BOOST TRANSFORMER

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jap

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I overheard an electrician today indicating he was going to install a 2p 60 amp breaker in a panel, feed a 208 to 240v single phase boost transformer and then to a small 8 circuit distibution panel to feed 2 seperate 20 amp 240v machines,but that he was going to extend a neutral from the 208v panel to the subpanel since the boost transformer does not create a neutral. In my mind you would end up with 120 to neutral on one of the phases in the subpanel and an odd voltage on the other phase to neutral. Is this type of installation even allowed ?

JAP.
 
Your concern is indeed valid. A quick violation would be 408.58
 
I would wait until the electrician finished the job. Then I would ask him "So, how did your job go?".

I had to do something like this before. There was 208 power on the shore and the client needed 240/120 on the end of a long dock. Due to derating and conductor fill we could only put 2 new wires in the existing conduit going to the end of the dock 350' away. So, the engineer had me put a buck boost by the 120/208 panel on land to get 240 and put a transformer at the end of the dock to derive a neutral(grounded conductor).
 
jap said:
I overheard an electrician today indicating he was going to install a 2p 60 amp breaker in a panel, feed a 208 to 240v single phase boost transformer and then to a small 8 circuit distibution panel to feed 2 seperate 20 amp 240v machines,but that he was going to extend a neutral from the 208v panel to the subpanel since the boost transformer does not create a neutral. In my mind you would end up with 120 to neutral on one of the phases in the subpanel and an odd voltage on the other phase to neutral. Is this type of installation even allowed ?

JAP.
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It may not be allowed for the reason that augie indicated, but your concern about the neutral is not necessarily an issue.
First, you would not have two phases, or even two ungrounded conductors at the subpanel. You would have a 240V L1 and grounded conductor.
If the subpanel is rated for 240V line to ground and if no 120V circuits are needed to be supplied from that panel, then the "neutral" is actually just the grounded conductor in a 240V two wire system with one side grounded. And pulling it in from the feeder panel is just fine.
But if the breakers in the subpanel or the subpanel itself are rated 120/240 (maximum 120V to ground) instead of 240 or if someone is likely to think that they can derive a 120V circuit by adding a new breaker to that panel, then there is a big problem.
 
GoldDigger said:
It may not be allowed for the reason that augie indicated, but your concern about the neutral is not necessarily an issue.
First, you would not have two phases, or even two ungrounded conductors at the subpanel. You would have a 240V L1 and grounded conductor.
If the subpanel is rated for 240V line to ground and if no 120V circuits are needed to be supplied from that panel, then the "neutral" is actually just the grounded conductor in a 240V two wire system with one side grounded. And pulling it in from the feeder panel is just fine.
But if the breakers in the subpanel or the subpanel itself are rated 120/240 (maximum 120V to ground) instead of 240 or if someone is likely to think that they can derive a 120V circuit by adding a new breaker to that panel, then there is a big problem.
Click to expand...

I'm not sure I'm following you. He indicated the 2 machines were strictly 240v (No Neutral). He extended 2 ungrounded legs and an EGC from the Boost Transformer to the Subpanel for distribution (so he wouldnt have to install a boost transformer for each machine individually like I would have done.) and then he installed a neutral from the 208v panel directly to the "Boosted" 240v Panel, since only 1 of the ungrounded legs in the Subpanel was "boosted" , the other simply made a connection to one side of the boost transforrmer and carried on to the subpanel at 120 volts. Its the voltage reading from the "Boosted" phase to neutral in the subpanel that I'm concerned about.if someone were to install a 1p breaker on the boosted leg in the subpanel not knowing what was ahead of it. The(2) 2p 20a breakers he installed in the subpanel to feed the (2) 240v machines would be ok since there is not neutral to them.

I think if it were me and I had to do this , I dont think I would install a Neutral from the 208 volt panel to the "Boosted" Distribution Panel at all, only an EGC, and install some kind of sticker indicating that it was a "Boosted" Panel and for no other purpose.
 
Sounds right to me.
But with the boosted leg being more than 120 V above ground I would still be concerned about the rating of the bus and breakers on that leg.
The effect would be small but a technical violation.


Tapatalk!
 
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GoldDigger said:
Sounds right to me.
But with the boosted leg being more than 120 V above ground I would still be concerned about the rating of the bus and breakers on that leg.
The effect would be small but a technical violation.


Tapatalk!
Click to expand...

That would be my concern also, I just didnt understand the part you had mentioned about not having 2 Ungrounded conductors in the Subpanel ?
How do you come up with only 1 240v L1 and Grounded Conductor ?
 
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