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 Post subject: Short Circuit Current Rating
PostPosted: Wed Oct 30, 2013 5:28 am 
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I am curious to see what other people are doing when performing a study that will include equipment duty evaluation when the equipment rating is unknown. This is particularly troublesome with panels and panelboards where the rating is unknown. The rating is usually on the inside of the panel so the front panel has to be removed. Depending on the age, many times the label is missing.
If the panel has a main breaker, that can be used as the panel rating. For example, a Square D NF panel at 480V, could be 35kA, 65KA, 100kA or 200kA but if the main breaker is a SQD EG, then the panel is 35kA.
But for MLO panels, what does one do? I have been defaulting to 22kA so there is a value to evaluate against. If these panels are overdutied, I will note in the report that it was a default value and that it is possible the panel may be rated higher but the manufacturer would have to be consulted.

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PostPosted: Wed Oct 30, 2013 7:18 am 
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1. Do not assume that the breakers are correct. It is fairly easy for the wrong breaker to be put in the wrong panel especially when they are all interchangeable with each other. This was a recent finding in a plant I'm aware of that had everything from 10 kA to 35 kA in the same panel.
2. If the whole point of doing the short circuit evaluation is to catch things like this, then you need to do a lot more research to find out what the panel rating is. And yes, this can be challenging. It is not difficult to do with some equipment and cabling. It can be prohibitively difficult with busbars and equipment that is sold old that the manufacturer is out of business (Westinghouse).

As a follow on, I have to frequently work with S&C expulsion fuses. Some are only rated to 12.5 kA and others are rated to 25 kA although the system short circuit potential is around 20 kA. But for the case of an expulsion fuse the way that it trips is to blow apart. If the fuse interrupting capability is less than the short circuit rating, obviously the fuse may blow apart. But from a practical point of view why would this even matter?


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PostPosted: Wed Oct 30, 2013 7:23 am 
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Thank you. When I said about the main breaker in the panel, I am not assuming the breaker but it has been verified in the field.
The only assumptions I am making is where the IAC rating is unknown. Then defaulting to 22k will give the program a value to work from and will show up in the equipment duty report.

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PostPosted: Thu Oct 31, 2013 11:31 am 
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wbd wrote:
Thank you. When I said about the main breaker in the panel, I am not assuming the breaker but it has been verified in the field.
The only assumptions I am making is where the IAC rating is unknown. Then defaulting to 22k will give the program a value to work from and will show up in the equipment duty report.

For 208 and 240V breakers it seems the minimum UL489 rating is 5kAIC.
For 480V the minimum seems to be 14kAIC.

IMHO, your 22kA level assumption is too high.


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PostPosted: Fri Nov 01, 2013 5:12 pm 
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True but I think practically speaking 10ka is probably the lowest that most manufacturers offer and that is usually in the 208V range whereas 22kA seems to be for 480V

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PostPosted: Mon Nov 04, 2013 7:32 am 
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wbd wrote:
True but I think practically speaking 10ka is probably the lowest that most manufacturers offer and that is usually in the 208V range whereas 22kA seems to be for 480V

wbd wrote:
True but I think practically speaking 10ka is probably the lowest that most manufacturers offer and that is usually in the 208V range whereas 22kA seems to be for 480V


Hey its your call.

Custom machine/control panels are often 5kA @480V.
Non-distribution panels, like those 400A and smaller and those often used as light switches, are usually 14kA @ 480V
Non-fused disconnect switches, typical for LOTO, are 10kA @ 480V unless they have a series rating with upstream devices.


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PostPosted: Mon Nov 04, 2013 9:15 am 
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I remember reading somewhere, if you don't know or information is missing you have to default to the lowest rating. I generally use 10kA if I do not know.


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PostPosted: Mon Nov 04, 2013 11:17 am 
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The really conservative answer is to quote National Electrical Code definition of:

Overcurrent Protective Device, Branch-Circuit -

[INDENT=1].....Such devices are provided with interrupting ratings appropriate for the intended use but no less than 5000 amps.[/INDENT]
It is on page 32 in the 2014 edition. This is often interpreted as meaning when no rating is known, the lowest possible rating is 5,000 amps. Except the obvious problem is this is extremely low and likely to be inadequate but at least it is traceable to a standard.

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PostPosted: Tue Nov 05, 2013 3:36 pm 
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Thanks Jim. I looked in several catalogs and it seems the lowest panel is 10kA. Probably just standard building techniques reault in this rating

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PostPosted: Tue Nov 05, 2013 6:00 pm 
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WBD, yes today 10 kA is the lowest. When I worked for a company in a past life, we used 5 kA to be conservative and traceable to a standard. From what I recall the reason was a long time ago some very old breakers with no markings were rated 5 kA. Great discussion!

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PostPosted: Thu Nov 07, 2013 9:10 am 
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I know when they were first introduced, Square D QO breakers were only 5kA. They did not become 10kA as standard, until maybe the mid to late 60's.


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