Additionl KVA to exisiting building

[dougjl]

I have a 2500 KVA transformer (23k/480 from utility) supplying 480V to a switchboard located inside the plant. I am adding a significant load to the plant and would like to increase the KVA of that transformer to 3750 KVA the largest 480 transformer supplied by my electric company. By doing this I raise the secondary current from 3000A to 4500A. Since the conductor size coming into the plant (4 X 1000 kcmil) will not cover the addtional current I will have to add 2 more 1000 kcmil conductors from the service to my switchboard.

I am considering installing a new switchboard to handle the new load. Will NEC code allow me to simply feed this new switchboard from the 2 addtional 1000 kcmil feeders and leave the exisiting installation as it is? Or would it be better upgrade the mains of the exisitng switchboard to handle the addtional current?

Opinions and comments please.

Thanks
Doug

 

[rbulsara]

Did you talk to your utility company?

They normally will not provide a transformer bigger than 2500kVA not would they install fan cooling.

You will be much better off, and less expnsive too, adding a second service and new switchboard.

Plus switchboard rated more than 4000A is an unsual animal, not mention problem you will run into to manage short circuit ratings. Also think of the construciton phasing problems.

 

[dougjl]

The utlity company is proposing upsizing the transformer to 3750KVA. This is why I am asking about it and trying to determine the best most efficient way to add the required power.
Thanks,
Doug

 

[dpc]

I agree with rbulsara - I would not want a service transformer that large. The available fault current to the existing equipment will be increased by quite a bit and this must be evaluated.

As for the legality of adding a second set of service conductors - since the transformer belongs to the utility, I think this would probably meet the letter of the NEC, provided the new feeder is outside the building.

The utility may prefer to simply install a larger transformer because this is the simplest solution for them.

 

[rbulsara]

Upgrading to 3750 kVA will increase the available SCC to 78,000A from say 52,000A for a 2500kVA transformer (assuming infinite utiltiy bus).

Verify the SCC rating of the existing switchboard, since it has to handle that too.

 

[rlpuck]

perhaps you can consider leaving the existing transformer as is and install a new seperate transfomer and new board? just a suggestion.

 

[peebee]

I do not believe that there is any UL-listed equipment available rated for over 4000 amps.

 

[advidana]

It not a good idea. Contact an engineering firm and have them design you several options with your power company. I agree with the rest of the comments.

 

[dougjl]

thanks for the comments, the concern about SCC is one I will have to take a closer look at. At this time I belive my switchboard can handle 65KA and the SCC currently available from the utility service is 37KA. I do not yet know what the SCC will be if the utlity upgrades the transformer. Assuming that the old gear can handle the revised SCC then what issues do I face by adding a a second switchboard. This would allow me to leave the old system as is and distribute the new power via another switchboard. As best as I can tell from code I am allowd to have the service go to as many as 6 fuses or disconnects.

As for UL ratings and gear available for the increased power that is not a problem:
QED-6 POWER-STYLE® Switchboard Class 2746
Switchboard ratings up to 150 kA Short Circuit Current Rating for services 1600 A to 5000 A.

A number of you have commented that it is not a good idea. Other than increased SCC what engineering reasons do you have to dismiss this arrangement?

Thanks,
Doug

 

[DanDel]

dougjl, check out NEC 240.21 completely. You may not be able to run two switchboards from one transformer without separate overcurrent protection at the transformer secondary for each circuit.

 

[jbartos]

Suggestion: Normally, the Utility will increase the size of the transformer and add current limiting fuses to limit the short circuit current. Actually, by adding the current limiting fuses, the existing short circuit level will be lowered. Therefore, the Utility approach is the standard Utility approach. The Utility probably has many places as yours and a good track of record or experience with what they are doing.
Alternately, additional transformer may be considered to have two transformers and two radial power distributions downstream. This works well if there is enough space for the second transformer and the larger kVA design margin required.

 

[dougjl]

DanDel,
I read Acticle 240 section VIII 240.92-B as allowing this since this installtion falls into the category of Supervised Industrial Installation. Would you not agree?

