Panelboard with no main in industrial facility

[diakonos1984]

Been perusing the forums, and haven't found the answer to this one yet:

We're looking at putting in a 2500 kVA 12470-480V transformer to feed a 3000A MLO panel with 5 or 6 large molded case breakers. It hasn't been determined whether it will be a Delta-Delta or Delta-Wye transformer yet.
This is in a very large industrial plant, and is a very small piece of their electrical system.
The cable length between the transformer secondary and the panel is roughly 65 feet.
We're planning to install CTs on the transformer secondary and use a protective relay on the primary side of the transformer to trip the primary side breaker for secondary side faults.

Does this satisfy NEC requirements for panelboard overcurrent protection (408.36) and for transformer secondary conductor overcurrent protection (240.21(C))?

Thanks!

 

[kwired]

Are these transformer secondary conductors considered service conductors? If so overcurrent protection is not the same as for feeders and there is no tap rules for service conductors. They need sized to handle calculated load but can and often do have overcurrent protection that is higher than ampacity when there is more than one disconnecting means.

Six disconnect rule comes into play - but looks like is not a problem from what is mentioned

 

[zog]

Your method will eliminate the typical "blind spot" for arc flash concerns as well.

 

[diakonos1984]

I don't think that these could be classified as service conductors.

You may have to help me out here, but this is a petrochemical plant, and this system is a long way from the utility service.
The panel will be installed in a powerhouse (a self-contained power building raised so many feet off the ground to get it out of a Class 1, Division 2 area) and feeds process equipment.
It's not "a building or structure served...supplied by only one service", unless I'm misunderstanding the definition of a "service conductor".

I was quite curious if someone could tell me whether or not the six throws of the hand rule comes into play.

 

[diakonos1984]

Zog,

Eliminating arc flash was our main purpose. We call this a "virtual main", and have used it to correct many existing installations where engineers decades ago saved money by not installing a main breaker in their switchgear. Now we're considering using it on a new installation to once again, "save money by not installing a main breaker". We know we can justify it from an arc flash standpoint; can we justify it per NEC?

 

[jim dungar]

We know we can justify it from an arc flash standpoint; can we justify it per NEC?

Absolutely.

NEC 2005 (what I have near me right now):
240.20(A) specifically says a CT and relay are equivalent to an overcurrent trip unit of a breaker.
240.92(B)(2)(3) discusses CTs and relays sensing secondary currents and then opening upstream devices.

 

[diakonos1984]

Thanks Jim, that is helpful.

Let me put a little twist on this to see if I understand correctly:

The 6 throws of the hand rule is for SERVICES.
My example is a SEPARATELY DERIVED SYSTEM.
I could have 10 feeder breakers in my MLO panel, and as long as my transformer secondary conductors are protected per articles 240.20 and 240.92 somehow (maybe just by being short, or having adequate primary protection) it would be code compliant. The 6 throws rule doesn't apply here.

Is that right?

 

[jim dungar]

...The 6 throws rule doesn't apply here.

Is that right?

Maybe.

240.21(C) is one area that covers the protection of trnsformer secondary conductors, some of the options do have a '6 device' limitation.
450.3 is really about sizing primay side protection based on if secondary protection exists, again there is a limit of '6 devices'.

240.92 is the area which may allow more than '6 devices'.

I regularly suggest and use a Virtual Main scheme for arc flash reduction. CT's and a relay definitley take up less floor space and also reduce maintenance costs, when compared to a 3000A 'main' breaker. Also I seem to be doing more 480V wye transformer secondariess with HRG.

 

[rbalex]

Except for a delta-delta, as described, none of the methods in 240.21(C) are applicable. As Jim mentioned though, 240.20(A) and 240.92(C)(2)(3) are. As such, six disconnecting means are not relevant since you are creating a "virtual" main.

I'd personally recommend an HRG although, since the design seems to be radial rather than some form of selective system, continuity of service can't be too important in the described application. If continuity is important and a delta secondary or HRG is used, I would avoid THHN/THWN for any conductors below #8. The insulation will become overstressed under a sustaiined ground-fault condition.

Be careful to observe 501.30 with your stubups into the powerhouse. With judicious design of the stubup you may be able to eliminate seals [See 501.15(B)(2) Ex 1, assuming you typically classify the underground as "unclassified"] but proper grounding/bonding is still critical.

 

[diakonos1984]

Thanks guys. I think I'm sufficiently straightened out. Wonderfully helpful once again.

Great forum.

And yes, we are using an HRG.