Three phase transformer and secondary panel single phase

[hhsting]

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

V48M28T5016 | Eaton General purpose ventilated transformer | Eaton

V48M28T5016 - Eaton General purpose ventilated transformer, Fungus proof, AL windings, DOE 2016, NEMA 2, 220°C, 50 kVA, Floor/wall mnt upright only, 3-phase, PV: 480V delta, SV: 208Y/120, NEMA ST-20, 2 at +2.5%, 4 at -2.5%, 150°C, 50 kVA

www.eaton.com

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?

 

[zbang]

Kinda silly, but I can't see a reason it would be compliant. Beware that the neutral is a current-carrying-conductor in this case (because there will always be some unbalance current).

 

[augie47]

compliant if properly protected. What size is the primary and secondary protection...

 

[hhsting]

compliant if properly protected. What size is the primary and secondary protection...

Primary breaker is 100A and secondary breaker is 200A.

 

[augie47]

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]

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?

 

[electrofelon]

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?

See table 450.3(B). The conductors need protection but that breaker also, per 240.21(C)

 

[hhsting]

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]

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)

 

[electrofelon]

Not going to work. Because your primary protection is over 125%, you are in the second row and the secondary can't be over 125% plus next size up.

 

[LarryFine]

In my opinion, either a 3ph panel or a 1ph transformer should be used. You're not going to get 240v with what you have.

 

[electrofelon]

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.

 

[hhsting]

In my opinion, either a 3ph panel or a 1ph transformer should be used. You're not going to get 240v with what you have.

System voltage secondary side i am looking for is 208V. The single phase panelboard is rated 120/240V. I dont think panelboard come in 208/120V

 

[hhsting]

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?

 

[hhsting]

The transformer is listed for 3 phase using it in secondary single phase panelboard would it be 110.3(b) compliance?

 

[LarryFine]

System voltage secondary side i am looking for is 208V. The single phase panelboard is rated 120/240V. I dont think panelboard come in 208/120V

Correct.. I would recommend a 3ph panel.

 

[hhsting]

Correct.. I would recommend a 3ph panel.

Is this in code anywhere or not compliance with code? Otherwise i cannot enforce it

 

[Dsg319]

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]

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]

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