, and if your answer is at or below 2500 KVA, then I think you can use Table 310.16 to size your secondary conductors. In that event, a set of 10 conductors, each being 500 MCM copper, will suffice for your installation.

 

[bsh]

This is an industrial installation and additional equipment will be added so the 2500 KVA transformer will probably be totally loaded even after assuming some diversity on the load.
NEC Appendix B discusses underground cable installation. IEEE 390 chapter 13 also discusses this. I know that Appendix B is included in the code for information and is therfore not required but I also know that the cables do not dissipate heat very well underground and wondered what experiences others had had with this type of installation.

 

[BAHTAH]

Cable Termination

Cable Termination

I would look at using a bussed pull section for termination of the cable. That also provides the installer with some room to pull all those cables and maintain the equal length requirement for parallel conductors.

 

[wasasparky]

...but what NEC article tells you that an underground installation must be derated? I know of no such NEC requirement. ...

This may be a bit of a stretch, but I believe 310.10 would come into play if an underground ductbank is anticipated to be highly loaded for long periods of time...???

 

[bob]

This is an industrial installation and additional equipment will be added so the 2500 KVA transformer will probably be totally loaded even after assuming some diversity on the load.
NEC Appendix B discusses underground cable installation. IEEE 390 chapter 13 also discusses this. I know that Appendix B is included in the code for information and is therefore not required but I also know that the cables do not dissipate heat very well underground and wondered what experiences others had had with this type of installation.

If you are going to try to derate the cables that are in an underground duct bank, I would suggest you let someone who has access to a software package that can calculate the ampacity of your installation. As was stated the NEC does not require you to derate the cable. The tables in B310 have factors, such as Rho, Load factor and Temperature that may or may not apply to your area. How did you arrive at 18 ckts?

 

[bsh]

The load of the equipment being installed will require a 2500 KVA transformer (this may have to be re-engineered if additional equipment is installed) The secondary current of the ransformer is 3007 amps. The circuit breaker being fed by the transformer will be sized at 3500 amps so the cable must be sized for this breaker (240.4(B)). Derating for IEEE 390 for a duct bank I will need 18-500 KCM copper cables per phase. Using EDSA the results are about the same (EDSA takes more time to input the information)

 

[Ragin Cajun]

Why not use a 3000A, 100% rated breaker? Those 7 amps don't mean squat. And yes, you can down size a secondary breaker. 3000A is a standard bus rating, it will save you big bucks. Be careful of the breaker interrupting rating.

I agree on a pull section just for the incoming cables. That's what we did the last time we had a service this size. The contractor loved it!

RC