Feeder Selection

[porf]

In selecting the proper size feeder to a transformer, should I consider the type of load the transformer feeds or should I just go by the primary current required? I am curious if one ever considers that the load may or may not be continuos or non-continous. I have a 3-Ph, 480V-208/277V, 45 kVA, general usage for lighting,hand tools amd other miscellaneous small loads - no motors. Thank you for your info.

 

[mpparent]

As long as you size the feeder for the ampacity required, you'll be ok. As you know, in most cases, the phase conductors and the neutral are sized the same. However, if you have loads that are high in harmonic content, then you may want to increase the size of the neutral conductor (usually 200%).

Mike

 

[cuky2000]

Sizing a feeder require to satisfy 3 criteria
1- Ampacity
2- Voltage drop.
3- Short circuit withstands.

For LV, ampacity and voltage drop may be the main factor in most cases.

In absence of known detail information of the load (presently and future) most people select the feeder ampacity base on the transformer size and a conservative power factor.

 

[peakload]

Thanks cuky2000 for your input.
Your last comment was precisely what I was thinking. Since our intended load varies by the tasks at hand, my preference was to go with the primary current required by the Xfmr in selecting the feeder size. Additionally, I was trying to adhere as close as I could with NEC 215.2, even though there cold be some diversity factors that could be considered, practically speaking.