240.21(c)

[A/A Fuel GTX]

I'm a little confused regarding protection of transformer secondary conductors. Case in point:

I have a 75kva transformer with a 480v primary and a 208/120v secondary. I've got a 125A breaker feeding the primary with #2's and 1/0's coming off the secondary feeding a 150A load. The load is a machine with a factory installed fused disconnect. The secondary is roughly 75' in length. Am I correct in assuming this is a compliant installation or do I need to protect the secondary conductors right at the exit point of the transformer?

 

[raider1]

The secondary conductors must be protected from overcurrent in accordance with one of the methods listed in 240.21(C).

There is not a method in 240.21(C) (except for outside secondarys) that permits the secondary conductors to be run 75' without protection.

Chris

 

[A/A Fuel GTX]

Thanks, Chris....So I'll have to put an OCPD within 10' of the transformer secondary exit point.....Correct?

 

[infinity]

As Chris mentioned, the OCPD for the secondary conductors need to be close enough to the transformer so that their conductor length is limited to 25' or less.

 

[raider1]

Thanks, Chris....So I'll have to put an OCPD within 10' of the transformer secondary exit point.....Correct?

Not necessarily, you could meet one of the conditions to be within 25' of the transformer as Infinity said.

Chris

 

[infinity]

Not necessarily, you could meet one of the conditions to be within 25' of the transformer as Infinity said.

Chris

Given the parameters outlined in the OP 240.21(C)(6) should apply.

 

[iwire]

Given the choice I would have placed the transformer close enough to the machine to eliminate the need for additional protection. It would also reduce wire costs.

 

[A/A Fuel GTX]

Given the parameters outlined in the OP 240.21(C)(6) should apply.

Is there a calculation to determine voltage ratio or is it simply 480/208?

 

[raider1]

Given the parameters outlined in the OP 240.21(C)(6) should apply.

I agree.

Chris

 

[A/A Fuel GTX]

Given the choice I would have placed the transformer close enough to the machine to eliminate the need for additional protection. It would also reduce wire costs.

Bob, I absolutely agree but this job is at a govenment site and money is no object

 

[raider1]

Given the choice I would have placed the transformer close enough to the machine to eliminate the need for additional protection. It would also reduce wire costs.

I agree, planning for something like this can really pay off.

Chris

 

[infinity]

Is there a calculation to determine voltage ratio or is it simply 480/208?

Using your parameters this is what I came up with:

(480/208) * (125 amps *1/3) = 96.15 amps minimum secondary conductor ampacity.

 

[ryan_618]

Is there a calculation to determine voltage ratio or is it simply 480/208?

No. Voltage ratio is the same as turns ratio. The turns (or voltage) ration for the panel you describe is 4.

Remember that we are comparing one phase to the other. Assuming a delta primary and wye secondary, the comparison of phase is 480 to 120, not 480 to 208.

 

[ronmath]

Bob, I absolutely agree but this job is at a govenment site and money is no object

I sure hope this was said "tongue in cheek" good design is good design.