exterior transformer - primary protection

[malachi constant]

I have an install going in with a 480V exterior pad-mount utility transformer feeding an adjacent connection cabinet. The connection cabinet has no overcurrent protection (which is typical for utility installs in our area). The cabinet will feed some chillers (which have service rated disconnects at them) and also will feed a 480:208V 500kVA step down transformer which in turn feeds the building. There is no disconnecting means in the plans on the primary of the step down transformer - I assume this is illegal and we need to get a primary disconnect ahead of it - that feeding it basically straight from the utility is prohibited.

Then from the stepdown transformer there is an underground run (1600A, not protected) into the building, at which point it immediately hits a 1600A service disconnect. I believe this is legal.

Am I correct that we need a primary on this transformer, and that the service entrance disconnect located within the building is sufficient for the secondary?

 

[Dale001289]

I have an install going in with a 480V exterior pad-mount utility transformer feeding an adjacent connection cabinet. The connection cabinet has no overcurrent protection (which is typical for utility installs in our area). The cabinet will feed some chillers (which have service rated disconnects at them) and also will feed a 480:208V 500kVA step down transformer which in turn feeds the building. There is no disconnecting means in the plans on the primary of the step down transformer - I assume this is illegal and we need to get a primary disconnect ahead of it - that feeding it basically straight from the utility is prohibited.

Then from the stepdown transformer there is an underground run (1600A, not protected) into the building, at which point it immediately hits a 1600A service disconnect. I believe this is legal.

Am I correct that we need a primary on this transformer, and that the service entrance disconnect located within the building is sufficient for the secondary?

450.3(B) - You'll need protection on the primary at either 125% or 250% of Xfmr FLA.
240.21(4)(C) The service entrance disconnect will be sufficient for the secondary.

 

[electrofelon]

450.3(B) - You'll need protection on the primary at either 125% or 250% of Xfmr FLA.
240.21(4)(C) The service entrance disconnect will be sufficient for the secondary.

While I agree, he also needs a service disconnect ahead of the transformer. The transformer protection requirements, and service disconnect can be the same piece of equipment if service rated.

 

[Dale001289]

While I agree, he also needs a service disconnect ahead of the transformer. The transformer protection requirements, and service disconnect can be the same piece of equipment if service rated.

Agreed - I should have clarified that in my initial post.

 

[kwired]

450.3(B) - You'll need protection on the primary at either 125% or 250% of Xfmr FLA.
240.21(4)(C) The service entrance disconnect will be sufficient for the secondary.

I assume you meant 240.21(C)(4)

That is addressing overcurrent protection of secondary conductors, basically puts no limit on length of secondary tap conductors if outdoors - with some requirements that must be met.

Does not address the fact that the transformer itself likely needs secondary protection

Two wire primary to two wire secondary and thee phase three wire to three wire transformers can have secondary protection provided by primary protection but multiwire secondary with a neutral usually needs secondary protection for the transformer itself.

 

[Dale001289]

I assume you meant 240.21(C)(4)

That is addressing overcurrent protection of secondary conductors, basically puts no limit on length of secondary tap conductors if outdoors - with some requirements that must be met.

Does not address the fact that the transformer itself likely needs secondary protection

Two wire primary to two wire secondary and thee phase three wire to three wire transformers can have secondary protection provided by primary protection but multiwire secondary with a neutral usually needs secondary protection for the transformer itself.

I don’t have my NEC in front of me but if I recall correctly xfmr can be protected by a single overcurrent device where the conductors terminate such as a panelboard MCB. The OCP doesn’t have to be located at the secondary windings. If outdoors the 2ndary conductors can be of unlimited length- if indoors restriction of 25 feet.


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[augie47]

Where does the utility identify the "service point"

 

[kwired]

I don’t have my NEC in front of me but if I recall correctly xfmr can be protected by a single overcurrent device where the conductors terminate such as a panelboard MCB. The OCP doesn’t have to be located at the secondary windings. If outdoors the 2ndary conductors can be of unlimited length- if indoors restriction of 25 feet.


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I believe you are correct, I might have misunderstood exactly how things are arranged in OP's installation ...

I have an install going in with a 480V exterior pad-mount utility transformer feeding an adjacent connection cabinet. The connection cabinet has no overcurrent protection (which is typical for utility installs in our area). The cabinet will feed some chillers (which have service rated disconnects at them) and also will feed a 480:208V 500kVA step down transformer which in turn feeds the building. There is no disconnecting means in the plans on the primary of the step down transformer - I assume this is illegal and we need to get a primary disconnect ahead of it - that feeding it basically straight from the utility is prohibited.

Then from the stepdown transformer there is an underground run (1600A, not protected) into the building, at which point it immediately hits a 1600A service disconnect. I believe this is legal.

Am I correct that we need a primary on this transformer, and that the service entrance disconnect located within the building is sufficient for the secondary?

This partly threw me off before. What you are describing isn't service disconnect. It is the main disconnect for the building that is supplied via feeder.

"Service" disconnect, conductors, etc. in the NEC are all supplied by the utility company.

On site power sources and separately derived systems (which is when the transformer is owned and maintained by the owner and not the POCO) are not "service" items.

So yes you can run those secondary conductors into the building before hitting an overcurrent protection device.

Yes you also do need service disconnecting means between the transformer primary and the service conductors.