Help Size XFMR and Temp Panel

[KD4315]

I was estimating a job last week that had some temporary power requirements. We excluded it but for my own knowledge I'm curious about calculating this service.
The existing site has 208 3 phase power to it. The GC's man lift and crane operate on 480. There is a 1000A 208V Temp board on site. The crane needs 400A service and the man lift needs 200A service. How big of a transformer and temp panel do I need to run both of those?

 

[copper chopper]

i believe a 500 kva xmer would be sufficient.. along with a 600 amp mb panel with a 400 and 200 amp breakers. I dont know what you will be feeding the primary with as far as breaker or disc.?

 

[templdl]

Compare the KVA of the 208v service entrance with the kva required for the 480v load to assure that you have enough KVA at the service entrance first.
KILOVOLT-AMPERES (kVA) = (VOLTS x AMPERES x 1.73)/1000=KVA

Your 208v. 3ph, 1000A service entrance should be 360KVA and your 480v, 3ph 600A load should be just short of 500KVa which means that you appear have a 140KVA problem.

 

[KD4315]

I figured a 500KVA transformer but I wasn't sure about the 600A panel. If the man lift and crane are being used at the same time will the 600A be enough?

 

[templdl]

KD4315
Simply use the formula that I gave you and you should find what the KVA required for each load will be. You omitted that both loads would not be used at the same time and as such if you could assure that both loads aren't going to be used at the same time then it would. Are you going to use some type of interlocking systim to assure that they can't? I don't know if an electrical interlock is acceptable but it is possible to use a N/C interlock that should be included in the crane contactor and the man life contactor, wiring it in series with the coil in the other control. If one contact is energize it would prevent the coil of the other contactor from being energize.

 

[hurk27]

Basicly a 350kva transformer is all your going to need since you are not going to be able to run both the manlift and the crane at the same time, pick the largest load of the two which is the crane and size from that, if other 208 volt loads are going to be used then you might even have a problem running the crane because it alone will use 923 amps of the 1k amp/208 service and that only leaves you 77 amps for other loads. maybe they need to check into another crane with its own prime mover for power?

 

[jusme123]

...he never stated they would not be used at the same time. It sounds like a construction site that has a man lift and a crane that require power, I have never been to a site where the crane and manlift could not be used at the same time, as time is money.

 

[templdl]

jusme123
That's why you CYA with some sort of interlock to assure that both can't be used at the same time. You know damned well when you tell them that they need to increase the size of the service in order to supply both loads simultaneously that they will say that both will not be used at the same time and you know that they will.