75 KVA XFMR primary feeder and OCP?

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T

T's C6

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I have an Engineer that wants me to feed a 75 KVA XFMR 480/208-120v with 3 #8's and a 40 amp breaker, the secondary has 3/0 and 225 MCB. I understand NEC 450.3(B) Table requires that the primary OCP be sized as the percentage of XFMR-rated Current which would be 250% for primary and secondary protection, I know that I can go down to 125% but he wants me to go lower then the rated primary of the XFMR which is 90 amps. The current load is minimal at this time, and the primary feeder will be increased later when other loads are added.

Will this be code compliant with 450.3?
 
C

cpal

Senior Member
Location
MA
T's C6 said:
I have an Engineer that wants me to feed a 75 KVA XFMR 480/208-120v with 3 #8's and a 40 amp breaker, the secondary has 3/0 and 225 MCB. I understand NEC 450.3(B) Table requires that the primary OCP be sized as the percentage of XFMR-rated Current which would be 250% for primary and secondary protection, I know that I can go down to 125% but he wants me to go lower then the rated primary of the XFMR which is 90 amps. The current load is minimal at this time, and the primary feeder will be increased later when other loads are added.

Will this be code compliant with 450.3?
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Table 450.3(B) is the maximum setting see the title. I do not see a Code violation, but I do not understand why you would not do the installation once.
 
T

T's C6

Member
cpal said:
Table 450.3(B) is the maximum setting see the title. I do not see a Code violation, but I do not understand why you would not do the installation once.
Click to expand...

MONEY funded for this job only, more money is funded in another project scope.
 
steve66

steve66

Senior Member
Location
Illinois
Occupation
Engineer
With a 40A breaker, you might have trouble with the breaker tripping when the power is first applied due to the transformer inrush current. (Although I'm not really sure the instantaneous ratings of a 40A breaker is any different than a 90A.)

Just something to consider.

Steve
 
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