TVSS wiring and OCPD

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muhandas

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I've searched the forum but couldn't find the answer, probably because it's so obvious that I just can't see it.

Im looking at a large (4000 A MCB main panel) commercial installation and a bunch of smaller distribution panels. Each distribution panel (other than the Main) has a TVSS mounted very close to the panel through a 60A breaker with AWG #10.

Now, here's my problem. I know that under normal conditions the TVSS draws no current so AWG #10 is obviously OK. However, the ampacity of #10 THWN is only 35A. So, in the case of a partial failure of the TVSS the current could be greater than 35A and less than 60A. That, to me, feels wrong because it is in excess of the conductor's ampacity, could continue indefinitely, and thus create a real problem. I am aware that with a complete TVSS failure the breaker would interrupt very quickly what is probably a massive overcurrent.

NEC Section 285 doesn't address this and the only basis I would have for my argument is section 240 but I'm not totally comfortable with it.

Should I worry about that partial failure and require that the CB be suitable for the conductor size? Or, if the TVSS manufacturer specifies a particular OCPD that an appropriately sized conductor be used?

Heinz R.
 
muhandas said:
I've searched the forum but couldn't find the answer, probably because it's so obvious that I just can't see it.

Im looking at a large (4000 A MCB main panel) commercial installation and a bunch of smaller distribution panels. Each distribution panel (other than the Main) has a TVSS mounted very close to the panel through a 60A breaker with AWG #10.

Now, here's my problem. I know that under normal conditions the TVSS draws no current so AWG #10 is obviously OK. However, the ampacity of #10 THWN is only 35A. So, in the case of a partial failure of the TVSS the current could be greater than 35A and less than 60A. That, to me, feels wrong because it is in excess of the conductor's ampacity, could continue indefinitely, and thus create a real problem. I am aware that with a complete TVSS failure the breaker would interrupt very quickly what is probably a massive overcurrent.

NEC Section 285 doesn't address this and the only basis I would have for my argument is section 240 but I'm not totally comfortable with it.

Should I worry about that partial failure and require that the CB be suitable for the conductor size? Or, if the TVSS manufacturer specifies a particular OCPD that an appropriately sized conductor be used?

Heinz R.
Click to expand...

IMO your right on. The man will tell you the breaker size. As far as I know there is nothing that would allow you use an under sized wire in that situation.
 
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Now, here's my problem. I know that under normal conditions the TVSS draws no current so AWG #10 is obviously OK. However, the ampacity of #10 THWN is only 35A. So, in the case of a partial failure of the TVSS the current could be greater than 35A and less than 60A. That, to me, feels wrong because it is in excess of the conductor's ampacity, could continue indefinitely, and thus create a real problem. I am aware that with a complete TVSS failure the breaker would interrupt very quickly what is probably a massive overcurrent.
Click to expand...

The TVSS will operate more efficiently if the conductors are sized PROPERLY (generally larger is better), are as straight as possible and bound tightly in a group.

I would READ the literature and see what the manufacture states is required, but the conductor should be protected at it's rated ampacity.
 
This is a plan review at this time.
I agree that bigger is better for surge suppression but I felt that whatever size conductor is used that it ought to be protected in accordance with 240. I was a little insecure because of the non-linear aspects of the TVSS and wasn't sure that other NEC requirements didn't apply different requirements to this particular beastie, although I couldn't find any.
You folks have made me feel secure once again.
Thanks for your help.
Heinz R.
 
muhandas said:
This is a plan review at this time.
I agree that bigger is better for surge suppression but I felt that whatever size conductor is used that it ought to be protected in accordance with 240. I was a little insecure because of the non-linear aspects of the TVSS and wasn't sure that other NEC requirements didn't apply different requirements to this particular beastie, although I couldn't find any.
You folks have made me feel secure once again.
Thanks for your help.
Heinz R.
Click to expand...

That's what Brian dose. Makes you feel all warm and fuzzy:grin:
 
Somebody has to.

My wife doesn't, my boss doesn't, the contractors and engineers whose work I sometimes have to take exception with don't, my kids don't, my friends do just the opposite, particularly when I'm late with the beer, and my cat bites me.

All kidding aside tho', when one is almost a lone wolf out there, a community such as this one hosted by Mike Holt is invaluable in being able to talk informally to one's professional colleagues when questions and doubts arise. There is almost always somebody, sometimes several somebodys, who have seen similar situations and are willing to share their knowledge and wisdom and as often as not will present angles I hadn't thought of.

Thanks guys and dolls.

Heinz R.
 
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