I am designing a control panel for a 208v 3ph heating load. I have been asked to add a 115v single phase blower motor by connecting one leg to 208v and the other leg to ground, (208/1.732=120v). I've never seen this done before and I can't believe it's permitted. Can you tell me what part of the NEC or NFPA 79 relates to this? If this is permitted why would anyone use control transformers?
115 volt motor on 208 volt 3 phase control
- Thread starter cpol
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- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: 115 volt motor on 208 volt 3 phase control
I presume your system does not have a grounded (i.e., neutral) conductor, and you are being asked to use the Equipment Grounding Conductor (commonly called the ground wire) in place of a neutral. If that is what you mean, then what is being suggesting to you is a death trap. I don?t know if the code has an explicit rule against creating death traps.
You can start with the discussion of ?objectionable current? in NEC 250.6(A). But in a quick look through the code, I could not find a rule that says you can?t use the EGC in place of a the neutral. But then, I don?t know of a rule from the Department of Motor Vehicles that prohibits driving at high speed, blindfolded, on the wrong side of the road.
Can't you just bring a neutral wire from the source of the 208 volts (i.e., to the 3-phase heaters), and connect your 120 volt load between a phase (pick one, it doesn't matter which) and that neutral?
Or do I misunderstand your description?
I presume your system does not have a grounded (i.e., neutral) conductor, and you are being asked to use the Equipment Grounding Conductor (commonly called the ground wire) in place of a neutral. If that is what you mean, then what is being suggesting to you is a death trap. I don?t know if the code has an explicit rule against creating death traps.
You can start with the discussion of ?objectionable current? in NEC 250.6(A). But in a quick look through the code, I could not find a rule that says you can?t use the EGC in place of a the neutral. But then, I don?t know of a rule from the Department of Motor Vehicles that prohibits driving at high speed, blindfolded, on the wrong side of the road.
Can't you just bring a neutral wire from the source of the 208 volts (i.e., to the 3-phase heaters), and connect your 120 volt load between a phase (pick one, it doesn't matter which) and that neutral?
Or do I misunderstand your description?
al hildenbrand
Senior Member
- Location
- Minnesota
- Occupation
- Electrical Contractor, Electrical Consultant, Electrical Engineer
Re: 115 volt motor on 208 volt 3 phase control
What about a 208/120 V stepdown transformer?
Or, what about replacing the motor with a 208 V motor?
What about a 208/120 V stepdown transformer?
Or, what about replacing the motor with a 208 V motor?
johndeereman
Member
Re: 115 volt motor on 208 volt 3 phase control
The "equipment grounding condutors" is just that & it should not carry current & it will carry current if you use it as a "grounded/neutral conductor" for that motor. If I'm under standing your question correctly?
The "equipment grounding condutors" is just that & it should not carry current & it will carry current if you use it as a "grounded/neutral conductor" for that motor. If I'm under standing your question correctly?
al hildenbrand
Senior Member
- Location
- Minnesota
- Occupation
- Electrical Contractor, Electrical Consultant, Electrical Engineer
Re: 115 volt motor on 208 volt 3 phase control
Charlie,
I think you've given a great reference with 250.6, especially 250.6(A). The grounding conductor will, very likely, be interconnected with other conductive materials (such as building steel, other circuit grounding conductors, exposed conductive surfaces of all manner of equipment that is electrically supplied, as well as any other conductive material(s) in incidental contact with that equipment.
A neutral current, introduced to the grounding conductor web at one point, will take all available paths back to the supply.
Depending upon the specific total assembly of the occupancy's web of grounding conductors and other conductive "noncurrent carrying" material, the return current from this 115 V blower motor will show up all over the facility. . .rather than being contained in an insulated (and inherently isolated) grounded conductor.
Charlie,
I think you've given a great reference with 250.6, especially 250.6(A). The grounding conductor will, very likely, be interconnected with other conductive materials (such as building steel, other circuit grounding conductors, exposed conductive surfaces of all manner of equipment that is electrically supplied, as well as any other conductive material(s) in incidental contact with that equipment.
A neutral current, introduced to the grounding conductor web at one point, will take all available paths back to the supply.
Depending upon the specific total assembly of the occupancy's web of grounding conductors and other conductive "noncurrent carrying" material, the return current from this 115 V blower motor will show up all over the facility. . .rather than being contained in an insulated (and inherently isolated) grounded conductor.
al hildenbrand
Senior Member
- Location
- Minnesota
- Occupation
- Electrical Contractor, Electrical Consultant, Electrical Engineer
Re: 115 volt motor on 208 volt 3 phase control
cpol,
2002 NEC 300.3(B) is another reference that implies that the blower motor return current has to be kept from scattering out onto a grounding web, when the wiring method incorporates ferrous materials.
cpol,
2002 NEC 300.3(B) is another reference that implies that the blower motor return current has to be kept from scattering out onto a grounding web, when the wiring method incorporates ferrous materials.
Re: 115 volt motor on 208 volt 3 phase control
Thanks for the feedback everyone, I'm going to review all the code sections you mentioned. We are trying not to use a transformer to cut costs. Charlie B I have a question regarding your reply, you mentioned bringing a neutral conductor back to the control panel and connecting one leg of the 208V to the motor and the other to the neutral. I think that is what we are trying to achieve. What is the source of the neutral wire? Is it the same neutral that is used with the 120 volt circuits derived from the 208 three phase? Is that permitted by code? If so why do people use control transformers? Why not just run this neutral to any 208v 3ph control and elliminate the cost of a control transformer? Forgive me if my questons seem stupid but I am not familiar with this method of eliminating control transformers and want to learn. If it is not permitted or a good idea I need information to back that up. Thanks.
Thanks for the feedback everyone, I'm going to review all the code sections you mentioned. We are trying not to use a transformer to cut costs. Charlie B I have a question regarding your reply, you mentioned bringing a neutral conductor back to the control panel and connecting one leg of the 208V to the motor and the other to the neutral. I think that is what we are trying to achieve. What is the source of the neutral wire? Is it the same neutral that is used with the 120 volt circuits derived from the 208 three phase? Is that permitted by code? If so why do people use control transformers? Why not just run this neutral to any 208v 3ph control and elliminate the cost of a control transformer? Forgive me if my questons seem stupid but I am not familiar with this method of eliminating control transformers and want to learn. If it is not permitted or a good idea I need information to back that up. Thanks.
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