# Transformer Primary protection



## billymac93 (Apr 4, 2015)

Don't know what the NEC says, but here in Canuckville, we wire the tranny based on tranny specs. Fusing code gives max. fuse size. You can choke it down from there. I'd make sure the wire is at least minimum size for tranny. What happens if someone sees undersized fuses, checks the tranny KVA and then installs fuses bigger than the wire's rating.


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## frenchelectrican (Mar 15, 2007)

rossi32s said:


> Have a 34KVA 208D to 480Y that with a 480 load @ 15A
> 
> Can I size and protect the primary using definite load calculations or do they have to be sized based on the transformer? Secondary will be protected using a fused disconnect.


I wrote a underline on that number please clear that one up.,,

it is a 35 KVA or 3.5 KVA ?? I never heard a 34 KVA so I was hopeing ya say 35 KVA.

and you have to becarefull with secondary netural due you say 208 Primary and 480 WYE so pay attetion to egc and bonding connection.

I will just size it to the nameplate rating because with inrush current if you used undersized fuse or breaker it will trip out when you heat it up. 

Yes.,, Ya can put OCPD on secondary side too.


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## rossi32s (Jan 6, 2009)

frenchelectrican said:


> rossi32s said:
> 
> 
> > Have a 34KVA 208D to 480Y that with a 480 load @ 15A
> ...


Secondary netural is bonded to enclosure and direct to ground rod on continuous ground wire. 3 conductors in NM from panel to primary. Hung a fused Disconnect on side of tranny. Inrush is around 30A. I sized primary for 40A for now, secondary is 15A fused, load is 10A.


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## frenchelectrican (Mar 15, 2007)

rossi32s said:


> Secondary netural is bonded to enclosure and direct to ground rod on continuous ground wire. 3 conductors in NM from panel to primary. Hung a fused Disconnect on side of tranny. Inrush is around 30A. I sized primary for 40A for now, secondary is 15A fused, load is 10A.


Ahhh I see that now after you posted the photo.,,

IMO the primary OCPD is too small for that size of transfomer because I just caluated the full load on 34 KVA I know 35 is very close to that number and with full load it will draw in about 91 amp on primary side but you say inrush is about 30 amps .,,

I know most 35 KVA's the inrush useally nail in about 115 to 175 amp inrush that basied on 208 side it will be little higher for real breif time.


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## rossi32s (Jan 6, 2009)

I'll bump up the wiresize and leave the OCPD as is.. It's a temp install so I can demo a machine for a week. I'm not seeing more than 35A inrush and when machine is running I have 10A on secondary and about 25A on primary


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