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#199645 03/03/11 07:14 PM
Joined: Aug 2001
Posts: 329
S
Member
Ya'll will love this set up. In one of my schools I have a 208Y120 3 phase panel with a 100A main. Whenever we loose power this main trips when the power comes back on. It is fed from a 75kva xfmr about 4 feet from it (that calculates to 208 amps max). At some point in the past this was a 45 kva xfmr but it was replaced to handle the two new panels they put in the room about 4 feet to the other side of the new 75 kva xfmr. To feed these new panels they ran a pipe from the first panel to the second and an offset nipple between the second and third. Now the good part ... they attached the feed to the bottom of the bus bars in panel #1. The second panel also has a 100 amp main and #3 is fed from a 3P60 in panel #2. Needless to say we now have 3 panels controlled by the 100 amp main. As per my amprobe the load is ~85-90amp steady and they couldn't figure out why it trips when we slam 90 amps (120 circuits) on at the same time thru a 100 amp main. BTW: before the second and third panels were added, it never tripped.
Now the question: I can find no code violation in removing the feed between the panels and sending 100 amps from the xfmr to (each of) panels 1 & 2 directly. (100a to panel 1 and 100a to panel 2, leave 3 as a sub of #2) That would not exceed the 75kva and would take 1/2 the load off the main in panel 1. Assuming we size the conductors correctly can anybody tell me of any violation here?

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Joined: Apr 2002
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Steve:
From my understanding of your post, I see no issues with what you propose. Double barrel lugs at the transformer secondaries, proper conductor size, and you're good to go.


John
Joined: Jan 2005
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Cat Servant
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I'm not sure of what it is you're trying to accomplish. Are you trying to prevent the main from tripping? If you are, you're on the wrong path.

Try disconnecting / turning off ALL of the secondary loads while the transformer is unpowered. Now energize the transformer - with NO load on it. You'll probably hear a big THUNK the moment you return power to the transformer, and you might trip the main anyway.

That 'thunk' is caused by the current inrush. Only after all the magnetic fields have stabilized does the current drawn by the transformer level off. This might take but a second or two, but that could well be enough to trip the breaker.

Your solutions are to use a bigger breaker, a breaker with a different trip curve, or change the 'instant trip' setting of the breaker (all depending on code rules and your specific equipment).

Apart from aging equipment, it's also likely that the transformer is fully loaded now - maybe even undersized. Those are other issues to examine.

Joined: Apr 2002
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Reno:
I think (thought) that he is tripping the secondary main @ 100 amp?



John
Joined: Jan 2005
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Cat Servant
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Maybe I did misunderstand.

Joined: Apr 2002
Posts: 7,333
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Or, maybe I did? Time will tell.


John
Joined: Aug 2001
Posts: 329
S
Member
Sorry for not getting back. The tripping, as far as we can tell is definitely inrush overload. The recorder shows almost 200 a on inrush. We do "assume" that is what's tripping the 100A main. With 2 separate mains we're hoping to split the inrush into 2 parts, each going thru a separate 100A main. Hopefully this will get it to a manageable level so neither main trips.


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