Control Transformer Ground Reference

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Raymond Hope

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Okaloosa Technical College
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Instructor
I have always referenced my secondary voltage on 480/240 to 120 control transformers to ground on one side. I can't find anything that shows this as correct, or incorrect. Does anyone have any insight on this. My reasoning is that by referencing ground, I keep voltage off of the push buttons, and HOA switches.
 
retirede said:
It depends on a number of factors you haven’t mentioned. Read article 250.20 and see if that helps.

I really don’t understand your reasoning.
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I found the clarification I was looking for in 250.20. Thank you. It seems that the secondary should indeed reference ground. Many of the factory pre-wired motor control boxes do not do this. By referencing ground, the voltage is no longer floating. This allows the Stop/Start/Jogg/HOA/etc buttons to be wired up with current flow through the conductors, but no voltage present.
 
Raymond Hope said:
I found the clarification I was looking for in 250.20. Thank you. It seems that the secondary should indeed reference ground. Many of the factory pre-wired motor control boxes do not do this. By referencing ground, the voltage is no longer floating. This allows the Stop/Start/Jogg/HOA/etc buttons to be wired up with current flow through the conductors, but no voltage present.
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The code does not require that the control power transformer be a grounded system. 250.21(A)(3) permits them to be ungrounded. I have worked on a number of facilities where that was the design requirement as a single ground fault does not open an OCPD and the equipment remains operational.

I have never seen the control devices you mentioned wired into the grounded circuit conductor. They are always on the ungrounded conductor and have voltage on them.
 
don_resqcapt19 said:
The code does not require that the control power transformer be a grounded system. 250.21(A)(3) permits them to be ungrounded. I have worked on a number of facilities where that was the design requirement as a single ground fault does not open an OCPD and the equipment remains operational.

I have never seen the control devices you mentioned wired into the grounded circuit conductor. They are always on the ungrounded conductor and have voltage on them.
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Thank you for responding. This added another level of clarification. Somehow I missed it when looking for the information, 250.20 gave me a much better idea, and 250.21(A)(3) spelled it out clearly.
 
If the one side is grounded it makes it a lot easier to troubleshoot. If they are left floating, you have to make sure you know where you are referencing you meter to when measuring voltages. If is grounded, I can check all the voltages pretty easily. Ground to Stop button, Ground to Start button, Ground to motor starter coil, etc.
 
RCC1 said:
If the one side is grounded it makes it a lot easier to troubleshoot. If they are left floating, you have to make sure you know where you are referencing you meter to when measuring voltages. If is grounded, I can check all the voltages pretty easily. Ground to Stop button, Ground to Start button, Ground to motor starter coil, etc.
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Yes, whenever I had to troubleshoot one of the ungrounded motor control circuits, I would install a jumper to temporarily make a grounded system, because as you say, the ungrounded control circuits are more difficult to troubleshoot.
 
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