# Transformer secondary question



## NolaTigaBait (Oct 19, 2008)

Say I have a 480 primary with 208y secondary... My question is about the secondary conductors. If I want to feed 2 panels off the secondary side, what are the rules for taps on the secondary? Can I just treat them as "service conductors" ?


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## raider1 (Jan 22, 2007)

No, they are not service conductors. 

The transformer secondary conductors must be protected from overcurrent in accordance with one of the rules in 240.21(C) for transformer secondary conductors.

Chris


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## Frasbee (Apr 7, 2008)

I'm curious to the answer of this too, we're learning about transformers in class.


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## NolaTigaBait (Oct 19, 2008)

So, then I woud have to put a disconnect, then follow the feeder tap rules in 240.21b?....I'm a little confused...


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## Greg (Aug 1, 2007)

Just make sure The Secondary is protected according to T450.3(B), in order to satisfy 240.21(C)(1)


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## raider1 (Jan 22, 2007)

NolaTigaBait said:


> So, then I woud have to put a disconnect, then follow the feeder tap rules in 240.21b?....I'm a little confused...


You would need to follow the transformer secondary conductor rules in 240.21(C). 240.21(B) is for feeder taps.

You would need to limit the secondary conductor lengths to one of the methods listed in 240.21(C) and terminate the transformer secondaries in an overcurrent device.

Most likely you can use the 25 foot rule in 240.21(C)(6). This would allow you to run the secondaries 25 feet before you would have to terminate them in a single overcurrent device such as a breaker or single set of fuses in a disconnect.

Chris


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## NolaTigaBait (Oct 19, 2008)

Greg said:


> Just make sure The Secondary is protected according to T450.3(B), in order to satisfy 240.21(C)(1)


Yeah, I understand that part...my question is more on how to feed 2 panels off the secondary conductors....Do I have to follow the (lets say its under 10feet from the transformer)...secondary tap rules?...I wire the secondary into a discconect, then I can make "feeder taps" into the two panels based on the 10 foot feeder tap rules in 240.21b1?


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## NolaTigaBait (Oct 19, 2008)

raider1 said:


> You would need to follow the transformer secondary conductor rules in 240.21(C). 240.21(B) is for feeder taps.
> 
> You would need to limit the secondary conductor lengths to one of the methods listed in 240.21(C) and terminate the transformer secondaries in an overcurrent device.
> 
> ...


So you are saying that I can't feed 2 panels off the secondary? It says they to terminate in a breaker or *single* set of fuses?


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## raider1 (Jan 22, 2007)

Greg said:


> Just make sure The Secondary is protected according to T450.3(B), in order to satisfy 240.21(C)(1)


240.21(C)(1) can not be used on a 208 volt Wye secondary.

Primary protection of secondary conductors can only be done on single phase 2 wire single voltage secondaries or 3 phases delta 3 wire single voltage secondaries.

If you are using a 208 volt wye secondary you must have overcurrent protection on the secondary and can't rely on the primary overcurrent protective device.

Chris


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## raider1 (Jan 22, 2007)

NolaTigaBait said:


> So you are saying that I can't feed 2 panels off the secondary? It says they to terminate in a breaker or *single* set of fuses?


You can run 2 sets of secondary conductors one to each panel provided that each set meets the rules of 240.21(C).

Chris


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## NolaTigaBait (Oct 19, 2008)

raider1 said:


> 240.21(C)(1) can not be used on a 208 volt Wye secondary.
> 
> Primary protection of secondary conductors can only be done on single phase 2 wire single voltage secondaries or 3 phases delta 3 wire single voltage secondaries.
> 
> ...


Yeah, yeah got that.:thumbsup:...Are you understanding my question? Sorry if Im being confusing:blink:


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## NolaTigaBait (Oct 19, 2008)

raider1 said:


> You can run 2 sets of secondary conductors one to each panel provided that each set meets the rules of 240.21(C).
> 
> Chris


Awesome...thank you:thumbsup:


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## raider1 (Jan 22, 2007)

NolaTigaBait said:


> Yeah, yeah got that.:thumbsup:...Are you understanding my question? Sorry if Im being confusing:blink:


You could run one set of secondary conductors to one panel and terminate them at a single overcurrent protective device and then run a second set of secondaries from the transformer to a second panel and terminate them in a another single circuit breaker.

Hope this makes sense.

Chris


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## NolaTigaBait (Oct 19, 2008)

raider1 said:


> You could run one set of secondary conductors to one panel and terminate them at a single overcurrent protective device and then run a second set of secondaries from the transformer to a second panel and terminate them in a another single circuit breaker.
> 
> Hope this makes sense.
> 
> Chris


Yes, it does...Thanks...Thats exactly what I was planning on doing...:thumbsup:...Thanks, Chris


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## Greg (Aug 1, 2007)

raider1 said:


> 240.21(C)(1) can not be used on a 208 volt Wye secondary.
> 
> Primary protection of secondary conductors can only be done on single phase 2 wire single voltage secondaries or 3 phases delta 3 wire single voltage secondaries.
> 
> ...


That's why I said he had have secondary OCP because of the 2nd paragraph in 240.21(C)(1).


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## raider1 (Jan 22, 2007)

NolaTigaBait said:


> Yes, it does...Thanks...Thats exactly what I was planning on doing...:thumbsup:...Thanks, Chris


Your welcome.

Chris


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## raider1 (Jan 22, 2007)

Greg said:


> That's why I said he had have secondary OCP because of the 2nd paragraph in 240.21(C)(1).


Remember that Table 450.3(B) only deals with overcurrent protection of the transformer windings themselves and does not have anything to do with secondary conductor protection.

Secondary conductor protection is dealt with in 240.21(C).

Chris


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## Mike in Canada (Jun 27, 2010)

I'm not sure about down there, but up here in Canada they're picky about the conductor length. If you're running to two panels from one transformer then the feeds from the transformer need to be the same length. It depends on the inspector, but some of them are really tight about it.

Mike


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## brian john (Mar 11, 2007)

Mike in Canada said:


> I'm not sure about down there, but up here in Canada they're picky about the conductor length. If you're running to two panels from one transformer then the feeds from the transformer need to be the same length. It depends on the inspector, but some of them are really tight about it.
> 
> Mike



where is the logic in that?


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## Mike in Canada (Jun 27, 2010)

brian john said:


> where is the logic in that?


 The notion is that if you're splitting the output you want the same power going to each box. If you have a '400A' transformer and you're powering two 200A panels then the cables should be the same length so that they each see the same voltage, etc. I know it's crap, since you're not going to have a balanced load on the panels, but that's what they tell us.

Mike


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## Bob Badger (Apr 19, 2009)

It is crap and it sounds like they took one to many hockey pucks to the temple.


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## NolaTigaBait (Oct 19, 2008)

If I run say 2 sets of 3/0 copper to two 200 amp panels, would I have to upsize the GEC to the nearest building steel or water pipe electrode? I'm assuming a yes on that one


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## raider1 (Jan 22, 2007)

NolaTigaBait said:


> If I run say 2 sets of 3/0 copper to two 200 amp panels, would I have to upsize the GEC to the nearest building steel or water pipe electrode? I'm assuming a yes on that one


Yes, the size of the GEC would be based on the sum of the area of the 2 3/0 conductors. 

Chris


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## raider1 (Jan 22, 2007)

Bob Badger said:


> It is crap and it sounds like they took one to many hockey pucks to the temple.


Agree 100%:thumbsup:

Chris


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