# MCCB Disconnect



## McClary’s Electrical (Feb 21, 2009)

Felidan said:


> Since MCCB are fast replacing the fused switch disconnects in control panels, does anyone knows of a quick calculator to size them?


 
Please clarify question. do you mean calculating the size of the disconnect for a motors control center?


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## Felidan (Jan 21, 2011)

It's a control panel for Industrial Machinery. I know for the "clasic" fused disconnect, I would get a 30 Amp unit and fit it with 25 Amp fuses, done!
The load would be : two motors, 3 Ph at 240VAC, 4.2 FLA, a DC drive pulling 10 Amp max and a control transformer, 350VA.


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## JRaef (Mar 23, 2009)

125% of the largest load FLC plus total FLC of all other possible connected loads, rounded up to the nearest standard size. It's in the NEC.

So 4.2 x 1.25 = 5.25 + 4.2 + 10 + about 1A for the CPT = 20.45A, round up to 25A. 

Well whadayaknow...


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## Felidan (Jan 21, 2011)

Is it 430.110 you are looking at?


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## JRaef (Mar 23, 2009)

Felidan said:


> Is it 430.110 you are looking at?


Huh, interesting. Yes that's apparently the section, but I remembered it as 125%, not 115%. I'll have to look at my 2002 book and see if that has changed, or maybe I'm remembering UL508 rules. Now I'm questioning where my brain came up with 125%...


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## McClary’s Electrical (Feb 21, 2009)

JRaef said:


> Huh, interesting. Yes that's apparently the section, but I remembered it as 125%, not 115%. I'll have to look at my 2002 book and see if that has changed, or maybe I'm remembering UL508 rules. Now I'm questioning where my brain came up with 125%...


440.33 is one of the placest you may have seen it


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## JRaef (Mar 23, 2009)

Nope, here's where I got it from:

430.24 Several Motors or a Motor(s) and Other Load(s).
Conductors supplying several motors, or a motor(s) and
other load(s), shall have an ampacity not less than 125 percent
of the full-load current rating of the highest rated motor
plus the sum of the full-load current ratings of all the
other motors in the group, as determined by 430.6(A), plus
the ampacity required for the other loads.

So you size the conductors feeding it per this rule, then size the feeder to protect the conductors. But the disconnect INSIDE the control panel could apparently be sized at 115% per the above.


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