110.26(C)(2) rating

[ryan_618]

110.26(C)(2) applies to large equipment (until 2005, anyway) that is rated 1200 amps or more.

Question: If I have a panel that is large, and is rated 1200 amps, but has a 1000 amp GFPE main breaker, does this equipment require 2 doors and panic hardware?

 

[charlie b]

Re: 110.26(C)(2) rating

Yes. The rating of an item is not altered by our choice of how to use it (provided, of course, that we use it within its limits and its listing). I think you would agree, so I am curious as to why you are asking.

The 2005 version does not change this part of the wording. It appears to have removed the six foot wide part of the requirement. So anything rated 1200 amps or more falls into this rule, regardless of its width.

 

[jtester]

Re: 110.26(C)(2) rating

Ryan
The way I read it, yes. If the switchboard was rated for 1200 amps and the largest switch was rated only 200 amps, I believe that you'd still need the 2 exits, etc., because of the equipment rating.
Jim T

 

[ryan_618]

Re: 110.26(C)(2) rating

I view the rating to be the governing factor as well, but think of it this way:

The intent of the rule, as well established in the ROC, is that a 1200 amp fault is a dangerous thing, regardless of the physical size of the equipment. So really, what governs the extra space and extra rules really is the size of the fireball that spews at you under a 1200 amp fault. The 1000 amp breaker is going to limit the size of fault, is it not? What if I had a panel that was rated 1200 amps, but I only had a 400 amp main breaker in it? I can't think that it would warrant the extra door and panic bars, do you?

 

[iwire]

Re: 110.26(C)(2) rating

I agree with Ryan's read on the intent.

 

[ryan_618]

Re: 110.26(C)(2) rating

I think I might try to come up with some code language to submit for the 08 cycle on this. I really do think the OCPD should govern, not the size of the bus bars.

 

[rbalex]

Re: 110.26(C)(2) rating

You already know I don't generally like "crystal-ball" enforcement, but one of the most common reasons a design would have a 400A main, in 1200A switchgear is specifically for "future" expansion with a lower initial installed cost.

The 1000A breaker in 1200A gear is a bit more "iffy;" but again, a very common industrial upgrade is to replace a main in such cases.

The answer your last question is that the primary limitation is the upstream impedance. The breakers' interrupting ratings(i.e., let-through) may be the same throughout the entire class of breakers.

I agree with Charlie and Jim.

Edited for gross spelling errors

[ January 05, 2005, 02:00 PM: Message edited by: rbalex ]

 

[jim dungar]

Re: 110.26(C)(2) rating

A "local branch" OCPD would not reduce the fireball that occurs on the internal buswork of the equipment. The fault would only be limited by the upstream feeder OCPD which is "rated 1200A or more", thus requiring the added egress requirements.

 

[jtester]

Re: 110.26(C)(2) rating

Ryan
I follow your logic, but I don't agree. First, unless you have a CL breaker, the same amount of fault current will flow thru a 400 amp breaker as thru a 1200 amp in the same application. If there is a failure of the switch, even a CL breaker will fault the amount available on the line side.
If you had a 1200 amp M.L.O. switchboard with a bunch of smaller breakers, a bus fault would be just as ugly as if you had one 1200 amp breaker.
Jim T

 

[charlie b]

Re: 110.26(C)(2) rating

Originally posted by ryan_618: The 1000 amp breaker is going to limit the size of fault, is it not?

No it will not; I have to agree with Bob on that point. The fault is limited by the instantaneous rating of the breaker, not the long time rating.

The amount of the fault, and the size of the fireball, are limited by the available voltage and the overall circuit impedance. The rating of the breaker has no impact here.

But I cannot see the size of the fireball as being related to the issue. If you are in front of the gear when the fault occurs, having two doors will not help you survive. The important thing is to be able to leave the room after the fault has occurred, presuming you had survived the fireball, without having to risk walking in front of, and close to, the gear. Having a door at both ends of the room allows you to get out, without walking in front of the gear. Having double the working clearance allows you to walk in front of the gear to the one and only door, without walking close to the gear.

[ January 05, 2005, 02:34 PM: Message edited by: charlie b ]

 

[tom baker]

Re: 110.26(C)(2) rating

Is not the size of a service based on the size of the ungrounded conductors, which are based on the calculated load? If the load is 1,000.6 amps you can round up to the next standard OC device which is 1200A (see 240.6(A). Is this a 1000 amp or 1200 amp service?
The arc blast is dependant on the fault current avaiable from the power transformer.

