480V to 120V

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EEPithon

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Need to figure out the correct sizing for a transformer that is stepping down 480V to 120V. Feeding 20 new circuits that are about 20A each. Need to size 120V panel for this. Not sure the regulations on sizing panels and transformers that follow code.
 
EEPithon said:
Need to figure out the correct sizing for a transformer that is stepping down 480V to 120V. Feeding 20 new circuits that are about 20A each. Need to size 120V panel for this. Not sure the regulations on sizing panels and transformers that follow code.
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There is a whole chapter on calculating feeder sizes. Chapter 215 I think.
 
Way too many "gotchas".... :)
You might want to verify the loads..do you want a full 20 amp capacity on all 20 circuits ??
Is your 480v supply 3 phase or single phase ?
Normal secondary would be 240/120 or 208/120. Is there any reason you can't use that ?
 
augie47 said:
Way too many "gotchas".... :)
You might want to verify the loads..do you want a full 20 amp capacity on all 20 circuits ??
Is your 480v supply 3 phase or single phase ?
Normal secondary would be 240/120 or 208/120. Is there any reason you can't use that ?
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Each breaker will feed 20A loads.

480V is 3 phase.

Going to to use 208/120 but need to properly size the kVA for the transformer. Also want to size the right amp rating for the 120V.

I'm new to all this so this question might be silly.
 
Using 21 circuits (divisible by 3), 21 x (20 x 120) = 50.4 kva.

Now, decide what kind of continuous loading, etc., you need to figure in.
 
Since 3 phase is available, the "usual" transformer would be a 480-208Y/120 with normal sizes of 45 and 75 kva. As Larry noted, if you truly have a full 20 amp load on all circuits (rare) it would be a 50kva load. A 75kva would provide power for 30 fully loaded 20 amp circuits (noncontinuous loading) or 25 20 amp continuous load circuits.
 
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LarryFine said:
Using 21 circuits (divisible by 3), 21 x (20 x 120) = 50.4 kva.

Now, decide what kind of continuous loading, etc., you need to figure in.
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The loads are not continous. All loads (each breaker) have 3 pieces of equipment that will never be used at the same time and would only be used a few minutes at a time maybe a couple times a day.
 
augie47 said:
Since 3 phase is available, the "usual" transformer would be a 480-208Y/120 with normal sizes of 45 and 75 kva. As Larry noted, if you truly have a full 20 amp load on all circuits (rare) it would be a 50kva load. A 75kva would provide power for 30 fully loaded 20 amp circuits (noncontinuous loading) or 25 20 amp continuous load circuits.
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How to do calculate the difference between continuous vs non continuous? All loads are not continuous, but FLA if all equipment was running would be 20A. This is very unlikely to happen though. I also want to make sure it is big enough for any extra loads to be added to the additional 10 breakers that will not currently be being used.
 
EEPithon said:
How to do calculate the difference between continuous vs non continuous? All loads are not continuous, but FLA if all equipment was running would be 20A. This is very unlikely to happen though. I also want to make sure it is big enough for any extra loads to be added to the additional 10 breakers that will not currently be being used.
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from article 100.

Continuous Load. A load where the maximum current is
expected to continue for 3 hours or more.
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