Dual Secondary Feeders from a Transformer

[rcstone]

I have a 4160V primary, 120/208V, 3phase secondary transformer. Does the NEC allow for multiple secondary feeders terminating in different locations? Southern California Edison does it all the time, but does NEC allow it?

thanks.

 

[raider1]

Welcome to the forum.

The NEC permits multiple transformer secondaries provided that each set of secondary conductors meet the requirements in 240.21(C).

Chris

 

[billyzee]

Xfmr Secondary Connections Additional Inquiry

Xfmr Secondary Connections Additional Inquiry

I have a similar and related question.

Suppose the transformer is a service transformer owned by a utility. Suppose the utility wants to feed 5 separate customers with that transformer. Each customer needs (4) parallel 500kcmil service conductors. Therefore, each secondary spade on the transformer will have 20 conductors attached.

Can anyone elaborate on the options for making these connections.
1. Can standard transformers accomodate this many conductors and support the weight? IEEE c57.12.22 may say something about this but I don't have a copy.
2. Are there listed enclosures available which expand the number of secondary connection points?

 

[Strathead]

I have a similar and related question.

Suppose the transformer is a service transformer owned by a utility. Suppose the utility wants to feed 5 separate customers with that transformer. Each customer needs (4) parallel 500kcmil service conductors. Therefore, each secondary spade on the transformer will have 20 conductors attached.

Can anyone elaborate on the options for making these connections.
1. Can standard transformers accomodate this many conductors and support the weight? IEEE c57.12.22 may say something about this but I don't have a copy.
2. Are there listed enclosures available which expand the number of secondary connection points?

I personally would suppose this would be unlikely. First that would be 5 separate 1200 Amp services (or more) in one location. That is figuring the secondaries using the NEC tables. Which is what they do in Florida. If it was sized by the Utility company, then the services would be much larger. But that said, a transformers lugging capacity is most likely going to be adequate for whatever design loads are capable of being fed from it.

 

[iceworm]

I have a similar and related question.

Suppose the transformer is a service transformer owned by a utility. Suppose the utility wants to feed 5 separate customers with that transformer. Each customer needs (4) parallel 500kcmil service conductors. Therefore, each secondary spade on the transformer will have 20 conductors attached.

Can anyone elaborate on the options for making these connections.
1. Can standard transformers accomodate this many conductors and support the weight? IEEE c57.12.22 may say something about this but I don't have a copy.
2. Are there listed enclosures available which expand the number of secondary connection points?

I'm guessing:
1. This is a 208Y secondary, maybe in the 2MVA range, does not exceed 2500kva.
2. Service to each customer is 1500A.
3. Customers are commercial not industrial
4. 5 customers, (4-500/phase, 4 - neutral, 4 grounding conductors)/customer = ~100 conductors in the xfm secondary

Not that any of this matters much, but it helps to get my head wrapped around the issues.

IEEE c57.12.22 addresses the xfm pad size and hole arrangement. It is not clear to me that it covers 208V xfm above 1500kva..

The weight wouldn't concern me - the conductors should be formed in place such there is no stress on the pads.

All of the enclosures with buses inside to expand the number of connection points, that I have seen, were locally fabricated and were not listed. I don't know of a requirement for being listed. But if your customer spec is to list, have them built by a NRTL panel shop.

Even if you get this enclosure, you still have to get 7500A (nominal) of capacity from the xfm secondary pads over to the enclosure. I don't see you gained anything. About the same number of cables in the secondary compartment.

Just curious - have you asked for a cut sheet/specs on the xfm being supplied? The utility surely has this since they are buying the xfm.

Stacking lugs are available such as thesehttp://www.burndy.com/products/product-by-category/mechanical/mechanical-aluminum.aspx

Another thought about the utility purchased transformer:
I highly suspect the utility has done this before. Call your buddy at the utility and ask for some education.

One more thought:
I am a real big fan of talking to journeyman that do this type of work. They tend to know a lot more than I do about hooking up wires. Call the contractors where your company has a working history. That should give an incentive to help. And if they are ones that could likely get the job, there should be plenty of incentive to help out

ice

 

[billyzee]

Iceman,

Your insight was helpful. I've asked the utility for xfrm spec but haven't gotten it. All I know is that they want to stay away from special order transformers. After talking with a few transformer manufacturers I found that, depending on the size, you get standard 6, 8, 10, 12, 16 or 20 hole spades.

Cooper Industries pad mount transformers 750kVA and bigger can be had with up to 20 hole spades. Sunbelt is not so generous. They quit putting 10 Holes or more when the transformer gets to be 1500kVA or less. This transformer happens to be 1500kVA, I know it sounds small.

Anyway, thank you for the suggestions. I will look into the stacked lugs and consider my options while I wait for the utility xfmr spec.

 

[iceworm]

Glad it was helpful, but I'd say you already have a pretty good handle on the issues.

ice