Transformer secondary - parallel OCPD question

[Pharon]

Is there anything in the NEC that prohibits multiple parallel OCPDs on the secondary of a delta-wye, non-service transformer?

In other words, let's say I have a 45kVA, 480V delta-208/120V wye transformer in a building, with a 70A primary OCPD, and want to feed four 42-pole 150A MCB panels, with each parallel, daisy-chained 150A MCB acting as the secondary transformer OCPD. Is this allowed?

Or would I have to install a dedicated secondary 150A enclosed circuit breaker OCPD and then fan off from there to each 150A OCPD?

 

[augie47]

Hopefully I understand.
As long as your primary is protected at 125% (as allowed in 450.3(B)), you can have multiple secondary feeds to multiple panels as long as each feed meets the requirements of 240.21(C)
Four sets of 1/0, one set to each of your panels would be permissible.

 

[Pharon]

So if I did 250% OCPD on the primary (70A) and 125% OCPD on each of the four MCBs 150A, that wouldn't be allowed?

And yes, let's assume all secondary taps are 100% (150A).

 

[augie47]

So if I did 250% OCPD on the primary (70A) and 125% OCPD on each of the four MCBs 150A, that wouldn't be allowed?

And yes, let's assume all secondary taps are 100% (150A).

You could not use anything over 125% for primary protection (adjusted up per 450.3) if you combined total secondary protection exceeded 125%
You 70 amp breaker is withing the 125% allowed for primary OCP with unlimitted secondary OCP.
250% primary protection (125 amps) or anything greater than 70 amps would not be allowed unless the secondary protection was 125% or less

 

[infinity]

Is there anything in the NEC that prohibits multiple parallel OCPDs on the secondary of a delta-wye, non-service transformer?

In other words, let's say I have a 45kVA, 480V delta-208/120V wye transformer in a building, with a 70A primary OCPD, and want to feed four 42-pole 150A MCB panels, with each parallel, daisy-chained 150A MCB acting as the secondary transformer OCPD. Is this allowed?

Or would I have to install a dedicated secondary 150A enclosed circuit breaker OCPD and then fan off from there to each 150A OCPD?

How will the daisy chained secondary conductors be protected when the total is now 600 amps?

 

[jim dungar]

...parallel OCPDs ...

Paralleled means both the line and load side are connected together - parallel OCPDs are not allowed by the NEC.
From the rest of your discussion, I assume you are wanting to install multiple secondary OCPDS.

Each secondary circuit must follow the rules of 240.21(C), for length and sizing.
As other have pointed out, after you have determined the characteristics of each secondary circuit, you would go to 450.3(B) and look at the transformer protection sizing requirements.

For the most part multiple secondary circuits typically result in the primary side OCPD being limited to 125% of the full load amps.

 

[Pharon]

You could not use anything over 125% for primary protection (adjusted up per 450.3) if you combined total secondary protection exceeded 125%

That's the part I was missing. Thanks.

How will the daisy chained secondary conductors be protected when the total is now 600 amps?

Good point. So if I do opt to have no single secondary OCPD, then the secondary feeder would have to be sized for the sum of all OCPDs that it feeds? Or, if it wasn't daisy-chained but instead fanned out individual feeders, the secondary transformer lugs would have to be rated for the sum?

 

[Pharon]

Paralleled means both the line and load side are connected together - parallel OCPDs are not allowed by the NEC.
From the rest of your discussion, I assume you are wanting to install multiple secondary OCPDS.

My mistake -- you are correct. I didn't know any other way to describe it and was too lazy to attach a drawing.