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Old 09-09-2009, 03:33 PM   #1
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Want to add a single phase Subpanel to three Phase Main


Here is my situation. I currently have a 3 Phase main 100 amp main with 12 slots. 12 slots ends up to be a few short as I have just purchased some new equipment and need to add a few recepticles. Most of my existing 3 phase panel is being used up by single phase items, lights, and two 220 outs. A larger 3 phase panel cost about 400 bucks I was wondering if it was possible to jump a single phase sub panel and run all the single phase off that box and the 3 phase off the existing main. Single phase main is about 100 bucks and would make it easier in the future to seperate 3 phase machines from single phase stuff. I have space right next to the existing panel for installation just need to reroute some conduit but no big deal. None of the machines operate at the same time but I want to have a seperate breaker for each panel will be far from overloaded amperage wise. Just want to get a seperate opinion on this as well as uncover any issues with code that might be present. I am under the assumption I should be able to jump two of the main legs from the 3 phase and the grounds from the other box, but once again want to get a second opinion here. Thanks for your help.

Adam

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Old 09-09-2009, 04:01 PM   #2
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Want to add a single phase Subpanel to three Phase Main


You also need a neutral wire!
Just put in a 2 pole breaker, sized for the subpanel, 4 wires, and don't forget the ground bar in the subpanel.

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Old 09-09-2009, 04:29 PM   #3
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Yeah forgot to add the neutral before...Yeah pretty much exactly what I was planning. Thanks for the confirmation.
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Old 09-09-2009, 04:51 PM   #4
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Want to add a single phase Subpanel to three Phase Main


You could put in separate single phase 120 volt subpanels here and there on a different phase each for better balance among the phases. Also 208 volt subpanels can be fed off of two of the phases each.

You would need to be careful about hooking up single phase 120/240 volt equipment to 120/208 volt lines and/or 120/208 volt subpanels namely hooking up to 3 conductor branch circuits with two phases and neutral. Heating elements will work acceptably. Single phase motors that use both hot wires and also the neutral will probably not work correctly.
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Last edited by AllanJ; 09-09-2009 at 04:54 PM.
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Old 09-09-2009, 05:45 PM   #5
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Want to add a single phase Subpanel to three Phase Main


Your plan sounds reasonable.

If your 3 phase is 120/240 4 wire delta, make sure you choose the proper two legs.


If it's 120/208 wye, all legs are 120V to ground and you won't have any issues.
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Old 09-09-2009, 07:34 PM   #6
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Want to add a single phase Subpanel to three Phase Main


Quote:
Originally Posted by AllanJ View Post
... Single phase motors that use both hot wires and also the neutral will probably not work correctly.
I've never seen a motor that uses both hot wires and a neutral.
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Old 09-09-2009, 07:52 PM   #7
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Want to add a single phase Subpanel to three Phase Main


There one thing you have to keep in your mind anytime you get any single phase load run to the subfeed panel make sure you have same size netrual as hot { phase } conductors are { espcally if you have 208Y120 system in there }

Merci,Marc
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Old 09-09-2009, 08:21 PM   #8
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Want to add a single phase Subpanel to three Phase Main


Quote:
Originally Posted by AllanJ View Post
You could put in separate single phase 120 volt subpanels here and there on a different phase each for better balance among the phases. Also 208 volt subpanels can be fed off of two of the phases each.

You would need to be careful about hooking up single phase 120/240 volt equipment to 120/208 volt lines and/or 120/208 volt subpanels namely hooking up to 3 conductor branch circuits with two phases and neutral. Heating elements will work acceptably. Single phase motors that use both hot wires and also the neutral will probably not work correctly.
I will contest you on that. Heating elements, as well as motors (But the former will be more noticeable in their underperformance) that are designed for single Phase, 240 Volts will underperform when hooked up to Two legs of a Three Phase system that will yield only 208V. They will run only @ 91.3% of their capacity. The solution is to hook up a "Buck-N'-Boost" Transformer which would bring up the voltage to the rating of the appliance. I've done that with Commercial freezers that were designed to run on 3Ph. 240V.!(No matter what)Don't Drink and Drive, Ever!!!
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Old 09-09-2009, 08:35 PM   #9
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Want to add a single phase Subpanel to three Phase Main


