Hello! Does NEC code allow for a transformer with secondary voltage of 115vac three phase with no neutral (delta)? The transformer will be used to power some 115vac outlets on an industrial machine. I don't think this is allowed by code due to the neutral connection on the outlet actually being a phase wire. This design is being proposed by my company counterpart in Austria.
three phase 115vac transformer
- Thread starter Rob E
- Start date
- Status
- Location
- Massachusetts
Re: three phase 115vac transformer
IMO 250.20(B) requires this transformer to have a grounded conductor.
Using a white wire for other than a grounded conductor is also a violation most times. 200.7(C)
IMO 250.20(B) requires this transformer to have a grounded conductor.
Using a white wire for other than a grounded conductor is also a violation most times. 200.7(C)
- Location
- Illinois
- Occupation
- retired electrician
Re: three phase 115vac transformer
You could ground one phase, making it a corner grounded delta and code compliant, but you would lose a 1/3 of your single pole circuits.
Don
You could ground one phase, making it a corner grounded delta and code compliant, but you would lose a 1/3 of your single pole circuits.
Don
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: three phase 115vac transformer
Don,
This is a delta secondary, there would be no 1 pole circuits in an ungrounded delta. The number of 2 and 3 pole circuits would remain the same even if one leg is grounded.
This 120V delta system probably needs to be corner-grounded per 250.20.
But
250.21(1) would not require grounding if the load is industrial "melting" equipment.
250.21(3) allows this system for control circuits in a supervised industrial installation.
Don,
This is a delta secondary, there would be no 1 pole circuits in an ungrounded delta. The number of 2 and 3 pole circuits would remain the same even if one leg is grounded.
This 120V delta system probably needs to be corner-grounded per 250.20.
But
250.21(1) would not require grounding if the load is industrial "melting" equipment.
250.21(3) allows this system for control circuits in a supervised industrial installation.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Re: three phase 115vac transformer
If the line voltage is 115 volts from phase to phase than why wouldn't Don be correct in saying that grounding one phase to make a grounded conductor be correct? In the original ungrounded state he would have:
A-B=115 volts
B-C=115 volts
A-C=115 volts
If he corner grounds using the B phase than he's left with:
A-Grounded conductor=115 volts
C-Grounded conductor=115 volts
and he loses 1/3 of his single phase circuits.
If the line voltage is 115 volts from phase to phase than why wouldn't Don be correct in saying that grounding one phase to make a grounded conductor be correct? In the original ungrounded state he would have:
A-B=115 volts
B-C=115 volts
A-C=115 volts
If he corner grounds using the B phase than he's left with:
A-Grounded conductor=115 volts
C-Grounded conductor=115 volts
and he loses 1/3 of his single phase circuits.
- Location
- Illinois
- Occupation
- retired electrician
Re: three phase 115vac transformer
Jim,
The original post says:
Don
Jim,
The original post says:
It looks like he wants to serve single phase 115 volt loads from this transformer. If so, the only code compliant way that I can think of, is making it a corner grounded delta.
Don
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Re: three phase 115vac transformer
A-B=115 volts
B-C=115 volts
A-C=115 volts
...which could all be used with 2- or 3-pole breakers.
True, if you're considering phase-to-ground only, but he would still also have:
A-B=115 volts
B-C=115 volts
A-C=115 volts
...which could all be used with 2- or 3-pole breakers.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Re: three phase 115vac transformer
But would it be code complaint to not have a grounded conductor?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: three phase 115vac transformer
Per the original post, as an ungrounded delta the possible voltages are:
A-B 115V using a 2 pole breaker
B-C 115V using a 2 pole breaker
C-A 115V using a 2 pole breaker
If this is corner grounded the possible connections are:
A-G 115V
C-G 115V
A-C 115V
Where is the loss of 1/3 of the circuits?
Unless the circuits fall under 250.21(1) or(3), a 115 ungrounded delta would not be allowed per 250.20(B)(1).
The use of a receptacle with a "neutral" blade should not be allowed in any case. There is no neutral in a corner grounded delta configuration.
Per the original post, as an ungrounded delta the possible voltages are:
A-B 115V using a 2 pole breaker
B-C 115V using a 2 pole breaker
C-A 115V using a 2 pole breaker
If this is corner grounded the possible connections are:
A-G 115V
C-G 115V
A-C 115V
Where is the loss of 1/3 of the circuits?
Unless the circuits fall under 250.21(1) or(3), a 115 ungrounded delta would not be allowed per 250.20(B)(1).
The use of a receptacle with a "neutral" blade should not be allowed in any case. There is no neutral in a corner grounded delta configuration.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Re: three phase 115vac transformer
You're right. There is no 1/3 loss. I was thinking...I don't know what I was thinking.
Trevor
Jim,
You're right. There is no 1/3 loss. I was thinking...I don't know what I was thinking.
Trevor
- Location
- Illinois
- Occupation
- retired electrician
Re: three phase 115vac transformer
Jim,
I was thinking that we would be using single pole breakers and that you could only use single pole breakers on the two ungrounded phases. If you use two pole breakers using all of the combinations, you are correct that there is no loss of circuits.
Don
Jim,
I was thinking that we would be using single pole breakers and that you could only use single pole breakers on the two ungrounded phases. If you use two pole breakers using all of the combinations, you are correct that there is no loss of circuits.
The receptacle needs a grounded conductor not a neutral. I don't see any problem using a standard receptacle on this corner grounded delta.
Don
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Re: three phase 115vac transformer
If your installation meets all of the following requirements you are OK.
250.21 3) Separately derived systems supplied by transformers that have a primary voltage rating less than 1000 volts, provided that all the following conditions are met:
a. The system is used exclusively for control circuits.
b. The conditions of maintenance and supervision ensure that only qualified persons service the installation.
c. Continuity of control power is required.
d. Ground detectors are installed on the control system.
If your installation meets all of the following requirements you are OK.
250.21 3) Separately derived systems supplied by transformers that have a primary voltage rating less than 1000 volts, provided that all the following conditions are met:
a. The system is used exclusively for control circuits.
b. The conditions of maintenance and supervision ensure that only qualified persons service the installation.
c. Continuity of control power is required.
d. Ground detectors are installed on the control system.
Re: three phase 115vac transformer
Thanks for all the input everyone.
This transformer is being supplied by Austria,
460v/115vac 3 phase.
My customer just reduced the 115vac duplex outlets from 20 to 10. With no knowing what the customer may plug in to these outlets and being a OEM, I think changing to a 1 phase may be best.
This machine also has to go thru a CSA inspection, since it is going to Canada.
Rob
Thanks for all the input everyone.
This transformer is being supplied by Austria,
460v/115vac 3 phase.
My customer just reduced the 115vac duplex outlets from 20 to 10. With no knowing what the customer may plug in to these outlets and being a OEM, I think changing to a 1 phase may be best.
This machine also has to go thru a CSA inspection, since it is going to Canada.
Rob
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