Disconnect Transformer

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Nietz001

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Location
St Paul MN
I'm installing a 75KVA transformer (480-120/208) The transformer will be located 150ft from the 480 volt switchboard and the 120/208 panel will only be 10 ft from the secondary of the transfomer. Do I need to install a disconnect ahead or behind the transformer?
Jeff
 

raider1

Senior Member
Staff member
Location
Logan, Utah
The 208/120 panelboard must have a main breaker installed in accordance with 408.36(B) and 240.21(C).

As far as a disconnecting means on the line side of the transformer a new section in the 2011 NEC now requires a disconnecting means located within sight of the transformer or the transformer must be marked with the location of the remote disconnecting means and the remote disconnecting means must be capable of being locked in the open position. (See 450.14 2011)

Chris
 

dkidd

Senior Member
Location
here
Occupation
PE
must have a main breaker installed in accordance with 408.36(B) and 240.21(C).
Not quite accurate. It could be fuses, and could be remote on the feeder.

And sometimes a single main device is not needed.
 
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raider1

Senior Member
Staff member
Location
Logan, Utah
Not quite accurate. It could be fuses, and could be remote on the feeder.
Correct, but the OPer mentioned that the secondary supplied a panelboard and the most likely way to comply with both 408.36(B) and 240.21(C) would be with a main breaker in the panelboard.

And sometimes a single main device is not needed.
To comply with 408.36(B) the overcurrent device would have to be a single breaker or set of fuses.

Chris
 

Nietz001

Member
Location
St Paul MN
length of secondary

length of secondary

Charlie,
The length of the secondary conductors should be less than 25ft. My main concern was do I need a main on the 120/208 panel board and/or a disconnect between the transformer and the panelboard. I thought conductors exceeding 25 ft would require the additional disconnect.
 
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don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Charlie,
The length of the secondary conductors should be less than 25ft. My main concern was do I need a main on the 120/208 panel board and/or a disconnect between the transformer and the panelboard. I thought conductors exceeding 25 ft would require the additional disconnect.
You need to comply with the rules in 240.21(C) that cover the required overcurrent protection of the secondary conductors. In general if the conductors exceed 25' you have to provide the overcurrent protection at some point on the transformer side of the 25' limit.
 
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