Feeding Panels

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patpappas

patpappas

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How is it possible to feed two 225 amp one 200 amp 3PH 208 panels off one 75 Kva tranformer, for me the math dones not work. I don't understand.
 
While there may not be any room for future expansion it would be relatively easy to have connected load small enough for the xfmr.

edit to add: I would not try that at home!:)
 
The connected load is what really matters. As Barbeer points out might not be of much use to the next guy who wants to add some additional loads but the code only requires that connected load be adequately served.
 
The ratings of the panels is not the important issue. What matters is the amount of "calculated" load that is connected to the panels.
 
So even though the connected load more than adds up much higher than the secondary load of the transformer as long as the secondary is monitored it is fine? Is there anywhere in the NEC that covers this.
 
patpappas said:
So even though the connected load more than adds up much higher than the secondary load of the transformer as long as the secondary is monitored it is fine?
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That is not what I said (and what Rob said just before me). The "connected" load is not defined by the size of the panel. You do not have a connected load of 650 amps here (i.e., 225+225+200). Tell us how many receptacle outlets are powered from these panels, and how many lights, and what AC or heating equipment, and what kitchen equipment (range, dishwasher, etc.), and what else gets its power from these panels. That is what comprises the "connected load." And you don't have to account for all of that being turned on at the same time. Article 220 gives you the method for determining the "calculated load."
 
It is often convenient to split up circuits so you can have a breaker to shut off a load without affecting other loads. My guess is that the reason for the number of panels is to get enough breaker space.

As long as the transformer is adequate for the calculated load, you can have as many branch circuits on it as you want.
 
charlie b said:

That is not what I said (and what Rob said just before me). The "connected" load is not defined by the size of the panel. You do not have a connected load of 650 amps here (i.e., 225+225+200). Tell us how many receptacle outlets are powered from these panels, and how many lights, and what AC or heating equipment, and what kitchen equipment (range, dishwasher, etc.), and what else gets its power from these panels. That is what comprises the "connected load." And you don't have to account for all of that being turned on at the same time. Article 220 gives you the method for determining the "calculated load."
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Computer is slow today, I did get your reply after I sent out my question, This does make sense now. I had an inspector add up breakers and tell me my connected load was higher than my feed and I stuck with it from then on.
 
If all three panels are connected directly to the secondary of the transformer (no main OPD), then the sum of all three MCB's cannot be more than 125% of the rated secondary current. (Table 450.3(B))
 
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