Three phase transformer and secondary panel single phase

hhsting

Senior Member
I have three phase 50kva transformer 480V delta primary and 208/120V secondary. Please see the transformer link below


The secondary panelboard provided is 240/120V single phase.

The designer has brought from the transformer secondary two legs two phase wires and one neutral wire and one equipment grounding conductor to the secondary single phase panelboard. The third leg/phase terminal of the transformer is left open nothing connected. Would this be NEC 2014 code compliant or not?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector
Looks like he sized the OCP based on a 50kva single phase.. That combination of OCP would not be correct in only loading 2 phases.

Before we fret thru 25+ posts, why don't you go back to whomever spec'ed the transformer and see if they made a mistake.
 

hhsting

Senior Member
Looks like he sized the OCP based on a 50kva single phase.. That combination of OCP would not be correct in only loading 2 phases.

Before we fret thru 25+ posts, why don't you go back to whomever spec'ed the transformer and see if they made a mistake.
I did ask before to provide the transformer he is using and post #1 link provided and plans show single phase panelboard secondary. However will ask again.

I am just curious how else one size the breaker correctly?
 

hhsting

Senior Member
See table 450.3(B). The conductors need protection but that breaker also, per 240.21(C)
So primary I have 480V three phase 50kva gives 60.1A. 100A primary Breaker sized is 166% so second row of table 450.3(B)

Secondary I have 208V single phase 50kva so that gives me 240A next size would be 250A breaker but 200A can also work table 450.3(b)



Am I correct or incorrect? What am I doing wrong?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector
Your numbers don't take into account that the 3 phases will not be equally loaded. Your three phase transformer is more like 3 single phase transformers and you are only loading two of them.
I re-checked my math and it.s possible with a 200 amp breaker it will meet Code but we need opinions from more knowledgeable folks here so stand by.
(It is, however, a bad idea in any event..he won't get 240, only 208 and if that's no problem, simply by going to a 3 phase panel he can get 1/3 morepower)
 

hhsting

Senior Member
Just want to clarify, in my previous post by "not going to work" I meant with the OCPD sizes as given. If OCPD sizes were changed to meet transformer protection and secondary conductor protection it would be code compliant.
How so? 200A secondary size is less than 125% thats what i get. Primary can be 250% max second row and is 166%. Can you please elaborate where do you see 125% max?
 
Last edited:

Dsg319

Senior Member
Location
West Virginia
Occupation
Journeyman Electrician
How so? 200A secondary size is less than 125% thats what i get. Primary can be 250% max second row and is 166%. Can you please elaborate where do you see 125% max?
I’m not a 100% sure but I’m thinking maybe they are saying you still need to size secondary protection with 3phase calculations 50kva/208(1.732) rather than single phase calculation.
If I’m incorrect someone chip in.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
How so? 200A secondary size is less than 125% thats what i get. Primary can be 250% max second row and is 166%. Can you please elaborate where do you see 125% max?
480V 50kVA 3 phase transformer nominal current is 60A.

208V nominal current is 139A.

100A primary OCPD is 166% and is acceptable if secondary OCPD is 125%.

Secondary OCPD would need to be 175A to be kosher.

Jon
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Is this in code anywhere or not compliance with code? Otherwise i cannot enforce it
The transformer is designed for up to 16.66 kVA load on each of 3 phases. The load could be zero on one of those phases.

Maybe there is an energy code violation because of extra losses, but not an NEC violation. Customer wants to buy a 50 kVA transformer and hobble it to only use 33kVA? It's their money.

Jon
 
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