Xfrmr Feeds

mstrlucky74

Senior Member
Location
NJ
If you have ATS feeding xfmr that feeds panel what are the primary and secondary conductors sized to? I have transformer schedule with prim/sec wire sizes that don’t match the ATS schedule. Thanks.

The ATS is 200A at 480v and call for it feeding a 400a 120/208 panel. The ATS schedule calls for 3#1 aluminum. In between them is a 75kva xfrmr. The 75kva transformer schedule primary(480v) calls for 3#2/0 & #4G aluminum and 3#500 with neutral secondary(120/208).
 
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Sounds like you needed a 150 KVA transformer.. but to make it all work with 75KVA transformer
75KVA: 90.2A potential on Primary, size the breaker in 200A ATS to 125A (3) #1/0+(1)#6GND--> Secondary 208.2A xfmr potential, main breaker in 400A 120/208 panel = 250A, (4) 250 kcmil + (1) #4G
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
If you have ATS feeding xfmr that feeds panel what are the primary and secondary conductors sized to? I have transformer schedule with prim/sec wire sizes that don’t match the ATS schedule. Thanks.

The ATS is 200A at 480v and call for it feeding a 400a 120/208 panel. The ATS schedule calls for 3#1 aluminum. In between them is a 75kva xfrmr. The 75kva transformer schedule primary(480v) calls for 3#2/0 & #4G aluminum and 3#500 with neutral secondary(120/208).
What size OCPD is in the panel?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
secondary of xfrmr feeding a 400a breaker panel.
So this panel has a 400 amp main with no other OCPD on the secondary of the transformer ahead of the panel? If this is the case then the 500 kcmil is too small. You need a 400 amp conductor which for one set would be 600 kcmil.
 

mstrlucky74

Senior Member
Location
NJ
So this panel has a 400 amp main with no other OCPD on the secondary of the transformer ahead of the panel? If this is the case then the 500 kcmil is too small. You need a 400 amp conductor which for one set would be 600 kcmil.
Correct. Actually riser says the panel is 300A but schedule says 400A. So you size the xfrmr secondary conductors based on the OCPD the xfrmr is feeding?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Correct. Actually riser says the panel is 300A but schedule says 400A. So you size the xfrmr secondary conductors based on the OCPD the xfrmr is feeding?
The secondary conductors require protection and the OCPD cannot be larger than the conductor ampacity. If you have 500's you cannot feed a 400 amp OCPD.
 
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