 

[iwire]

Re: 110.26(C)(2) rating

Obviously I am over my head here so I will ask some questions

Originally posted by charlie b:
The amount of the fault, and the size of the fireball, are limited by the available voltage and the overall circuit impedance. The rating of the breaker has no impact here.

Understood.

I would also think the number 1,200 on the label will not change anything either.

Originally posted by charlie b:
But I cannot see the size of the fireball as being related to the issue. If you are in front of the gear when the fault occurs, having two doors will not help you survive.

Also understood.

It seems that the important issue is what Tom brought up, what is the fault current available form the source, it seems it has nothing to do with the gear size or the OCPD size.

If I am correct with my above statement wouldn't make more sense that 110.26(C)(2) would base the requirement on the available fault current?

Please help me understand.

Bob

 

[jtester]

Re: 110.26(C)(2) rating

Bob

The logic seems to be "...the fault currents that are typically available at the bus of a utility service provided for a customer who has 1200 amps of gear, are sufficiently high to make 2 exits desirable...". You are exactly right that the driving force, pun intended, for additional safety measures should be the fault current value. There is probably a strong relationship between 1200 amp services and the threshold of objectionably large fault currents. It is easier for an inspector or consciencious installer to verify the rating of a piece of gear, than to determine the fault current available at the service.
Jim T

 

[rbalex]

Re: 110.26(C)(2) rating

Originally posted by iwire:
...
If I am correct with my above statement wouldn't make more sense that 110.26(C)(2) would base the requirement on the available fault current?
...
Bob

Yes it would. And, indirectly, it does. Using relatively standard equipment, especially transformers, 1200A implies a certain minimum available fault level. You could possibly make an "under engineering supervision" exception with lots of supporting 70E calcs.

 

[iwire]

Re: 110.26(C)(2) rating

Thanks guys that makes sense, just a way to make enforcement easier.

Bob

 

[gary b]

Re: 110.26(C)(2) rating

Ryan?s comments have merit. Herein is the dilemma. The service installation from the XFMR to the MDP is wired with 4-parallel runs of 255 kcmil THHN conductor. The service is a 480/277V, 4-wire system. The utility XFMR is located approx. 57-feet from the MDP. The calc load of this installation is 678 A. PQ issues have been considered because to the heavy solid state loads in the building.

The service is sized at 1000A (MCB) GFPE. The available Main switchboard panel has a 1,200 amp rating. The question here is what determines the ?rating? as required by 110.26(C)(2) in this case, the MCB or the nameplate rating of the equipment? Do the short circuit calcs help with this problem?

Note: If I install a 225-load center (subpanel) in a building and feed it from a service main breaker rated at 125A what is the rating of the load center? The load center has no additional main CB in it, MLO, and supplies BC loads.

 

[ryan_618]

Re: 110.26(C)(2) rating

Welcome to the forum Gary.

For those of you who don't know Gary B, he is a very qualified electrical instructor. You might even find his name in the opening credits to some of Mike Holt's books. I have much to thank Gary for, so I would like to be the first to extend to him a warm welcome.

 

[engy]

Re: 110.26(C)(2) rating

I would say 1200A bus with single 1000A non-adjustable trip main, should be considered rated as 1000A. I don?t think 1200A is a (the only)magic number, since if it is less than 6? wide, then no second exit is required.

 

[gary b]

Re: 110.26(C)(2) rating

It looks to me that this can go either way with the code laungage. It does create some confusion. As an installer or service person I think I would perfer to have the second exit and would tend to support the more conservitive look based on the equipment rating of 1200-A. I understand the economics of the added door, but what is the price of a life?

 

[sandsnow]

Re: 110.26(C)(2) rating

I've been pondering this since it was posted. I would have to go with the the nameplate on the switchboard.
Although if the applicant wanted to apply to the Chief Building Official for a variance to the code on this, a compromise could be reached based on the arguments about what is actually going to be supplied from the POCO. Conditions would have to be in writing, etc.

Without anything in writing backing up the lack of 2nd door, the inspector in the field as well as the designer and contractor leave themselves open to liability should there be an accident.