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Old 06-10-2009, 09:27 AM   #1
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3 ph compressor hook up


I am wanting to hook up a 15hp, 230V, 3 ph, 3-wire plus ground compressor to an existing subpanel that is fed from a main panel with a 100A, 3 pole breaker plus ground. In the subpanel, two feed wires go to the main lugs, one wire labeled white goes to one ground bar and the ground wire to the other bar. There is an existing 2 pole 70A breaker in the subpanel. Can I hook up red and black from compressor to 2 pole breaker with white to same ground bar as feeder and ground to other bar? Is this configuration a delta system? Is it more advantageous or necessary to convert the subpanel to three main lugs and add a 3 pole breaker?

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Old 06-10-2009, 09:30 AM   #2
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3 ph compressor hook up


The panel and supply you discribe is single phase. You would need a phase converter to use the 3 phase compressor.

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Old 06-10-2009, 09:55 AM   #3
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3 ph compressor hook up


Wait... A 3 pole breaker feeds the subpanel, but one of the wires is white and lands on a neutral bar? Are you sure all three wires come from the 3 pole breaker? There is a type of 3-phase system where this is possible, but I don't want to over complicate things just yet. It sounds right off hand that you have a single phase subpanel, but the 3 pole breaker adds a little doubt.

This might be a corner grounded delta system. You should take some voltage readings between each phase, and then each phase to ground, and get back to us....
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Old 06-10-2009, 09:56 AM   #4
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3 ph compressor hook up


I agree this sounds like a single phase system. Another option is a VFD. A compressor would love this setup. Regulated starting, and superb OL protection. A VFD is much less money than a phase converter and can do much more.

Check
www.automationdirect.com
www.baldor.com
www.wegelectric.com

Search AC drives by HP, then narrow you selection down by compressor motor current. Very easy install and quick setup.
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Old 06-10-2009, 09:58 AM   #5
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3 ph compressor hook up


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Originally Posted by InPhase277 View Post
Wait... A 3 pole breaker feeds the subpanel, but one of the wires is white and lands on a neutral bar? Are you sure all three wires come from the 3 pole breaker? There is a type of 3-phase system where this is possible, but I don't want to over complicate things just yet. It sounds right off hand that you have a single phase subpanel, but the 3 pole breaker adds a little doubt.

This might be a corner grounded delta system. You should take some voltage readings between each phase, and then each phase to ground, and get back to us....
This MUST be checked out first as InPhase said.
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Old 06-10-2009, 10:21 AM   #6
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3 ph compressor hook up


Thanks for your reply. I should have clarified that the main panel is definitely 3ph and the 100A 3 pole breaker feeding the subpanel has one wire marked "white" and this marked "white" wire goes to the neutral bar in the subpanel, which is also labeled 240V, 3 ph. I am going to the shop later today to check voltages per your suggestion. Thanks again.
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Old 06-10-2009, 10:33 AM   #7
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Quote:
the 100A 3 pole breaker feeding the subpanel has one wire marked "white" and this marked "white" wire goes to the neutral bar in the subpanel
I think you are mistaken.

Maybe 2 of the 3 poles feed the panel but if the neutral bus is fed from the 3P breaker......yikes!
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Old 06-10-2009, 11:29 AM   #8
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3 ph compressor hook up


Maybe the breaker that feeds the sub is a GFI?

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Old 06-10-2009, 11:45 AM   #9
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Maybe the breaker that feeds the sub is a GFI?

You might be on to something Scott, But this looks like a single phase system to me. If one hot leg of a three phase system were connected as he describes, all the neutrals would be hot. 240 volt recepts ect.....

OP......Measure the voltage from each of the hot main breaker wires to the neutral bus. If you read 120 you have a single phase service. If you read 208 or 240 you have a 3 phase system that is not installed properly. This would be a serious and deadly installation.
Double check were that white taped wire originates. Where its hooked up in the main panel.

