Industrial control panel OCPD rating

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mjmike

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I am working on a project where there is an industrial control panel with a 600A horizontal bus serving 3 buckets. The original design drawings showed a 100A main fused disconnect bucket but the installed unit is actually only main lugs. It is served off a tap from a wireway from a 480V 3-phase service disconnect with 400A fuses. We need to select the OCPD and add a main disconnect beside the unit. The buckets are as follows:

#1: 100A-3P thermal mag CB serving a 25HP motor and 150VA control transformer.
#2: 100A-3P thermal mag CB serving a 25HP motor and 150VA control transformer.
#3: 50A-2P thermal mag CB serving a 7.5 KVA transformer with a 120/240V sec. this is for a small load center in the bucket. The transformer is actually in the bottom of the control panel which could be considered a 4th bucket.

I am considering this an industrial control panel per the definition even though it looks like a motor control center. Per 409.21.C, I sized it as follows:

100A + 34A (motor #2), + 0.31A (control transformer #1), 0.31A (control transformer #2), 15.6A (7.5KVA/480V for transformer) = 150.22A.

I would select a 150A rated OCPD because NEC 409.21.C says I can't go over the calculated load of 150.22 which means I select the next size down.

Id I calculated it out with the actual panel loads as opposed to the transformer size, I get 147.98A meaning I would need a 125A OCPD (next size down).

This is a struggle because the current OCPD is technically 400A, the bussing is 600A main lug, the original design drawings called for a 100A fused main disconnect bucket, and the panel data tag indicating the amps is blank.

When sizing the OCPD, it seems odd you would add the loads up, and pick the next smaller breaker? Just does niot sound right.

Any advise would be appreciated.
 
What are you trying to protect?
This is the way that I would look at it.
since breakers are used to protect conductor, the conducted has been sized to carry the calculated load.
You have established where the breaker is to be located then has it been sized to protect the conductor(s) down stream?
 
Why is it that selecting a breaker which is the next size *down* from the calculated load seems wrong to me?
Next size down from the calculated wire ampacity, that makes more sense. And if that ends up to be below the calculated load, the wires are too small.


Tapatalk!
 
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templdl said:
What are you trying to protect?
This is the way that I would look at it.
since breakers are used to protect conductor, the conducted has been sized to carry the calculated load.
You have established where the breaker is to be located then has it been sized to protect the conductor(s) down stream?
Click to expand...

Because an industrial control panel requires a main OCPD and there is not one now. Per the definition of an industrial control panel, even though it looks like a motor control center, it appears to be an industrial control panel.
 
mjmike said:
Because an industrial control panel requires a main OCPD and there is not one now. Per the definition of an industrial control panel, even though it looks like a motor control center, it appears to be an industrial control panel.
Click to expand...

Where is this to be found in the NEC?

In any case, there is considerable overlap in functionality between ICP, MCC, and switchboards.
 
petersonra said:
Where is this to be found in the NEC?

In any case, there is considerable overlap in functionality between ICP, MCC, and switchboards.
Click to expand...

409.21.A Indicates the control panel shall be provided with overcurrent protection. 409.21.B is the location; either an OCPD exterior to the control panel or one in the control panel.

I see where it could be considered a MCC as well per the definition of a MCC in article 100. It actually does appear to be more of a MCC than an ICP. If it is a MCC, then per 430.94, the OCPD can be anything as long as the common power bus amperage isn't exceeded. Meaning, with my bus being 600A rated, the 400A service disconnect for the wireway is properly protecting this panel. We could then place a 200A OCPD just for the panel and that should suffice. I am basing this on the following calculation:

25HP motor = 34A x 2.5 = 85A = 90A CB
25HP motor #2 = 34A
2 control transformers = 150VA x 2 = 300va x 1.25 = 375/480 V = 0.78A
power transformer = 7.5KVA x 1.25 = 9375VA / 480V = 19.53A

90A+34A+0.78A+19.53A = 144.31A. So a 200A OCPD should be adequate.
 
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IMO, it is a MCC if it has a UL sticker on it saying it is a MCC.

It is an ICP if it has a UL sticker on it saying it is an ICP.

if there is no UL sticker on it, than it can be whatever you want it to be.
 
petersonra said:
IMO, it is a MCC if it has a UL sticker on it saying it is a MCC.

It is an ICP if it has a UL sticker on it saying it is an ICP.

if there is no UL sticker on it, than it can be whatever you want it to be.
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I think that is the salient point here. Once you forego the idea of having an NRTL listing of the panel/MCC, why are you then concerned for regulatory compliance?

If this has "buckets" that stab into a common bus bar, I don't see how that can be anything except an MCC, at least if UL is involved, which would be listed under UL 845. The closest thing UL 508A has to an MCC concept are the IEC style combination starters that plug onto bus bars, but not in separate "buckets", they all just line up next to each other on the bus, which goes into one common cabinet. Because of the style, those will usually need "Type 2 Coordination" which will require, at some point, a maximum size OCPD ahead of it somewhere, but its usually larger than the maximum bus rating of those systems anyway, so that just leaves the maximum bus size as the determining factor for the OCPD.

You can ALWAYS go smaller with the OCPD, but at the risk of nuisance tripping, not regulatory compliance. So I'm not sure if you are interpreting the 409.21.C correctly, something doesn't sound right, but I don't have my NEC here with me right now.
 
mjmike said:
Because an industrial control panel requires a main OCPD and there is not one now. Per the definition of an industrial control panel, even though it looks like a motor control center, it appears to be an industrial control panel.
Click to expand...

So, you need an OCPD. As I asked before what are you trying to protect? Or is it a main disconnect?
Should you use a 15at are 1200at OCPD? Or, should we pull a rating at random out of a hat?
 
I think it is Rittal that makes a pricey enclosure system for UL508a ICP that looks a lot like a MCC. It has individual compartments for starters or feeders that connect to a bus system behind it.

Why anyone would want to use it is beyond me, but it seems to have at least some people who like it.
 
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