Overcurrent Protection of incoming service

Status
Not open for further replies.

mshields

Senior Member
Location
Boston, MA
A number of parts to this question...

1) I've got a 1500KVA, 13.2kV to 480Y/277V pad mounted transformer with under oil short circuit fuses plus full range bayonett fusing on the primary. This feeds a 2500A 480V service feeder into my facility (which happens to be a hospital). My protection for my secondary conductors is the main breaker of the interior main service switchboard.

a) Does it matter that I am primary metered and that the transformer is owned by the facility?

b) I assume I'm ok thus far insofar as I've employed Article 230.90 (A) to protect these secondary conductors.

Now - We're being asked if we can add a Zenith ZTG switch down stream of the transformer but prior to the service entrance circuit breaker to act as a manual transfer switch. The purpose of which is to create a point at which we could plug a temporary generator if the transformer failed on us. Now it seems to me, I've muddied the waters and I need to have some overcurrent protection as part of this switch.

The switch is made from contactors not breakers so this would be mean the addition of a 2500A breaker.

Thoughts? Do I need that breaker?
 

dbuckley

Senior Member
Does it matter that I am primary metered and that the transformer is owned by the facility?
Only that you're paying a premium of a few percent on your electricity bill for keeping your transformer (which you paid for) warm and humming(*).

And when the tranny does fritz it's your problem to find a replacement, the replacement of an item with a lead time measured in significant fractions of a year, and in the meantime you need to keep the hospital powered, presumably by renting gensets. Make it the PoCos problem, and they can probably find a replacement transformer that'll do and have it installed, all by lunchtime.

*) Humming,. of course, because the transformer forgot the words the moment it fell out of warranty...
 

Cold Fusion

Senior Member
Location
way north
Where in your distribution that you describe is your service switch? Is it the main breaker of the interior main service switchboard. Is there a n-g bond there?
rom -
Jut a guess, but since the customer owns the xfm, and it's primary meered, I'd say the service is on the 13.8kv xfm primary. But it also could be that the utility and the customer have a negotiated agreement puting the service on the xfm secondary - that's rare, I've only seen that once.

cf
 

Cold Fusion

Senior Member
Location
way north
Only that you're paying a premium of a few percent on your electricity bill for keeping your transformer (which you paid for) warm and humming(*).

And when the tranny does fritz it's your problem to find a replacement, the replacement of an item with a lead time measured in significant fractions of a year, and in the meantime you need to keep the hospital powered, presumably by renting gensets. Make it the PoCos problem, and they can probably find a replacement transformer that'll do and have it installed, all by lunchtime. ...
The customer pays for the xfm and heating it whether the poco owns it or not. If the customer want sthe poco to have a xfm laying there waiting - then the customer pays for that as well. None of this is free.

All the customers I seen with primary metering get a significantly better rate - and that is because the pooe is not paying for all the stuff you mentioned.

cf
 

Cold Fusion

Senior Member
Location
way north
A number of parts to this question...

1) I've got a 1500KVA, 13.2kV to 480Y/277V pad mounted transformer with under oil short circuit fuses plus full range bayonett fusing on the primary. This feeds a 2500A 480V service feeder into my facility (which happens to be a hospital). My protection for my secondary conductors is the main breaker of the interior main service switchboard.

a) Does it matter that I am primary metered and that the transformer is owned by the facility?

b) I assume I'm ok thus far insofar as I've employed Article 230.90 (A) to protect these secondary conductors.

Now - We're being asked if we can add a Zenith ZTG switch down stream of the transformer but prior to the service entrance circuit breaker to act as a manual transfer switch. The purpose of which is to create a point at which we could plug a temporary generator if the transformer failed on us. Now it seems to me, I've muddied the waters and I need to have some overcurrent protection as part of this switch.

The switch is made from contactors not breakers so this would be mean the addition of a 2500A breaker.

Thoughts? Do I need that breaker?

First: Where is the service point? Is it on the trnsformer primary? If so, the trnsformer secondary conductors are not a service - they are a feeder.

This answer significantly impact the design.

cf
 

mshields

Senior Member
Location
Boston, MA
Response to questions

Response to questions

I had planned to bond at the transformer and bring in a neutral and a ground. Does that make the secondary a feeder and therefore does that mean I need overcurrent protection within 25 feet of the transformer?

If so, I'll bond at the switchboard and run only the Service Grounded Conductor along with the three phase.

Forgetting about the Manual Transfer Switch for the moment, do you agree with the above?
 

dahualin

Senior Member
First: Where is the service point? Is it on the trnsformer primary? If so, the trnsformer secondary conductors are not a service - they are a feeder.

This answer significantly impact the design.

cf

Hi Cold Fusion,

Would you please explain a little bit more on how it will impact the design?

There are two situation. First one is service transformer owned by utility and the transformer secondary conductors are called service-entrance conductors. The other one is service transformer owned by customer and the transformer secondary conductors are called feeder. I am wondering what the difference is for designing the service conductor and building service equipment and designing the feeder and "building service equipemnt"? Is there any different term for "building service equipment" at second situation?

Thanks,

David
 

Cold Fusion

Senior Member
Location
way north
I had planned to bond at the transformer and bring in a neutral and a ground. Does that make the secondary a feeder (?) ...
The bond location has nothing to do with it.
... Where is the service point? Is it on the trnsformer primary? If so, the trnsformer secondary conductors are not a service - they are a feeder. ...

... does that mean I need overcurrent protection within 25 feet of the transformer? ...
I don't know. Put the the OCP where it needs to be. This is not a case where the design is by NEC minimums.

... Forgetting about the Manual Transfer Switch for the moment, do you agree with the above?
No.

cf
 

Cold Fusion

Senior Member
Location
way north
...There are two situation. First one is service transformer owned by utility and the transformer secondary conductors are called service-entrance conductors. The other one is service transformer owned by customer and the transformer secondary conductors are called feeder. I am wondering what the difference is for designing the service conductor and building service equipment and designing the feeder and "building service equipemnt"? ...

If the service point is on the xfm secondary, then the secondary conductors are covered by Article 230.

If the service is on the xfm primary, then the xfm is an SDS and the conductors are a feeder. These are covered by 240. generally 240.21.

Test question: If the service point is on the transformer primary, what section do you use to size the secondary feeder Equipment Grounding Conductor?

The kid and I were falling trees yesterday, and out fishing all day today, and its late, and all from memory. If someone wants to add/educate a bit - I won't feel hurt at all.

cf
 
Judging by your original description, I assume that the service disconnect is on the load service side of the transformer, where the "under oil short circuit fuses plus full range bayonet fusing" is. If that is the case, and assuming that there is no direct contact between primary and secondary conductors (including the neutral), then the secondary conductors are feeder conductors and, as Cold Fusion said, 240.21 applies.

Now, if the service disconnect is "the main breaker of the interior switchboard", then the secondary conductors are service entrance conductors and Art. 230 applies; in particular, 230.82, or 230 VII.

So, in answer to a), it does matter that "you are primary metered and ... owned by the facility" because that determines where the service point is and the classification of the equipment and conductors that you are working with. Different classification, of course, requires different treatment.

The answer to part b) depends, again, on where the service disconnect is. Since you used 230.90(A) to protect the secondary conductors, it seems to me that there is no disconnect on the service side of the transformer but this would conflict with my introductory assumption.
 
Status
Not open for further replies.
Top