Multiple Xfrmrs on a single breaker

[MD88]

Hello,

The situation is this: a single 5kV breaker feeds a transformer. The transformer has an unfused disconnect for isolation purposes. Now, if we tap off that line to feed another transformer, do we need to put fusing on the transformer primaries (of the existing and/or the new transformer)?

Thanks!

 

[erickench]

NEC 240.4(F) gives the rules for secondary conductors protected by primary OCPD. Is the transformer a 2-wire single phase? Is it a delta-delta? Also, NEC 240.21 gives the rules for transformer taps.

 

[MD88]

Each transformer is a Delta-Wye, 2000kVA, 4160/480V transformer. Thanks though, I'll check that section!

 

[Pierre C Belarge]

Hello,

The situation is this: a single 5kV breaker feeds a transformer. The transformer has an unfused disconnect for isolation purposes. Now, if we tap off that line to feed another transformer, do we need to put fusing on the transformer primaries (of the existing and/or the new transformer)?

Thanks!

NEC 240.4(F) gives the rules for secondary conductors protected by primary OCPD. Is the transformer a 2-wire single phase? Is it a delta-delta? Also, NEC 240.21 gives the rules for transformer taps.

He mentions a 5kv breaker supplying all of the transformers.
I do not believe that 240.4(F) applies in this installation.

 

[pete m.]

You may (should) want to check 450.3(A)

Pete

 

[augie47]

I agree with Pete. Check 450.3(A) for the applicable section for your install. If each transformer is protected per that Table, you are good to go.

 

[MD88]

Agreed, but the question really is, do they need INDIVIDUAL protection, or can a single protective device handle them both.

 

[MD88]

240.101 Additional Requirements for Feeders.

(B) Feeder Taps. Conductors tapped to a feeder shall be
permitted to be protected by the feeder overcurrent device
where that overcurrent device also protects the tap
conductor.


That looks promising?

 

[MD88]

I think as long as this is satisfied I can do it. To me, this means I would just need to put a fuse on the disconnect at the new transformer we're adding, and the original transformer is ok without any protection (recall that at the moment, the original transformer is fed from a breaker and I'm tapping off that feeder cable to feed another transformer). Any comments?

240.21B(5) Outside Taps of Unlimited Length.
Where the conductors are located outdoors of a building or structure, except
at the point of load termination, and comply with all of
the following conditions:
(1) The conductors are protected from physical damage in
an approved manner.
(2) The conductors terminate at a single circuit breaker or
a single set of fuses that limit the load to the ampacity
of the conductors. This single overcurrent device shall
be permitted to supply any number of additional overcurrent
devices on its load side.
(3) The overcurrent device for the conductors is an integral
part of a disconnecting means or shall be located immediately
adjacent thereto.
(4) The disconnecting means for the conductors is installed
at a readily accessible location complying with one of
the following:
a. Outside of a building or structure
b. Inside, nearest the point of entrance of the conductors
c. Where installed in accordance with 230.6, nearest
the point of entrance of the conductors

 

[Pierre C Belarge]

If the conductors are the same size, the place you are calling a tap, would be a splice, not a tap.
More than one transformer is permitted to be supplied/protected by a single overcurrent device. Providing the overcurrent device is properly sized.



From your last post, are we to assume this is all located outside the building?

 

[Volta]

Remember that the 'tap rules' protect the conductors, and you have to figure the transformer protection separately, though that may be able to be done with the same equipment.

 

[MD88]

Actually I just looked at it again, and it's more like 2 totally separate feeders off one breaker, due to the cabling. It would be be 2 runs of 3C-500 coming off the breaker, and really each run would be feeding one of the transformers - there'd be no need to splice anything since each transformer will be fed with a single 3C-500. Just a way to save a breaker I guess... could this be allowed??

 

[Volta]

It could. What size is the new transformer and the existing breaker?

 

[augie47]

Pierre, I don't see any sign of the "rust" you worried about

MD88, Keep in mind you must address two Code Sections.
Art 450 addresses the transformers and you must make sure each is protected according to 450.(3)(A). As you can see, with proper secondary protection, you are allowed to use primary protection that could possibly feed both transformers. The need for individual protection would depend on the size of the transformers.
After you satisfy 450, then you must satisfy Art 240. As far the primary is concerned, if you don't reduce the wire size the, as Pierre notes, it's not a tap and will be protected by a properly sized OCP.
If you do reduce the primary wire size then you will need to address Art 240.21 for the primary.
In any event you need to address Art 240.21(C) for the secondary.