OH.....Where are you located? In the US?
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Old 06-10-2009, 12:20 PM   #10
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OP......Measure the voltage from each of the hot main breaker wires to the neutral bus. If you read 120 you have a single phase service. If you read 208 or 240 you have a 3 phase system that is not installed properly. This would be a serious and deadly installation.
Double check were that white taped wire originates. Where its hooked up in the main panel.

OH.....Where are you located? In the US?
This could be the little understood "corner grounded delta", where one leg of the service is grounded. In that case, there is no neutral, but there is a white wire. Most electricians have never dealt with such a beast, but they exist and are perfectly fine if installed properly. There will be 240 V between each phase, and 240 V to ground from A, and 240 V from B to ground, but 0 V from C to ground.

We won't know for sure until the OP gets us some measurements and maybe some pics.
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Old 06-11-2009, 09:01 AM   #11
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3 ph compressor hook up


3 pole 100 amp breaker in main panel is reading 245V on each of two legs and wire marked "white" on third leg is zero volts. In subpanel, two main legs are reading 245V each and wire marked "white" on neutral bar is zero volts. Is this a safe configuration to hook up my 4-wire pigtail....red/black to 2 pole 70 amp breaker, white to neutral bar and ground to ground? My compressor is a rotary vane type and rotation must be correct. I usually just switch red and black to obtain correct rotation. Thanks.
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Old 06-11-2009, 10:20 AM   #12
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3 pole 100 amp breaker in main panel is reading 245V on each of two legs and wire marked "white" on third leg is zero volts. In subpanel, two main legs are reading 245V each and wire marked "white" on neutral bar is zero volts. Is this a safe configuration to hook up my 4-wire pigtail....red/black to 2 pole 70 amp breaker, white to neutral bar and ground to ground? My compressor is a rotary vane type and rotation must be correct. I usually just switch red and black to obtain correct rotation. Thanks.
3 pole breaker. 2 hot wires and one zero volts. This makes no sense. Where are you located. Are you located in the US? This is very important.
Post some close up pictures of the breaker inside the panel and the inside of the sub panel. Show the open whole panel, so we can see what you describe.
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Old 06-11-2009, 10:20 AM   #13
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3 ph compressor hook up


This sounds like a corner grounded system. But you still didn't say what the voltage was between each pole of the breaker. But I believe it is a corner grounded system. So, yes, hook the four wires up like you describe. The white is not really a neutral, even though it is white and grounded. But I don't want to confuse things.

Also, are you wiring the cord straight to the panel? Depending on what type of cable it is, that may be a Code violation. But overall, a 15 HP, 230 V motor should be fine on a 70 A breaker.
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Old 06-11-2009, 10:24 AM   #14
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3 pole breaker. 2 hot wires and one zero volts. This makes no sense. Where are you located. Are you located in the US? This is very important.
Post some close up pictures of the breaker inside the panel and the inside of the sub panel. Show the open whole panel, so we can see what you describe.
J.V., it is a corner grounded system. It isn't very common any more. I suspect this place may be an old installation. If the utility supplies you with a 240 V delta service, you only have two ways to ground it: either a center tap on one of the phases, giving a high leg of 415 V, or grounding one of the phases, giving you 240 V three phase, but allowing you to use double pole breakers. One phase is white.
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Old 06-11-2009, 05:51 PM   #15
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3 ph compressor hook up


If the voltage readings were taken with one probe on ground, and the other on the breaker, white, etc., then this system is almost certainly a 240 3 phase corner grounded delta.

This system looks like single phase, but the voltage from phase to ground is 240, as well as phase to phase. There is no 120.

Voltages of 245 from either hot leg to ground certainly isn't single phase.

A 3 phase motor is connected as follows;

T1 to one side of the two pole breaker, T2 to the other side of the breaker, and T3 to the white bus. As always, swap any two leads to reverse rotation.

Rob

P.S. As stated above, a 70 amp breaker is about right for a 15 HP motor. A 110 amp would be the codebook maximum.

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