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Discussion Starter #1
First of all i am not well at english.so please excuse me.
When i was working on a motor control panel which is 120V control, i found some neutral connections are taken from supply transformer instead of control transformer(380V/120V) to complete the control circuit.
My question is, is it necessary to run neutral connection from control transformer to control all the motors?? If yes, why?
 

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First of all i am not well at english.so please excuse me.
When i was working on a motor control panel which is 120V control, i found some neutral connections are taken from supply transformer instead of control transformer(380V/120V) to complete the control circuit.
My question is, is it necessary to run neutral connection from control transformer to control all the motors?? If yes, why?
Assuming here it is a grounded neutral, it's all the same. Local codes may have something to do with it, I have no idea what the codes are in Saudi Arabia.

Here in the US we can create a new neutral in a control panel by grounding one side of a control power transformer. This is often done because for us, we can have a Wye supply or an ungrounded Delta supply, or even if its a Wye supply, the neutral may not be brought out to a motor control panel if the neutral is not going to be used; we may not know in advance. So using a CPT (Control Power Transformer) with a grounded secondary to create its own neutral for just the control circuit makes it safe no matter what.

In most of the rest of the world, Delta power grids are non-existent, everything is Wye so there is a neutral conductor run to every panel. In that case, the control circuit neutral can be connected to it. In different countries, there are also different standards as to HOW ground connections and neutral connections are made and separated, so that's why your local code may have REQUIRED not using the grounded secondary of a CPT as the neutral. This often gets confusing for people outside of North America when they get equipment built here, because they are not used to 120V control circuits.
 

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Discussion Starter #3
Thank you sir for your kind reply. Yes we can take neutral from CPT as you told. I have one more question.what happens if we connect some of neutral connection from CPT and some other from Supply transformer's neutral for same motor control panel? I am asking this because i found control wiring like this in one motor control panel.
 

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Assuming that the neutral from both transformers were grounded (earthed), the 120 volt control equipment would function just fine, but it would not be a good idea to have the ungrounded conductors and the neutral (grounded conductor) to originate from different sources, even if they are bonded to the same path to ground (earth). Using a neutral from a source separate from the ungrounded conductors would be a code violation in many locations.
 

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Discussion Starter #7
360max said:
do you wire according to our National Electric Code (USA) or do you have your own code in Saudi Arabia?
Yes, they do have their own code called 'Saudi arabian distribution code'. But i just want to know whether it is technically inappropriate to perform these kind of wiring in control panel and i got my answer. Thank you for your concern.
 
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