Indoor transformer 75kVA, not single phase 2W or delta-delta 3W, can MCB qualify as OCP for the secondary side?

[CelectricB]

We're putting in a dry type 75kVA transformer with a fused disconnect on the secondary side, followed by a distribution panel. The onsite electrician wants to get rid of the fused disconnect and just use an MCB on the distribution panel instead. Does the NEC allow this?

 

[jap]

What would make you think a Main Breaker in the panel wouldn't be an acceptable means of overcurrent protection?

JAP>

 

[augie47]

If the MCB meets the requirements ( size and distance) of 240.21)(C) it would be permissible.

 

[EC Dan]

I've found relying on the MCB for secondary OCP often results in excessively high arc flash incident energy in that panel since the arc flash clearing time relies on the primary OCP. Up to the engineer whether the extra cost of external secondary OCP is worth the reduced arc flash rating.

 

[jap]

I've found relying on the MCB for secondary OCP often results in excessively high arc flash incident energy in that panel since the arc flash clearing time relies on the primary OCP. Up to the engineer whether the extra cost of external secondary OCP is worth the reduced arc flash rating.

Main Breakers in Panelboards are being fed directly from utility transformers in most all modern services across the country.

JAP>

 

[jim dungar]

Main Breakers in Panelboards are being fed directly from utility transformers in most all modern services across the country.

JAP>

And the arc flash incident energy at incoming lugs of those service entrance panelboards is almost always in excess of 8cal/cm*2 and typically over 40cal/cm*2.

 

[jap]

And the arc flash incident energy at incoming lugs of those service entrance panelboards is almost always in excess of 8cal/cm*2 and typically over 40cal/cm*2.

A breaker is an acceptable means of overcurrent protection for the secondary conductors of a transformer.

Which is what this post is about.

JAP>

 

[don_resqcapt19]

Main Breakers in Panelboards are being fed directly from utility transformers in most all modern services across the country.

JAP>

And that is one of the reasons why the 2020 code is requiring service OCPDs to be in separate compartments.
In addition this thread does not seem to be about a service. It appears to be to be about a SDS. As stated by Dan in post 4, it may be good engineering practice to install an OCPD upstream of the panel for arc flash and safety reasons. In the industrial plants I have worked in, some of the highest incident energies are in panels that are directly fed from the secondary of a transformer.

 

[jap]

And that is one of the reasons why the 2020 code is requiring service OCPDs to be in separate compartments.
In addition this thread does not seem to be about a service. It appears to be to be about a SDS. As stated by Dan in post 4, it may be good engineering practice to install an OCPD upstream of the panel for arc flash and safety reasons. In the industrial plants I have worked in, some of the highest incident energies are in panels that are directly fed from the secondary of a transformer.

I know you guys stick together, and, I agree completely if we're going to turn this into an arc flash discussion instead of an acceptable means of overcurrent protection for transformer secondary conductors.

JAP>

 

[EC Dan]

I wasn't trying to go off on a tangent, but if the onsite installation electrician is requesting a change to design it's important for everyone involved to understand why it may have been designed a certain way in the first place. If the OP already considered that and didn't think it was a concern, then original question asked and answered.

 

[jap]

I'm with ya, but,

I would think if this were a designed installation where a fused disconnect was being installed for arc flash purposes, the design engineer wouldn't be visiting an electrical forum asking if a Main Circuit breaker qualifies for overcurrent protection on the secondary side of a transformer.


Seems to be just a simple discussion between an installing electrician and the company he's doing the work for.


JAP>

 

[jim dungar]

Seems to be just a simple discussion between an installing electrician and the company he's doing the work for.

And that discussion should cover all of the issues that are important not just whether the install is to the NEC. Maybe the designer didn't consider arc flash initially, but might want to now that it has been mentioned.

 

[jap]

Makes me wonder why designers haven't jumped on the fused disconnect wagon on the secondary side of a transformer if that's the case.

