Two 208/120 Secondary Feeds to Same MDP

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eeee

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I have a facility with a MDP that is receiving two wye connected 3 phase secondary feeds in to this MDP from three single phase transformers, 100KVAea, 12KV primary Wye to 208/120 volt secondary wyes.

I have never seen this before. Usually a 3 phase transformer bank may feed one of it's secondaries to one building and another secondary to another building, but not to the same MDP in the same building.

Why was this done??
 
Re: Two 208/120 Secondary Feeds to Same MDP

Is this a parallel feed to the same buss bars? Or are you saying two seperate sets of buss bars?
 
Re: Two 208/120 Secondary Feeds to Same MDP

It sounds like parallel conductors to me. That's pretty common for larger services. With (3) 100KVA transformers, you would have a 830 amp capacity at 208V. It would be very difficult to get all that through one set of wires. A reasonable upper limit for a single set of wires is usually at about 500KCM or 600KCM at about 400A.

Or am I completely misunderstanding your question?

Steve
 
Re: Two 208/120 Secondary Feeds to Same MDP

Is this a Main-Tie-Main setup? This is where there are 2 sets of busbars that are able to be connected together through a Tie breaker or switch. There is a main on each set of the busbars so that they can be fed by either or both transformers when the tie breaker is closed.

In normal operation, tie open, each set of busbars runs independently.

Might this be what you have?
 
Re: Two 208/120 Secondary Feeds to Same MDP

The second secondary is tapped off of the first secondary just before the service entrance. One set of four 500MC feed in to a tap point where then two sets of four 300MC feed the MDP. There is only one main breaker in the MDP and the drawings appear to show only one bus bar. Only one secondary carries all of the current for almost the entire 300 foot run.

I therefore must assume it is a single bus bar since there is only one main 800 amp breaker in the MDP. I also can not assume the second secondary is used for carrying a large amp load since only one secondary carries the current most of the way.

I will need to get an appointment to shut the power off to the building to look inside. Existing drawings do not indicate enough information.

I believe the second secondary is used for future growth maybe should another bus bar and circuit be desired in the future???
 
Re: Two 208/120 Secondary Feeds to Same MDP

If it is parallel feeds, then it better have two bus bars jb I would think. I'll know more later when I get inside the MDP or ask an electrician that knows this panel.
 
Re: Two 208/120 Secondary Feeds to Same MDP

Originally posted by eeee:
If it is parallel feeds, then it better have two bus bars jb I would think. I'll know more later when I get inside the MDP or ask an electrician that knows this panel.
Click to expand...
If it is a parallel feed, then they need to go to the same set of buss bars. not seperate ones.
 
Re: Two 208/120 Secondary Feeds to Same MDP

JB,

I wouldn't see any advantage for one secondary to run 280 feet simply to tee off and become two 3 phase wye overhead secondaries feeding a set of bus bars connected together in a MDP. You are not getting more amp carrying capability as the previous responder stated since only one 3 phase secondary carries the load for the majority of the run. I could see more amp carrying capacity if it were two secondaries emanating from the 3 phase transformer and then entering the MDP on a set of bus bars connected together.
 
Re: Two 208/120 Secondary Feeds to Same MDP

conductors run in free air have a different ampacity than conductors run in conduit.

conductors controlled and owned by a utility are not governed by the nec. the utility engineers decide the size.

if the demark is at the junction that you are questioning. then parallel feeds may be needed by the building owner, that the utility did not need.
 
Re: Two 208/120 Secondary Feeds to Same MDP

Thanks JB. The demark for the utility is probably at the transformer or substation. This is a secondary off of the transformer as I am sure you know.

You have given me some ideas. The junction I am talking about is just above the ceiling of the serive entrance. Two weatherheads are there emanating from the split where the four 500MCM cables are converted to four 300MCM cables each having a weatherhead and separate conduit entrance in to the MDP. The drawings do seem to indicate one bus bar though, not two bus bars connected together in the MDP.

It might have been done for a reason of technicality.
 
Re: Two 208/120 Secondary Feeds to Same MDP

One set of 500's is not rated for 800A :confused:
But neither is 2 sets of 300 :confused:

JB has a couple of ideas why there might be a change from 500 to 300's. I might also guess that either the 500's or 300's were a replacement for something older that was demolished.

Steve
 
Re: Two 208/120 Secondary Feeds to Same MDP

It is actually a 700 amp breaker, but the panel is rated for 800 amps I noticed after another survey.
 
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