NEC Req

Status
Not open for further replies.
J

jryan

Member
I have a 2500KVA+FC XFMR Sub going in with a 4000A secondary Bus and associated switchgear.

Originally I had 1-4000A MCB, 2-1200A, 1-600A Breakers in the switchgear. Due to costs, I have decided that I can delete the MCB as I am not required to have it per NEC450-3. I have added a high resistance ground to protect the secondary bus. On the 13.8KV side, I have a ground mounted air-switch. So I have a way to lock out the 480V secondary to do work inside the Gear.

Vender tells me that I can not do this per code.

Can I? I thought I could have no more than 6 feeder breakers without a MCB. I have only 3 and maybe 1 Future, so that leaves me wiggle room.
What thought does anyone have on this? :)
 
Re: NEC Req

J
I would ask the vendor what the violation is. The
installation appears to meet 450.3.A note 2 requirements.
Have you considered using current limiters on the
transformer terminals to protect the bus rather that the ground resistance method?
What happens if you have a phase to phase fault?

[ May 21, 2004, 01:02 PM: Message edited by: bob ]
 
Re: NEC Req

Well we have a ungrounded 3 phase 13.8KV system and there are current limiting fuses on the Primary side so I think we are ok there.

As for the vendor, I told him to read his code book and make sure before he brings it up again. See the breaker that I have removed from the Scope is around 45k, so he has a vested interest in me buying it. I do not need it and removing it brings me closer to my budget.

:provide a set of E-rated, full range, current limiting fuses, sized for the fan cooled rating of the transformer. :)

:provide a set of metal oxide type surge arrestors


So I think I am covered. Thing is I have always wanted a MCB, but in this case I believe we can do without it and save some dollars in the process.


Thanks for your help.

JRyan
 
Re: NEC Req

PS :

Include a high resistance neutral grounding resistor assembly, with a ground alarm contact, and the ability to pulse the ground current level to allow tracing of any ground fault. Nominal ground fault current shall be 5 amperes.

This is around a $5k addition but with a ~$40k savings, I think a good trade... :)
 
Re: NEC Req

According to Table 450.3.A

- Supervised locations Note 1
- 250% for the primary
- Not required for secondary

But, since 240.21.C
Subpart 1:
Since I have a delta/wye connected xfmr, then
"transformer secondary conductors are NOT considered to be protected by the primary overcurrent device"

I do not think I meet this but I do meet 450.3.A

I also meet 240.21.C.2

The secondary switchgear will have individual breakers to protect the secondary circuits.
But I do meet 240.21.C.3
(1) the conductor sizes and ampacities are in line
(2) All the secondary OC devices are grouped in the swicthgear
(3) The Secondary conductors are in RGS conduit.
 
Re: NEC Req

I think I am ok now. Thanks for your help everyone. This is a great forum and I will stop in now and again.
 
Status
Not open for further replies.
Top