3 phase


Old 07-14-06, 07:37 PM
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3 phase

I have a 3 phase 100 amp circuit terminating in a knife switch box. What it use to operate is taken out of service. Can I put a sub panel and take 2 legs of the 3 phase and run a neutural to the box to get a 220 30 amp circuit for an air conditioner and may be some 110 20amp circuits as well.
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Old 07-14-06, 07:45 PM
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That depends on the voltage of the 3-phase and the size of the wire.
Are you qualified and insured to do electrical work in a commercial setting?
Do you own this building?
Old 07-14-06, 08:11 PM
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3-phase what..? voltage.? 3-wire 4-wire.? Off the bat you should have a fused disco for the things you mentioned.
Where does it originate from? Could this somehow feed a sub panel?
Old 07-19-06, 10:35 AM
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3 phase

I will explain it better:

Current config

100 amp VAC on a breaker in main panle with other circuits.
going to a knife switch terminating in a sub panel with 2 branch circuits. All 3 phases are 120 vac

New config

I am disconnecting the sub panel adding a new sub panel and 30amp breaker with a 240 30amp vac delivered on a 10/3 solid wire BX to a 30 amp receptacle with a ground going to a main upright I-beam in building drilled to 5/16 hole brushed to bare metal. Applying a non corrosive electrolytic paste with a #8 crimped lug which is connected to sub panel ground. The original question stands. Are the legs of the 3 phase stable voltage and can they be split this way
Old 07-19-06, 03:46 PM
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Is there a neutral in the main panel?

and why are you running the ground to building steel?
Old 07-20-06, 04:40 AM
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Something doesn't add up here.
Old 07-20-06, 07:06 AM
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Hmmmmm... I've been following this thread and because of the number of knowledgable people replying have just kinda read along. But I must say that there is some concern that you fully understand what you are doing in order to have a safe installation of your sub-panel. Some of things you are saying is causing that concern.
First you need to verify (be certain) what three phase supply you are dealing with. The only configuration I can think of that would have 120 volts on all phases, as you state, would be 208Y120 three phase. That would require a neutral in the panel serving the phase conductors if 120 volt branch circuits are desired. Also I'm not sure what you are testing the phase conductors in respect to.... ground or a neutral wire? Please look at this link and verify with us which 3 phase you have. You may have to call your utility to be certain. You are going to need a 3 phase four wire configuration to do what you want. Specifically the last two delta configurations shown on the link would give you the capability for 240 volt loads and 120 volt loads from a sub-panel....please note that you must identify the phase that is "high leg" as it is unusable for phase to neutral loads. The wye configuration (208Y120) will only give 208 volts on any two legs but you will have 120 volt loads utilizing the phase to neutral voltages.


As Bob says things dont add up with what you are describing.

Also NAP brings up an observation about connecting your ground (equipment ground) to building steel....you do not connect the fault path ground to building steel intentionally. At the main panel you may bond the building steel to the grounding electrode system but you would not connect the equipment ground of a sub-panel to building steel.

Go back to the panel that serves this knife switch disconnect and read the panel supply description as to what three phase it is configured. This will answer a lot of concerns in what you are wanting to do. Right now we are just speculating as we need answers to all the questions.


Last edited by Roger; 07-20-06 at 09:40 AM.

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