Transformer Cable Secondary Protection

[mull982]

I'm looking at an application where a 112.5 KVA 480pri - 208sec has (2) sets of 400MCM copper cables coming off of the secondary of the transformer and termininag into a 1600A breaker about 200ft away.

I believe this is a violation of 240.31(C) (I believe this is the right referene) in that the secondary protective device rating must not exceed the rated ampacity of the secondary cables. In this case the (2) sets of 400 MCM cables has an ampacity of 670A. So the way I see it, either the cables sizes would have to be increased to allow for greater ampacity or the breaker would have to be changed to a 600A breaker.

Am I correct in my thinking?

 

[mull982]

Wye-Delta Starter feeder cable protection

Wye-Delta Starter feeder cable protection

I'm looking at an application where a 1600A breaker is protecting (3) sets of 400 MCM copper cables which are feeding an enclosuere containing a wye-delta starter. I could not gain access to the starter but am told there is a wye-delta starter in the enclosure. The load the starter is starting is aproximately a 566 kW compressor.

At first glance it does not appear that the (3) sets of 400 MCM cables with a combined ampacity of 1005A are adequately protected by the 1600A breaker. Is there a provision for feeding a wye-delta starter that allows the cable to be rated less than the upstream breaker?

 

[iggy2]

reference is not correct - there is no 240.31. You mean 240.21(C) I think.

I agree that this is a violation. I do not see a condition which meets what you have described. You can change the conductors, or the CB, or both, but the length is still an issue, unless the sec conductors are outside. Even in that case, you have other conditions to comply with.

 

[augie47]

I have a few questions that might be pertinent.
1. Are the conductor and transformer under POCO control ?
2. Are the conductors outside ?
3. Does the 1600 amp breaker actually have a 1600 amp trip unit or is it just a 1600 amp
frame ?
4. Are motors involved ?

 

[rbalex]

If this is a true 2-wire (single-voltage) secondary, 240.21(C)(1) may apply.

 

[don_resqcapt19]

Yes, just like any other set of motor conductors the conductors are sized at 125% of the motor FLA and the OCPD sized per 430.52. If this is a hermeticlly sealed compressor the rules in Article 440 would apply.

 

[don_resqcapt19]

If this is a true 2-wire (single-voltage) secondary, 240.21(C)(1) may apply.

208 is rarely a two wire system.

 

[augie47]

(two threads...same topic, same OP merged)

 

[mull982]

Yes, just like any other set of motor conductors the conductors are sized at 125% of the motor FLA and the OCPD sized per 430.52. If this is a hermeticlly sealed compressor the rules in Article 440 would apply.

Would the breaker and cable I am talking about be considered a motor feeder or motor branch circuit.

If it is considered a branch circuit then I guess the breaker can be sized at 250% of motor FLA and the cables dont necessarily have to be protected by the breaker but just need to be sized for the motor load. In this case it would be o.k. to have (3) sets of 400 MCM cables protected by a 1600A breaker.

However since the breaker and cable feed a wye-delta starter enclosure I am thinking this may be considered a feeder? I'm guessing that the wye-delta starter may have a breaker and definitley has overloads so everthing after the starter would be considered a branch circuit and anything before it would be a feeder circuit?

 

[mull982]

1. Are the conductor and transformer under POCO control ?

No

2. Are the conductors outside ?

No all indoors

3. Does the 1600 amp breaker actually have a 1600 amp trip unit or is it just a 1600 amp frame ?

1600A Frame and Trip unit

4. Are motors involved ?

No motors

 

[mull982]

One other question I had related to the transformer secondary protection. Based on a 112.5kVA transformer is it correct that if the transformer primary portection is rated at 125% or less then no secondary transformer protection is needed? However if the primary protection is 125%-250% then a secondary transformer protection device is needed.

So if a 112.5 kVA 480pri 208sec transformer has a 150A breaker on the primary than no secondary protection is required, however if the primary breaker is a 200A breaker then secondary protection of the transformer is required?

 

[rbalex]

One other question I had related to the transformer secondary protection. Based on a 112.5kVA transformer is it correct that if the transformer primary portection is rated at 125% or less then no secondary transformer protection is needed? However if the primary protection is 125%-250% then a secondary transformer protection device is needed.

So if a 112.5 kVA 480pri 208sec transformer has a 150A breaker on the primary than no secondary protection is required, however if the primary breaker is a 200A breaker then secondary protection of the transformer is required?

No. As the merged threads now indicate, 240.21(C)(1) doesn't apply.

Edit add: Section 240.21(C)(1) only applies to two-wire primary/ two-wire secondary, single-phase or delta-delta, three phase applications.