An existing building (probably 25 years old max) has two grounded B phase 3p4w 480-volt services (one for HVAC, one for general use). The building formerly was a medical occupancy with a large x-ray load. The entire building is gutted (except for the service) and will be redeveloped into bank offices. My belief is that grounded B phase services have not for a long time been allowed to supply premises wiring or premises wiring systems and this system should be replaced with a wye type. But 250.26 (4) still references grounded phase conductors and this building isn't comparatively old. Where did the code sections go that prohibited grounded B phase services except for certain industrial applications?
Grounded B phase service
- Thread starter dschomer
- Start date
- Status
- Location
- Illinois
- Occupation
- retired electrician
Re: Grounded B phase service
As far as I know the NEC has never prohibited the use of grounded phase systems in any occupancy.
Don
As far as I know the NEC has never prohibited the use of grounded phase systems in any occupancy.
Don
Re: Grounded B phase service
I do not think the code will prevent you using this voltage. However I would suggest that you get with the utiltiy and see if they will provide 120/208 or 277/480 volt service. It is the perferred method of service to this type of load.
I do not think the code will prevent you using this voltage. However I would suggest that you get with the utiltiy and see if they will provide 120/208 or 277/480 volt service. It is the perferred method of service to this type of load.
- Location
- Illinois
- Occupation
- retired electrician
Re: Grounded B phase service
dschomer,
Yes, you will have a tie between the primary and secondary grounded conductors via the equipment grounding conductors. Do you see this as a problem?
Don
dschomer,
Yes, you will have a tie between the primary and secondary grounded conductors via the equipment grounding conductors. Do you see this as a problem?
Don
Re: Grounded B phase service
I'm just not comfortable with the grounded conductor of the primary system being one of the phases used to drive the separately derived system and then being linked to the neutral of the secondary system when in a conventional wye primary the phase conductors are truely isolated from the secondary phase conductors. In one sense, the configuration becomes similar to an autotransformer. It just seems like I'm creating a recipe for poor power quality on the secondary. Hypothetically, it seems like a fault on the primary could seek a return path through the secondary system (via someone's computer?) on its way to the grounded phase conductor.
I noticed that in this building I'm looking at all the secondary systems are single phase 120/240V with the primaries being derived from A and C phases. Someone must have had the same concern.
I'm just not comfortable with the grounded conductor of the primary system being one of the phases used to drive the separately derived system and then being linked to the neutral of the secondary system when in a conventional wye primary the phase conductors are truely isolated from the secondary phase conductors. In one sense, the configuration becomes similar to an autotransformer. It just seems like I'm creating a recipe for poor power quality on the secondary. Hypothetically, it seems like a fault on the primary could seek a return path through the secondary system (via someone's computer?) on its way to the grounded phase conductor.
I noticed that in this building I'm looking at all the secondary systems are single phase 120/240V with the primaries being derived from A and C phases. Someone must have had the same concern.
curt swartz
Electrical Contractor - San Jose, CA
- Location
- San Jose, CA
- Occupation
- Electrical Contractor
Re: Grounded B phase service
dschomer
If this was a conventional installation you would have 480/277Y as you incoming voltage and install a 480 delta - 280/120Y transformer. You would ground X0 of this transformer which would be connecting it to the neutral, grounded conductor, XO terminal or what ever you want to call it of the 480/277Y system you are feeding it with. This is not an isolated system.
I don't see a problem with installing a 3 phase transformer on this system but depending the loads you will be adding it might be more cost effective to change the service to a different type.
dschomer
If this was a conventional installation you would have 480/277Y as you incoming voltage and install a 480 delta - 280/120Y transformer. You would ground X0 of this transformer which would be connecting it to the neutral, grounded conductor, XO terminal or what ever you want to call it of the 480/277Y system you are feeding it with. This is not an isolated system.
I don't see a problem with installing a 3 phase transformer on this system but depending the loads you will be adding it might be more cost effective to change the service to a different type.
BAHTAH
Senior Member
- Location
- United States
Re: Grounded B phase service
If I am reading this correctly you have a corner grounded delta service and are looking to create a 480/277v 3ph4w system. You may want to consider removing the ground from B-Phase ( I think it is only to help stablize the system with a ground reference) and install a Zig-Zag transformer to establish a Neutral and get 277V that could be used for lighting. Then your 480-208y/120 would be connected to an ungrounded delta primary to get your 120V.
If I am reading this correctly you have a corner grounded delta service and are looking to create a 480/277v 3ph4w system. You may want to consider removing the ground from B-Phase ( I think it is only to help stablize the system with a ground reference) and install a Zig-Zag transformer to establish a Neutral and get 277V that could be used for lighting. Then your 480-208y/120 would be connected to an ungrounded delta primary to get your 120V.
- Location
- Illinois
- Occupation
- retired electrician
Re: Grounded B phase service
dschomer,
Grounded conductors of different systems are connected together in some way, normally via the equipment grounding conductors. The grounded conductor, if there is one, of the utility high voltage distribution is directly connected to the grounded conductor for the building serivce. Faults on the primary must return to another conductor on the primary, not to ground. There are no more hazards in connecting your transformer to a grounded phase system then there are in connecting to a conventional system.
Don
dschomer,
Grounded conductors of different systems are connected together in some way, normally via the equipment grounding conductors. The grounded conductor, if there is one, of the utility high voltage distribution is directly connected to the grounded conductor for the building serivce. Faults on the primary must return to another conductor on the primary, not to ground. There are no more hazards in connecting your transformer to a grounded phase system then there are in connecting to a conventional system.
Don
- Status