Thanks for your thoughts,
Doug

 

[BJC]

dougjl
For a service that big I would go with 2- 2,500 kVA transfromers. In a new design I would go with a double ended switchboard with a main-tie-main arrangement.
2,500 kVA transfromers are almost "off the self" if you need a replacement. If one should fail you can overload the other for a short while ( maby a long while depending the load cycle and the ambient temperature ).
Check the lead time on getting another 3750 vs getting a 2500.
You didn't say how much room you have but you could add a second switchboard with a cable connect to your existing unit to make a main-tie-main arrangement.

 

[peebee]

I'd try to avoid current limiting fuses if possible. Assuming you're currently using air power breakers, the installation of CL fuses would increase the frequency/liklihood of power outages.

You ARE going to get a licensed electrical PE to help you with this, aren't you? This is not exactly a Home Depot weekend warrior DIY kind of project.

 

[dougjl]

peebee,
What engineering are you basing your statements on? Do you have some test data that shows current limiting fuses are unreliable? And as for your home depot reference I am an electrical engineer and work with an electrical PE. I come to this forum for ideas and caveats that others have experienced not for you to imply I am incapable because I ask a question. If you have nothing nice to say.....

Doug

 

[peebee]

Re. current limiting fuses: The whole point of these is that they will trip out first on a large overcurrent fault, before downstream OC devices sustain damage. The whole point of air breakers is that they will try to stay connected through a fault, maintaining power to the facility, until a downstream OC device can clear the fault; this is known as coordination. CL fuses will NOT coordinate with downstream devices, they go first. Currently, assuming you're using air breakers, if you experience a fault it's likely that most of your facility will stay up and running. If you go to CL fuses, though, and experience a fault, it's likely that you will drop ALL LOADS fed from the switchboard.

Re. PE assistance: I'm not the only one here so far that's suggested you contact an engineer for assitance on this project. I'm glad to hear that you are in fact working with a PE. The motivation for my statement was not to slight you. The motivation was SAFETY. It's clear that you do not have a lot of experience with such installations, and that's OK, and questions are welcome. However, your questions gave the strong impression that you were trying to wing this on your own. It's not about being nice, it's about keeping people from dying and buildings from burning down.

 

[dpc]

In addition to problems associated with increased fault current, the 3750 kVA transformer will be something of a white elephant - very difficult to replace if it fails. A 2000 kVA or 2500 kVA unit would be much easier to swap out if necessary.

When computing the fault current, you need to take into account motor contribution as well as utility source current. With a common 480V secondary, you will see motor contribution from the entire 480V system.

Reliability will be sacrificed if you don't have main breakers on each bus. A bus fault on either bus would result in complete loss of power. Main breakers at this rating are extremely expensive, so most of these massive secondary systems don't use them.

It may be that in your particular situation, your proposed solution makes sense when all factors are considered.

I'm not convinced that there would be any code violations in the installation you are proposing. But that doesn't necessarily make it a good idea.

Good luck.

 

[rbulsara]

dougjl:

As dpc said, feeding two boards from directly from one transformer without OCPD is not a code violation, as long as the service feeders fall under definintion of 'outside' building and properly protected from physical damage.

Plus having multiple service is not a code violation either. But these two issues are independet of each other.

As for the uses of fuses, I beleive the discussion about CL fuses is misplaced here. A regualr Class L fuses will make the main switch rated for 200,000AIC, provided the bussing is braced for available SCC (say 100,000AIC in this case). Also most regualr breakers are UL listed (ask the mfrs.) for series rating of 200,000AIC when used in series with Class L fuses. This again does not necessarily guarantee coordination.

I kinda agree with peebee for his commens on CL fuses, but that is not the issue here. Also I beleive UL does not recognize using CL characteristics to enhance SCCR or limit SCC. It however lists series ratig after successfully testing two components in series.

 

[DanDel]

dougjl, can I then assume that you are relying on 240.92(B)(2)(4) - 'engineering supervision', than the previously mentioned '6 fuses or disconnects' which is mentioned in 240.92(B)(2)(2), but which restricts the multiple overcurrent devices to be: 'mounted in a single enclosure, in a group of separate enclosures, or in or on a switchboard', effectively limiting the devices to one switchboard instead of two, as you mentioned?