Unbalanced 3 phase systems

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Cody K

Senior Member
Location
Texas
Hello folks, I have a question on systems we work on which consist of
480Y/277 service from either transformer banks or generator. These systems never have a 4th conductor installed (other than ground wire) between service and distribution panel. The loads consist all of 3 phase 480v motors, except for a 15kva 480/240 transformer tied in for control power. This transformer is what makes me think the service should have some sort of neutral conductor.

Lets assume the service is a 100kva transformer bank and the control transformer is the 15kva previously mentioned. Can yall please help me figure what the neutral current would calculate to under full load.

Thanks
 

roger

Moderator
Staff member
Location
Fl
Occupation
Retired Electrician
Cody, 480v single phase or three phase does not use a neutral.

The KVA of the service or any downstream transformers would not have any bearing on this.

Roger
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
Cody K said:
Lets assume the service is a 100kva transformer bank and the control transformer is the 15kva previously mentioned. Can yall please help me figure what the neutral current would calculate to under full load.
To say Roger's point another way. . .

The 15kVA transformer is not connected to the neutral, it is only connected line to line. All the current in the transformer is in the lines.

If you add a neutral from the supply, the current will still be zero.
 
Hope this helps

Hope this helps

Cody, I think roger is right in his reply to your question 480v single or three phase does not use a neutral. The 15 kva transformer will have no bearing on the 480v. I am assuming you are making a neutral with the control voltage transformer doing this you will use a delta to wye transformer and the current will go to xo on this transformer.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
John Burt said:
I am assuming you are making a neutral with the control voltage transformer doing this you will use a delta to wye transformer and the current will go to xo on this transformer.
I got the impression that the control transformer is single-phase, but it is important to mention that the secondary should have a conductor grounded, and OCP applies. This is an SDS, and 15Kva is enough to treat seriously.
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
Cody K said:
..the control transformer is the 15kva previously mentioned. Can yall please help me figure what the neutral current would calculate to under full load.

If you needed the low side of that 480/240 corner grounded Delta, I'm showing the full 15kva burning 36.08A for each Line-to-Grounded(neutral), if control wiring does not require a 1.25 factor for continuous operation. And, I see three #8cu ccc's in conduit operating @ 56?C.
 
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Cody K

Senior Member
Location
Texas
LarryFine said:
I got the impression that the control transformer is single-phase, but it is important to mention that the secondary should have a conductor grounded, and OCP applies. This is an SDS, and 15Kva is enough to treat seriously.

Larry, I grounded the centertap portion of the secondary where x2 and x3 connect (neutral). OCPD's on all the 120v and 240v circuits. Upstream of primary is protected by 2 30 amp fuses. Is this what you were referring to?

Cody
 

W6SJK

Senior Member
This transformer is what makes me think the service should have some sort of neutral conductor.

When you say "service" are you really referring to the feeder to the motors and control xfmr? I must be missing something here. Your service ground conductor _is_ (or should be) a grounded conductor (neutral), not a ground wire.

Grounded Y services NEVER have a ground wire installed. To do so would put current on it, as it would be in parallel with the neutral (Gnd and neutral bonded at xfmr and service disconnect.

A grounded conductor (a neutral in the case of a Y service) is always installed and is required by NEC 250.24 B. Otherwise there is no return path to the xfmr for phase to ground faults in the building. The neutral is run to the service and bonded to the ground bus. It might have no loads connected.
 
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Smart $

Esteemed Member
Location
Ohio
Cody K said:
Larry, I grounded the centertap portion of the secondary where x2 and x3 connect (neutral). OCPD's on all the 120v and 240v circuits. Upstream of primary is protected by 2 30 amp fuses. Is this what you were referring to?

Cody
Sounds like this is a 1?, dual-voltage-secondary transformer, with X1, X2, X3, and X4 secondary terminals. If so, yes.
 
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