Quote:
Originally Posted by frenchelectrican View Post
There one thing you have to keep in your mind anytime you get any single phase load run to the subfeed panel make sure you have same size netrual as hot { phase } conductors are { espcally if you have 208Y120 system in there }

Merci,Marc
You're right. Mais Oui Monsieur! At least the same size, if not larger, Due to (Triplen) Harmonics. (No. it is not a Musical instrument,...) Where the current in the Neutral does not get canceled out by the Phases. Rather it multiplies. It happens mainly when we run non-linear loads (Electronically controlled office equipment like copiers and Computers) Or Motors with Large Capacitors (Not built-in the Motors) (I don't think DIY'ers come across this problem too much.)(No matter what) Don't Drink and Drive, Ever!!!

Last edited by spark plug; 09-09-2009 at 08:37 PM. Reason: Typographical Error (TYPO)
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Old 09-09-2009, 08:49 PM   #10
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Want to add a single phase Subpanel to three Phase Main


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Originally Posted by 220/221 View Post
Your plan sounds reasonable.

If your 3 phase is 120/240 4 wire delta, make sure you choose the proper two legs.


If it's 120/208 wye, all legs are 120V to ground and you won't have any issues.
Most Mixed use (lights & appliances & Motors) Services are 120/208 WYE) Unless a Wye/Delta Transformer is inserted. Then again. The power Companies don't hook up Residences to such Service. (No matter what) Don't Drink and Drive, Ever!!!
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Old 09-09-2009, 08:57 PM   #11
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Want to add a single phase Subpanel to three Phase Main


Oh Yes! If you're running ANY 3Phase Motor, be absolutely certain that it's spinning in the right direction. Look at the ARROW at the base of the Machine. (Compressor, Etc.) The Motor has to be disengaged from the Machine and left to spin freely. If it spins in the WRONG direction, switch around ANY Two (of Three) wires. If this procedure is not followed, the Machine will be destroyed!!! Don't Drink and Drive, Ever!!!
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Old 09-09-2009, 09:42 PM   #12
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Want to add a single phase Subpanel to three Phase Main


So what is voltage rating of this service?
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Old 09-09-2009, 10:57 PM   #13
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Want to add a single phase Subpanel to three Phase Main


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I will contest you on that. Heating elements, as well as motors (But the former will be more noticeable in their underperformance) that are designed for single Phase, 240 Volts will underperform when hooked up to Two legs of a Three Phase system that will yield only 208V. They will run only @ 91.3% of their capacity.
Worse than that. Heating elements will only provide 75% of their rated power. P=IV, and I=V/R, so P=V^2/R. The change in power is therefore proportional to the change in voltage squared. 208^2/240^2=0.751, or 75%.

Motors are not resistive loads and will increase their current draw (within reason) while producing equal power output. The higher current causes higher losses in the windings, thus lower efficiency and more heat. If the motor isn't rated for 208V operation and is loaded to near rated capacity, it will likely overheat.
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Old 09-10-2009, 08:37 AM   #14
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Want to add a single phase Subpanel to three Phase Main


OT: I believe that calculating the performance of heating elements at different voltages has to be done by actual measurement. The element's resistance is lower when the temperature is lower so at equilibrium (I=V/R) with power applied, the current draw is greater and therefore the wattage consumed is greater than that (according to the square of the voltage) suggested above. The manufacturer may have done the math for you or taken figures from watts/volts curves (graphs) for that kind of heating element, publishing the wattage output in the instructions.
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Last edited by AllanJ; 09-10-2009 at 08:39 AM.
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Old 09-10-2009, 11:09 AM   #15
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Why the moderators continue to allow these professional arguments on a DIY forum are beyond comprehension. We have professional forums to discuss and argue our trade. This is not the place for it. The OP will get some help at first and then we all get into a discussion that the OP has no clue about. I said "we", because I am guilty too.

Moderators: End the thread once we start getting to complicated or get way over the head of the OP. At least chime and and remind us where we are.

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