transformer secondary conductors

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snjeza

Member
Would the following installation be acceptable:

After transformer two phases and neutral go to two pole switch. Second three pole switch is fed with one phase from same transformer and two other phases from two pole switch.

I never seen this but can't find anything in code that prohibit that.
Please help.
Thank you.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Re: transformer secondary conductors

Are you saying that,</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Current from one phase to the three phase loads travels from the transformer to the 3-pole switch, and</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Current going to the other two phases of the three phase loads travels through the two-pole switch and then through the 3-pole switch?</font>
<font size="2" face="Verdana, Helvetica, sans-serif">What this would create is a situation in which opening the 2-pole switch would de-energize two of the three phases in all three-phase loads. Do I have that right?

Where are the overcurrent devices that are protecting the secondary of the transformer? If these two "switches" are in fact circuit breakers, then you have a violation of 240.20(B).

I have to believe there is a similar prohibition against a disconnect switch (fused or unfused) opening some, but not all, of the ungrounded conductors. In a quick look through the book, I could not find it.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Re: transformer secondary conductors

I don't like it. Someone may think the first switch is shutting off all three phases, but it isn't. Allowed or not, I wouldn't include that in a design.

Why would you want to do this? The only thing I can imagine is that the two pole switch is existing, and you are changing a 1 phase xformer to a 3 phase xformer. IF that is the case, why not demo the 2 pole switch?

Steve
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Re: transformer secondary conductors

Assuming that there is proper three phase overcurrent protection upstream, the installation could be code compliant.

A motor controller is not required to open all of the phase conductors while a disconnecting means is. So, the two pole switch could be a motor rated controller (430.84) and the three pole switch could be a motor disconnect switch (430.103).
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Re: transformer secondary conductors

What if this were a corner-grounded Delta? Couldn't the first disconnect open only the two ungrounded phases legally?
 
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