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Old 09-17-2012, 10:50 PM   #1
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single phase subpaneling from 3 phase main panel.


I'm looking to build a living quarter section on my shop. Shop is supplied with 200 amp 3 phase service. There is plenty of room in the main panel. All of the appliences in the apt. will be gas so I'm only looking for receptacles, ceiling fans, lights and a washer. Can I just run 50 amps or so from 2 phases of the main panel to a subpanel and then wired the apt. like a standard single phase? I understand I will need to make sure I'm not using the high leg of the system. How can I make sure I keep the phases balanced? Thanks so much for your time

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Old 09-17-2012, 11:26 PM   #2
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single phase subpaneling from 3 phase main panel.


Are you sure you have a high leg service or is it true three phase?
If its a high leg service then yes you would just use the 120/240 part of the panel to feed your sub.

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Old 09-17-2012, 11:48 PM   #3
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single phase subpaneling from 3 phase main panel.


No I guess I'm not sure. What's the best way to check? Phase to ground?
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Old 09-18-2012, 12:31 AM   #4
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single phase subpaneling from 3 phase main panel.


Normally the wild leg if you have delta system they will be marked in orange colour.

On the breaker panel the wild leg lugs will be every third spot when you go down the panel you will tell the differnce on non wild leg and wild leg part.

If you have voltmeter and read line to ground or netural the normal phase to ground is typically 120 volts while wild leg to ground even netural it will useally read 208 volts so if you have any 120 volts load try to advoid this one otherwise you will make magic smoke pretty fast.

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Old 09-18-2012, 08:05 AM   #5
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single phase subpaneling from 3 phase main panel.


Take the two non-high legs and the neutral and a ground to the subpanel. Measure voltage from hot to neutral (aka phase to neutral). Do the wiring in the same manner as you would wire up a subpanel to a single phase 120/240 volt panel.

For a symmetric (120/208 volt wye) 3 phase system you can use any two hot legs.

Make an educated guess for balancing the various subpanel branch circuits (subbranch circuits if you insist) between the two legs coming into the subpanel. It does not have to be exact. It is not necessary to think about it or turn on something else on the other leg each time you turn on tools or televisions or washing machines to be sure the load is balanced.

Be sure the zoning is correct and you get the proper electrical and plumbing and occupancy permits for an additional dwelling unit (above your shop) on your property.
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Last edited by AllanJ; 09-18-2012 at 08:15 AM.
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Old 09-18-2012, 01:09 PM   #6
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single phase subpaneling from 3 phase main panel.


Why set a sub panel? Just pull the needed circuits from the main panel. The living area is located under the same roof.

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