trasnformer&load calc

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bill@usps06492

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I need to size a transformer(kva), ocp, and panel. Pr@480 3ph to sec@ 120/208 3ph. I calced it out to be a 45kva@60a on the pri and that would allow me a 125a panel @120/208 on the load(sec). 60ax480vx1.73/1000=49.824kva. On the load side 45kwx1000/208vx1.73=125a. Am I on the right track? This is a dry type xf installed indoors.
 
Re: trasnformer&load calc

@ 45 kva Pri. 480v.... Sec. 208
Pri. amps 54..... Sec. 125amps
Pri. OCPD @250% 150 amps.... Sec. OCPD @125% 175 amps.

[ January 13, 2006, 04:34 PM: Message edited by: Jhr ]
 
Re: trasnformer&load calc

Though 250% (with proper larger wire) is usually legal on the primary, you will probably find that 70A or 80A pri. OCP is much more economical & will handle inrush just fine.

I agree that 175A is a fine choice for the sec. as it has the guts to carry the full rated 100% 125A capacity of the xfmr sec.
 
Re: trasnformer&load calc

Though 250% (with proper larger wire) is usually legal on the primary, you will probably find that 70A or 80A pri. OCP is much more economical & will handle inrush just fine.
Click to expand...
But then you would not be code compliant according to T-450.3(B)NEC 2002, my 2005 is in my van, and its cold outside
 
Re: trasnformer&load calc

But then you would not be code compliant according to T-450.3(B)NEC 2002, my 2005 is in my van, and its cold outside
Click to expand...
Why not?
Actually pri 54 amps x 1.25 = 60 amp breaker. Unless you know of a problem no need to go to 250%. Don't forget 240.21C
 
Re: trasnformer&load calc

I see how you came up with the pri ocpd 54ax250%=135a sized up to 150a or a ocpd on the pri set @ 60x250%=150a, but I don't see where you are getting the 175a on the sec. 125ax125%=156a, are you sizing up the ocpd on the sec to 175a? Per art240.4(B)&(F)and per art240 can I use the ocpd on the pri to protect the panel on the sec side?
Bill
 
Re: trasnformer&load calc

Bill you really have separate questions.

Protection of the transformer.

Protection of the conductors

Protection of the panel.

As for the 45 KVA transformer, I agree jhrs first response.

480 Primary could be up to 150 amps.

208Y/120 Secondary could be as high as 175 amps.

Those are the maximums and nothing says you have to go to the max.

A 100 amp 480 breaker will be most times much cheaper than a 175 amp breaker.

You said you want a 125 amp panel, if you keep the panel close to the transformer (secondary conductors less than 10' long) the panel and the secondary conductors could all be protected by a 125 amp main breaker in the panel.

So it seems reasonable that you could use a 100 amp primary breaker with 3 awg copper conductors.

On the secondary side you could use 1 AWG conductors suppling a panel equipped with a 125 amp main breaker.
 
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