Overcurent protection for Transformer Secondary over ten feet.

[PWRUP]

I'm installing a 45KVA transformer and its' secondary conductors will be just under 25feet long. Section 240.21(c)(6)(2) states that the secondary conductors terminate in a single circuit breaker or set of fuses. Does the single circuit breaker have to be in its own enclosure or would a main breaker of a branch circuit panel be acceptable?

 

[Fitzdrew516]

An MCB in a panel is a common practice for a transformer secondary disconnect. I do it all the time; almost exclusively even.

 

[kwired]

Would be more common to see the individual overcurrent device located near the transformer in cases where the panel supplied is going to be more then 25 feet away, otherwise a main breaker panel is likely the preference for most.

 

[Fitzdrew516]

Would be more common to see the individual overcurrent device located near the transformer in cases where the panel supplied is going to be more then 25 feet away, otherwise a main breaker panel is likely the preference for most.

This is also an option, but there's nothing wrong with an MCB in a panel as long as it's under 25', right? That's how I've always read it.

 

[ron]

I've had lots of trouble over the years getting a clear acceptance of whether a particular commercial building is an industrial installation, as required for the 25' allowance. I stick to 10' or less

 

[kwired]

I've had lots of trouble over the years getting a clear acceptance of whether a particular commercial building is an industrial installation, as required for the 25' allowance. I stick to 10' or less

I only see the "industrial" part in the over 25 foot rule. The wording doesn't exactly say "industrial" but the description of requirements strongly leans that direction.

For some large transformers 10 foot rule can be to short to even get out of the transformer and into the supplied equipment with 10 feet or less of conductor.

 

[david luchini]

I've had lots of trouble over the years getting a clear acceptance of whether a particular commercial building is an industrial installation, as required for the 25' allowance. I stick to 10' or less

I only see the "industrial" part in the over 25 foot rule. The wording doesn't exactly say "industrial" but the description of requirements strongly leans that direction.

For some large transformers 10 foot rule can be to short to even get out of the transformer and into the supplied equipment with 10 feet or less of conductor.

There is a 'Not Over 25 ft' rule for "Industrial Installations" in 240.21(C)(3) and a 'Not Over 25 ft' general rule in 240.21(C)(6). Nothing in 240.21(C)(6) suggests to me that it must be an "industrial installation" to use that 25 ft rule.

 

[ron]

There is a 'Not Over 25 ft' rule for "Industrial Installations" in 240.21(C)(3) and a 'Not Over 25 ft' general rule in 240.21(C)(6). Nothing in 240.21(C)(6) suggests to me that it must be an "industrial installation" to use that 25 ft rule.

Thanks for the reminder. The calculations of pri-sec voltage ratio x 1/3 rating of pri ocpd usually scares people away. A 480-208/120V, 45kVA D-Y transformer with a pri FLA of 54.13A and a large 125A pri breaker would have to result in a small single secondary breaker of 100A (smaller than usually wanted).

 

[david luchini]

Thanks for the reminder. The calculations of pri-sec voltage ratio x 1/3 rating of pri ocpd usually scares people away. A 480-208/120V, 45kVA D-Y transformer with a pri FLA of 54.13A and a large 125A pri breaker would have to result in a small single secondary breaker of 100A (smaller than usually wanted).

I think you are applying that provision wrongly. (480/208)*(125/3)=96.2. The secondary conductors must have an ampacity that is not LESS than 96.2A. A 175A secondary c/b with #2/0 conductors would be fine.

 

[ron]

I think you are applying that provision wrongly. (480/208)*(125/3)=96.2. The secondary conductors must have an ampacity that is not LESS than 96.2A. A 175A secondary c/b with #2/0 conductors would be fine.

Thanks, I guess that is a result of staying to distances 10' or less

 

[jim dungar]

Thanks for the reminder. The calculations of pri-sec voltage ratio x 1/3 rating of pri ocpd usually scares people away. A 480-208/120V, 45kVA D-Y transformer with a pri FLA of 54.13A and a large 125A pri breaker would have to result in a small single secondary breaker of 100A (smaller than usually wanted).

240.21(C)(6) establishes a minimum rating of the secondary conductors, not an upper limit.
Given your example, the secondary conductors must not be rated less than 96A. You are certainly allowed to go up in size.

 

[PWRUP]

Thanks for all the replies.
It's always great to have many thoughts on a subject.

 

[kwired]

There is a 'Not Over 25 ft' rule for "Industrial Installations" in 240.21(C)(3) and a 'Not Over 25 ft' general rule in 240.21(C)(6). Nothing in 240.21(C)(6) suggests to me that it must be an "industrial installation" to use that 25 ft rule.

I agree and tried to say that with my second paragraph in what I posted.

I was replying to Ron's suggestion that it is hard to determine whether a particular commercial building is an industrial installation.

The conditions there don't directly state industrial, but sort if by nature is limiting them conditions mostly found in certain industrial applications.