Secondary Protection of Transformers

[Samardas]

Hi, I have a design situation as explained below:

It is a light industrial warehouse which a new owner has purchased to rent it out. Although he is upping the utility service power from 1200A to 4000A, he wants to put minimum power distribution to the facility for rental purposes. The new tenant will then do their Tenant Improvement (TI) work as per their needs. Therefore the owner wants to use the existing sub-panels, transformers etc to distribute power to the facility for permit purposes.

In the Single Line Diagram (SLD) I have shown a disconnect switch (on the wall) off of the switchboard on the primary side of the transformer and attached sub-panels on the load side of the transformer. I first thought the sub-panels have main breakers which would also protect the secondary of the transformer. However, upon investigating further I found out that the sub-panels are Main Lug Only (MLO). My question is, can I use another switch on the wall on the secondary side of the transformer just before the sub-panels to meet the code? Or, do I have to convert the MLO to Main CB inside the sub-panels, which I am not sure if it can be done, being the panels to be quite old. I can send you the SLD if needed. Transformers and the sub-panels are in the same vicinity. Your prompt response will be greatly appreciated. Thanks a LOT for your help! - Samar, P.E.

 

[jumper]

Assuming that your install meets the parameters of 240.21(C)(6), yes you can provide OCP at a switch before the MLO panel and not install a main breaker.

 

[charlie b]

You can use a single fused disconnect on the secondary of the transformer to protect the transformer's secondary windings. But that fuse is going to be sized for the transformer's rating. You will have a number of feeders to the subpanels, and the fuse is not likely to be able to protect all of them. Nor will it be able to protect the sub panels themselves. It all depends on whether the panel ratings, the wire sizes, the fuse rating, and the distances all fit within one or more of the "tap rules" from 240.21.

A one-line diagram will help me understand your situation. My guess is that the best solution is to put a distribution panel immediately downstream of the transformer. It would have a main breaker sized for the transformer, and feeder breakers sized for each sub panel. That way, you would not need to have main breakers on the sub panels.

 

[charlie b]

I think Derek and I have a different understanding (or lack thereof) of the proposed installation. So perhaps we do need a one-line diagram.

Are you talking about putting two disconnects for the service transformer, one at the primary and one at the secondary, and using the one at the secondary to feed all the sub panels?

OR

Are you talking about each individual 480V to 120/208V step down transformer in a prospective tenant's area getting a primary disconnect and a secondary disconnect, with the sub panels being fed from the secondary disconnect?

 

[Samardas]

Secondary Protection of Transformers

Thanks a Lot to both Charlie and Derek,
Attaching the SLD for your reference. Thanks again.

 

[jumper]

So, panels A and B are MLO only and instead of adding a main breaker, you want to install disconnects before the panels to provide OCP?

 

[don_resqcapt19]

You may be able to use the rule in 240.21(C)(2) for the protection of the transformer secondary conductors, but 408.36 will require an OCPD between the transformer secondary terminals and the line side lugs in the panel.

 

[mike7330]

i would try to get a 100 amp breaker for the panel and back feed the buss
You would need to get a lock down kit for the breaker if it's not a bolt on.
Or just add disconnect between xfmr and panel

 

[augie47]

i would try to get a 100 amp breaker for the panel and back feed the buss
You would need to get a lock down kit for the breaker if it's not a bolt on.
Or just add disconnect between xfmr and panel

Based on your one-line, this would be my suggestion also.

Curious, is 27kva the actual transformer size. Not sure I have ever seen a 27 kva

 

[don_resqcapt19]

The only 27kva transformers that I have seen were drive isolation transformers and they had 138Y/240 volt secondaries.

 

[Samardas]

Secondary Protection of Transformers

Thank you all for your invaluable suggestions. So, in summary, I can use a disconnect (a CB or a Fused Disconnect) outside the sub-panel to protect the secondary of the transformer, which should be no greater than the size of the sub-panel, right? Also, I was as curious as anybody else about the size of the 27KVA, 480-208/120V, 3 Ph transformer. I confirmed, it is indeed a 27KVA transformer. Thanks again.

 

[wbdvt]

One thing to consider as the service is increasing from 1200A to 4000A is that the available fault current is increasing and your old, existing, electrical equipment may be overdutied. Also it will change any previous arc flash hazard analysis.