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  1. #1
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    Default RPC Wiring question

    Hey guys with the help of the forums and a few different designs i have put together a 10hp RPC. I have a question in regards to the neutral line and the ground line.

    my setup consists of:
    my main 240v single phase panel has the neutral line and grounds separated. the bare ground goes to a rod in the ground.

    i have 240v single phase going into my RPC box to power everything. I brought the Neutral line and ground line into the RPC box and are separated still. i use the neutral because my 2 contactor coils are 120 single phase.

    then i send the 3 phase legs from the RPC box to a 3 phase panel which has the breakers for the different machines. what i noticed i did was i brought the neutral line down into the 3 phase panel instead of the ground line.

    what should i bring into the 3 phase panel? neutral or the bare ground? and why?

    i have 3 things running on this RPC. i only run 1 at a time. 1st is a forklift battery charger, 2nd is a makino rmc55 mill which power goes through a transformer to go from 230v to 200v and 3rd is a hurco mill which also goes through a 230v to 208 transformer.

    thanks for the help in advance

  2. #2
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    Your 3 phase lines should have a safety ground (green wire).

    A small control transformer would allow for 240vac to 120vac, neutral not needed.

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    Quote Originally Posted by rons View Post
    Your 3 phase lines should have a safety ground (green wire).

    A small control transformer would allow for 240vac to 120vac, neutral not needed.
    +1

    If any of the downstream loads need a neutral, then bring both. But the green wire
    groundING conductor always needs to be there.

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    Quote Originally Posted by alienturtle View Post
    Hey guys with the help of the forums and a few different designs i have put together a 10hp RPC. I have a question in regards to the neutral line and the ground line.

    my setup consists of:
    my main 240v single phase panel has the neutral line and grounds separated. the bare ground goes to a rod in the ground.

    i have 240v single phase going into my RPC box to power everything. I brought the Neutral line and ground line into the RPC box and are separated still. i use the neutral because my 2 contactor coils are 120 single phase.

    then i send the 3 phase legs from the RPC box to a 3 phase panel which has the breakers for the different machines. what i noticed i did was i brought the neutral line down into the 3 phase panel instead of the ground line.

    what should i bring into the 3 phase panel? neutral or the bare ground? and why?

    i have 3 things running on this RPC. i only run 1 at a time. 1st is a forklift battery charger, 2nd is a makino rmc55 mill which power goes through a transformer to go from 230v to 200v and 3rd is a hurco mill which also goes through a 230v to 208 transformer.

    thanks for the help in advance
    Neutral and ground are connected together in you panel unless you have ground fault. But neutral and ground are not the same though they are connected at the same place. You asked WHY? OK ground can NOT carry current. Neutral can. Reason why as you noticed the ground is bare though it don't have to be and is usually a green insulated wire. It is intended to provide safety if it is carrying current and all the machinery frames are connected to the ground and for some reason the conductor is broken then all grounded items before the break will be hot to ground and become a shock hazard. If you already have a neutral then tape it with green tape and use it as ground but not to carry current such as a 120VAC light on the machine.

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    This will display my lack of fundamental understanding, but could someone elaborate more on methods of deriving 120vac 1ph, (control transformer vs pulling a neutral from the main panel) and what the preferred method is and/or what applications would require a specific method?

    Currently, my main issue is determining the proper procedures for hooking up used equipment to an RPC, specifically things that incorporate single phase components (vertical bandsaw, mig welder, mill w/ lighting & 1 ph controls, etc).

    Thanks in advance.

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    Quote Originally Posted by EW57 View Post
    This will display my lack of fundamental understanding, but could someone elaborate more on methods of deriving 120vac 1ph, (control transformer vs pulling a neutral from the main panel) and what the preferred method is and/or what applications would require a specific method?

    Currently, my main issue is determining the proper procedures for hooking up used equipment to an RPC, specifically things that incorporate single phase components (vertical bandsaw, mig welder, mill w/ lighting & 1 ph controls, etc).

    Thanks in advance.
    This is a typical method of effectively creating a neutral with an isolation transformer. One leg of the transformer secondary is connected to ground. That leg may then be used as a neutral.

    I have two RPC's in my shop. On one of them, I used this method to provide a neutral for a red power lamp, a Hobbs meter and also to operate contactors. On the other RPC, I pulled in a regular neutral from my sub panel.

    preview.jpg

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    A couple things.

    Bring the ground and not the neutral. The ground is not supposed to have enything flowing in it other than fault currents. Neutral has other current in it.

    The neutral is ONLY the neutral for the single phase input. It is NOT a 3 phase neutral, and cannot be used as that. The generated leg is at a different voltage from the single phase neutral than the other two lines are. If it were a 3 phase neutral, then all the 3 phase lines would have the same voltage to it, but they do not.

    There is no reason to bring it into the 3 phase distribution, and there is a very good reason not to.... because it is NOT a 3 phase neutral, and it is not a ground.,


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