electrical service 4

[Reesh14]

I am working on an electrical service upgrade at an administration building. The power utility company is scheduled to provide 800A, 120/208V, 3phase service. An 800A MDP is being installed and will feed the other panels in the building. After design development, a server room was added and it increased the load approx. 250A. A 400A panel w/ shunt trip main breaker will be installed in the server room and serve all the IT equipment and HVAC equipment. The MDP does not have anymore breaker space nor can fit a 400A breaker. The plan is to provide a 400A enclosed breaker and tap into the main lug of the MDP or into the incoming service. I am assuming for now that the increase in load for the server room does not require us to increase the incoming service. The contractor suggested to tap the feed ahead of the main. Right now the plan is to use 2 parallel 500 sized wires from the utility transformer and into the MDP. I am not sure of how this should be tapped. If the server room panel should tap into the main lug of the MDP or at the incoming service (ahead of the main), will the 500's be enough to handle this or should they be increased to parallel 750's? I am not familiar with how feeds can be tapped and at what point this should be done. Any help would be much appreciated, thanks!

 

[jraef]

Several issues involved here that I see off the top of my head.

1) Are the main lugs on the MDP rated for being tapped off anyway? They might not be, so that would limit your choice.

2) Without information on voltage drop calcs, insulation, run method etc., it's not possible to remotely determine ampacity requirements for the cables. A decent rule of thumb however is if in doubt, upsize it. Whoever pays for the wire may not like that though.

3) Assuming you are in the US (120/208V service), read the applicable sections of the NEC (Article 230 I believe) regarding multiple service drops. I'm too lazy to search for it right now, but as I recall the NEC only allows multiple service drops into a single location under a somewhat narrow set of rules, none of which appear to apply by what you described. You may still be able to do it however, under the "by special permission" clause, which means at the whim of the AHJ, and your electrical contractor is probably already familiar which what they will want. The best course IMHO is to just call your AHJ and ask!



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[l3city]

Hi there!, I recommend consulting this with the Utility Co. In my opinion, they shall now what is being done with their incoming service entrance cable/wire. The Util. Co. might have special rules, and as well (as "jraef" stated), the NEC provides you with info on tapping rules and service entrance sections. Regards

 

[EEJaime]

You state that the current planning is to run parallelled 500kcmil conductors "from the utility transformer to the MDP", well if this is indeed the utility companies' secondary service drop, then it is probably not metered at this point and you will not be allowed to tap into it.

Your tap point must be downstream of the metering devices and tapping the distribution buss is one way of accomplishing this. The possibility of doing this depends on buss location, wire bending space, space for additional lugs, etc....

I might suggest however that perhaps a single 400A feed to single panel which feeds BOTH-IT and HVAC loads may not be the best approach? We go to many lengths to isolate motor loads from panels feeding IT equipment. If at all possible could you split this into two feeds for which you could fit two 200A shunt trip breakers in your MDP?

Did you state that the 800A MDP cannot accomodate a 400A breaker, even if there were space? That seems difficult to believe. If that is in fact the case however, then you are left with a somewhat expensive fix if tapping the distribution buss does not work out. You will need to look at adding distribution capacity by addition of another distribution panel that can take a 400A c/b, moving some of your existing feeder c/b's to this to make room for tapping and extending feeders to the new panel. Re-working feeders to accomodate the relocated c/b's etc.... Messy and expensive, but that is what happens when pre-planning fails.

 

[VAElec]

I would consider a tap box of some sort after the meter and ahead of the MDP. Check NEC 240.21(B) for the 10 foot tap rule. Size the feeders for the MDP and Server Room Loads for the combined load and then tap off the feeder for each individual load. The disconnecting means for each becomes a service disconnect so bond accordingly. As long as you don't have more than six disconnecting means (not counting a remote fire pump disconnect) you should be kosher regarding the NEC.

Also worth checking, who owns the transformer secondary? Makes a difference on sizing the secondary feeders.