# Sizing Main Disconnect For Control Panel?



## Voltshead (Mar 18, 2008)

I am looking for a quick rule of thumb when sizing the main breaker or fusing in control panels. Do you add up all the supplemental breakers and fusing and add 10% to get the main breaker size? All the components do not operate at the same time so the actual load will be much less than the sum of all the supplementary circuits. My back gound is stronger in control voltage, PLCs and VFDs. Power distribution is not my strongest point yet.


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## nolabama (Oct 3, 2007)

this may be a dumb question, but what is the control panel controlling? motors, lights, machines?


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## Voltshead (Mar 18, 2008)

I guess that would be a good thing to list. I have 8 each 1 HP frequency drives, 1KVA transformer, 480 to 24vdc power supply. The supply voltage is 480 3PH. I like using calculators to make designing move along faster. Do you have any suggestions for sizing breakers?


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## nolabama (Oct 3, 2007)

sorry; way, way out of my league. Unfortunately outside of the ability levels of the people i work around so i go to this place.


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## LGLS (Nov 10, 2007)

Voltshead said:


> I am looking for a quick rule of thumb when sizing the main breaker or fusing in control panels. Do you add up all the supplemental breakers and fusing and add 10% to get the main breaker size? All the components do not operate at the same time so the actual load will be much less than the sum of all the supplementary circuits. My back gound is stronger in control voltage, PLCs and VFDs. Power distribution is not my strongest point yet.


You size the main to protect the feeder, not the subsequent loads.


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## John (Jan 22, 2007)

Voltshead said:


> I am looking for a quick rule of thumb when sizing the main breaker or fusing in control panels. Do you add up all the supplemental breakers and fusing and add 10% to get the main breaker size? All the components do not operate at the same time so the actual load will be much less than the sum of all the supplementary circuits. My back gound is stronger in control voltage, PLCs and VFDs. Power distribution is not my strongest point yet.


2.1 amps per motor times 175% = 4 amp and use FRN-R-4 fuses for each motor. This times 8 = 32 amps, so fuse it for a 30 amp duel element time delay fused main panel disconnect. Make sure you ground the 24 VDC secondary.


Carry On!


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## davdug (Feb 5, 2008)

I would like to know what size disconnect for a resistive load bank rated for 500KW at 380 volt,3 phase,3 wire, 50hz, and 1.0 power factor. I'm thinking simple knife switch that is unfused.


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## HARRY304E (Sep 15, 2010)

davdug said:


> I would like to know what size disconnect for a resistive load bank rated for 500KW at 380 volt,3 phase,3 wire, 50hz, and 1.0 power factor. I'm thinking simple knife switch that is unfused.


 IDK if you will get an answer the thread is from 3 years ago:laughing:


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## davdug (Feb 5, 2008)

I beleive you're right. I'll try to post again.


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