Transformer not used to capacity

[nathanslc]

This is the nameplate of a transformer that is to feed a piece of equipment. The secondary feeder will land on a 160 amp breaker that feeds the equipment. The engineer that has designed the install, has sized the transformer to its max capacity and is requiring feeders much larger than 160 amp. I physically do not have enough space to do this however; he insists that the conductors must be sized to the nameplate of the transformer. Call me crazy, but there is nothing that requires me to use 100% of the transformer and I should be able to size my secondary conductors off of the 160 amp breaker, correct? no other loads will be fed by this transformer. He is also insisting that the primary conductors would be sized for 270 amps which is 125% of the nameplate. But again, if Im not using the transformer to its capacity, am I required to have OCPD and conductors sized to the nameplate?

 

[nathanslc]

I just realized that is a horrible picture. It is a german Transformer 480V primary 400 V secondary. 180/185 KVA. Pri I 230A. Sec I 267A.
Schaltung Ya0. I dont know what Shaltung Ya0 is but the engineer is also insisting that the primary and secondary require a nuetral. I have no where to land nuetrals in the transformer or the equipment. He is saying that it is a YY transformer.

 

[petersonra]

I just realized that is a horrible picture. It is a german Transformer 480V primary 400 V secondary. 180/185 KVA. Pri I 230A. Sec I 267A.
Schaltung Ya0. I dont know what Shaltung Ya0 is but the engineer is also insisting that the primary and secondary require a nuetral. I have no where to land nuetrals in the transformer or the equipment. He is saying that it is a YY transformer.

if it is german it probably is Y-Y.

It likely does not need a neutral coming in but the secondary probably does need to have a neutral that is grounded.

I don't recall off the top of my head for sure, but I think the language of the code requires one to have both primary conductor protection and primary transformer protection. I don't recall there is a minimum ampacity for the conductors or a minimum rating for the OCPD on the primary.

OTOH, if the engineer wants it done this way, maybe he knows something about future needs you don't or just wants it done that way. It is his call to make.

 

[nathanslc]

if it is german it probably is Y-Y.

It likely does not need a neutral coming in but the secondary probably does need to have a neutral that is grounded.

I don't recall off the top of my head for sure, but I think the language of the code requires one to have both primary conductor protection and primary transformer protection. I don't recall there is a minimum ampacity for the conductors or a minimum rating for the OCPD on the primary.

OTOH, if the engineer wants it done this way, maybe he knows something about future needs you don't or just wants it done that way. It is his call to make.

Normally I would agree and just do it. In this case, I physically cannot install how it was designed. The end user just wants the equipment to run the way it was.

 

[DrSparks]

Normally I would agree and just do it. In this case, I physically cannot install how it was designed. The end user just wants the equipment to run the way it was.

You do not have to size the wiring to the maximum capacity of the Transformer. You size the wiring according to the ocpd and load requirements

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[DrSparks]

Just always remember this rule of thumb. Everything we do as far as service and feeder calculations as well as branch circuit selection has to do with the total connected load

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[nathanslc]

Just always remember this rule of thumb. Everything we do as far as service and feeder calculations as well as branch circuit selection has to do with the total connected load

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Thanks for the reply! So in this case, my OCPD on my secondary will be 160A and I can size my wiring on that. Working backwards from secondary to primary, would I then use a voltage ratio to get my primary OCPD and feeder sizes?

 

[DrSparks]

Thanks for the reply! So in this case, my OCPD on my secondary will be 160A and I can size my wiring on that. Working backwards from secondary to primary, would I then use a voltage ratio to get my primary OCPD and feeder sizes?

Yup but remember you can round up to the next higher standard ocpd rating if under 800a

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[mayanees]

Thanks for the reply! So in this case, my OCPD on my secondary will be 160A and I can size my wiring on that. Working backwards from secondary to primary, would I then use a voltage ratio to get my primary OCPD and feeder sizes?

You could have a problem with an energization trip if you size that primary breaker to just meet the dedicated load. But, in order to increase the breaker size you'll have to increase the size of the wire. 450.3(B) gives you as much as 250% for inrush tolerance.
Perhaps bench test the energization before wiring it permanent.

 

[DrSparks]

You could have a problem with an energization trip if you size that primary breaker to just meet the dedicated load. But, in order to increase the breaker size you'll have to increase the size of the wire. 450.3(B) gives you as much as 250% for inrush tolerance.
Perhaps bench test the energization before wiring it permanent.

Standard inverse time curve should allow for inrush current

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[sameguy]

If I recall, transformers run best when loaded to 80% &+.

 

[DrSparks]

If I recall, transformers run best when loaded to 80% &+.

They do have an optimal efficiency had a specific load. It should be in the specs

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[nathanslc]

Thanks all! I found another thread that describes my situation almost exactly.

http://forums.mikeholt.com/showthread.php?t=143652

 

[Ingenieur]

Schaltung means wiring or circuit
Ya0 is the vector group, basically the euro configuration designation

are you sure it's Ya0 ?

Y means wye primary
a = ?? (v?)
0 no phase shift, 0 deg

 

[petersonra]

Yup but remember you can round up to the next higher standard ocpd rating if under 800a

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only on the secondary side.

 

[electrofelon]

he insists that the conductors must be sized to the nameplate of the transformer. Call me crazy, but there is nothing that requires me to use 100% of the transformer and I should be able to size my secondary conductors off of the 160 amp breaker, correct?

You do not have to size the wiring to the maximum capacity of the Transformer. You size the wiring according to the ocpd and load requirements

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True for <600 volt transformers, but note 215.2(B)(1) requires feeder to be greater than or equal to namplate capacity. OP, You might point him toward 215.2(A) which has no such requirement.

 

[electrofelon]

Yup but remember you can round up to the next higher standard ocpd rating if under 800a

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Actually, I dont see that there is an 800 amp limit for use of the next size up rule for transformer protection.

only on the secondary side.

I can use next size up if I am using 125% primary protection, but not if I am using 250% primary (with secondary protection).

 

[Jraef]

"Schaltung" just means "circuit", Ya0 is the transformer "vector group" designation, an IEC standard numbering system we don't use here (it comes from the very organized Germans).
Ya0 vector group describes the relationship and configuration of the primary and secondary.
"Y" means the primary is a wye (star) configuration,
"a" means secondary is an autotransformer (so is the same as the primary, wye)
"0" means the phase displacement between primary and secondary is zero degrees,

The 0 seems obvious if it's an autotransformer, but those Germans like to be consistent...

So you have a Y-Y autotransformer, there is probably no "neutral" connection terminal for it either, but if there is, it's only going to be one.

 

[Ingenieur]

if it has a neutral it will have an N or n (prim/sec) after the winding designation

eg Dyn0

 

[Jraef]

if it has a neutral it will have an N or n (prim/sec) after the winding designation

eg Dyn0

Yep, you are right. So there will be no Neutral connection on that transformer, and as an autotransformer, I don't see why it would anyway.

Back to the original question about capacity, there is nothing saying you MUST size protective devices for anything MORE than the intended load. It says you CAN, but not MUST. the NEC only says it MUST NOT be any larger than a certain value. So if you are only going to use 160A off of that transformer, you can use a 160A breaker if you want to, so long as that transformer is capable of 160A or more.