Transformer Feed Point

[Mr. Pickle]

Thoughts and Opinions: We are having a bit of debate on what is the better way to feed the input of a transformer. Assume a 480Y, 75KVA transformer with a 208Y/120V secondary. Transformer to be located in a small electrical room about 200' from main service switchboard. In the smaller electrical room there will be a 480Y/277V panelboard and 208Y/120V panelboards. The transformer will be used to feed the 208Y/277V panelboards. The 480Y/277V panelboard will be used to power mainly 277V LED lighting and a couple misc. mechanical exhaust fans or other small mechanical loads.

Question 1: Would it be best to feed the transformer from the service switchboard 200' away (so there would be two feeds from the switchgear; 1 for transformer and 1 for 480V panelboard) or from the nearby 480Y/277V panelboard located in the small electrical room next to transformer (only 1 feed from the switchboard in this case)? Would you be concerned with possible harmonic with the LED lighting and transformer load at the local 480V panel?

Question 2: Do you feel the size of the transformer makes a difference in determining which means to feed it? What size transformer would you believe is the "breaking" point that it would be better to feed it from one of the sources and not the other?

Question 3: Would you be concerned with a possible oversized transformer introducing harmonics into the system? If Yes; what would be the oversize or "spare" capacity you would start to be concerned with?

 

[ActionDave]

Question 1: Would it be best to feed the transformer from the service switchboard 200' away (so there would be two feeds from the switchgear; 1 for transformer and 1 for 480V panelboard)

No. You would be using twice as much wire to do the same amount of work.

or from the nearby 480Y/277V panelboard located in the small electrical room next to transformer (only 1 feed from the switchboard in this case)?

This would be my choice.

Would you be concerned with possible harmonic with the LED lighting and transformer load at the local 480V panel?

I don't worry about harmonics. I'm sure there are places where they need to be paid attention to, data center maybe, but in a normal office type building I have yet to see a problem when good wiring practices are employed.

Question 2: Do you feel the size of the transformer makes a difference in determining which means to feed it? What size transformer would you believe is the "breaking" point that it would be better to feed it from one of the sources and not the other?

I feel an obligation to do things to spec first and as efficiently as possible second.

Question 3: Would you be concerned with a possible oversized transformer introducing harmonics into the system? If Yes; what would be the oversize or "spare" capacity you would start to be concerned with?

If the scope of the work is beyond your understanding or confidence it's worth it to bring a good electrical engineer on board.

 

[augie47]

agree 100% with ActionDave's response

 

[Jraef]

I agree too and can only add that if the 208Y120 panel is going to feed a lot of plugs that feed computers or any kind of electronics, or 120V LED / CFL lighting etc., get a transformer with a rating of K-4, that makes the neutral connections bigger and includes a little more iron to absorb the harmonics. You didn't mention the size of your 480V transformer but given that it is feeding mostly 277V lighting, I would use a K-4 rated transformer there as well.

 

[Mr. Pickle]

In your opinion what makes electrical engineer good and how do you find them?

 

[ActionDave]

There are some good ones that are members of this forum.

 

[Cow]

I would feed it from the panel in the room and eliminate the extra disconnect you would need if you had fed it from the service switchboard.

 

[Fnewman]

In general, I would base that decision on the size of the new transformer(given in this case) vs the available capacity in the existing panel, along with any consideration for future growth.

i agree that harmonics should not be an issue, but would be more likely influenced by the active components of the loads attached.