Every one-line I've seen over the years, and, believe me that's several, show exactly what the installing electrician suggested.

Put a breaker in a 480v HV panel, down to a 75kva transformer, which feeds a 225 or 250a Main Breaker Low Voltage panel.

A setup that's common in most every electrical room in the country with a 480v service, and, hasn't changed on blueprints to this day.

JAP>

 

[jim dungar]

Makes me wonder why designers haven't jumped on the fused disconnect wagon on the secondary side of a transformer if that's the case.

Fuses may not be any more help than a breaker would be. What is important is some physical separation between the secondary protective device and the panelboard.

Most designers do not care what the incident energy is at equipment. It would fall on the owner to have it as part of the criteria given to the designer.

 

[jap]

Fuses may not be any more help than a breaker would be. What is important is some physical separation between the secondary protective device and the panelboard.

Most designers do not care what the incident energy is at equipment. It would fall on the owner to have it as part of the criteria given to the designer.

If the fused disconnect is no more help than a breaker would be, I guess I don't understand what you're saying.

What would you gain by having a fused disconnect immediately on the secondary side of a transformer feeding a Main Lug panel vs. having a Main Breaker in the panel assuming both panels are located immediately next to the transformer?

JAP>

 

[don_resqcapt19]

If the fused disconnect is no more help than a breaker would be, I guess I don't understand what you're saying.

What would you gain by having a fused disconnect immediately on the secondary side of a transformer feeding a Main Lug panel vs. having a Main Breaker in the panel assuming both panels are located immediately next to the transformer?

JAP>

A OCPD between the transformer and a panelboard removes the high incident energy from the panelboard and relocates it to the first OCPD. That permits you to turn off the first OCPD and then not be exposed to high incident energies in the panelboard. With the main in the panelboard that high incident energy remains in the panelboard, even when the main it turned off.

This is becoming a more common design for applications where there is a strong electrical work safety culture. The hazard is reduced or eliminated by design.

Of course you can also accomplish this by locking out the primary circuit that feeds the transformer.

 

[jap]

A OCPD between the transformer and a panelboard removes the high incident energy from the panelboard and relocates it to the first OCPD. That permits you to turn off the first OCPD and then not be exposed to high incident energies in the panelboard. With the main in the panelboard that high incident energy remains in the panelboard, even when the main it turned off.

This is becoming a more common design for applications where there is a strong electrical work safety culture. The hazard is reduced or eliminated by design.

Of course you can also accomplish this by locking out the primary circuit that feeds the transformer.

You took away my response I had already typed up and was ready to hit send.

This was fixing to go from an acceptable means of overcurrent protection discussion, to an arc flash discussion, then to a proper procedure and where to disconnect for safety discussion.

JAP>

 

[jap]

A OCPD between the transformer and a panelboard removes the high incident energy from the panelboard and relocates it to the first OCPD. That permits you to turn off the first OCPD and then not be exposed to high incident energies in the panelboard. With the main in the panelboard that high incident energy remains in the panelboard, even when the main it turned off.

This is becoming a more common design for applications where there is a strong electrical work safety culture. The hazard is reduced or eliminated by design.

Of course you can also accomplish this by locking out the primary circuit that feeds the transformer.

That was very well put by the way.

JAP>

 

[EC Dan]

Correct me if I'm wrong but the ability to lock out the primary side doesn't necessarily negate the benefit of a secondary OCPD outside the panelboard. The primary OCPD for a transformer may be sized as high as 2.5X of FLA to compensate for inrush, therefore the arc flash current as a multiple of rated current of the primary OCPD is likely to be much lower (and hence much higher clearing time) than that for a secondary side OCPD. I bring this up because there may be occasion to work live in the secondary panel.

 

[jim dungar]

This was fixing to go from an acceptable means of overcurrent protection discussion, to an arc flash discussion, then to a proper procedure and where to disconnect for safety discussion.

I feel these are interrelated items.
Installations made to code minimum often become a hassel for maintenance and safety.