We are installing a new maintanenace transformer that will be fed from 4 different 15kV circuits.
Only one circuit at a time will ever power this transformer. Most of the existing feeders to the existing transformers are fed with aluminum pipe. And that is what we want to do here. The circuits are 15kV phase to phase.

I'm looking at the NEC Table 490.24 that indicates to to get a BIL of 110kV, we need at least 10.5 inches between the phases and 10 inches to a grounded surface.

It's been suggested a couple of different configurations. One would give us more space but would involve bringing one of the phases underneath the other two, and not exactly an equal distance away from the other phases. It seems there may be some type design flaw we aren't thinking of? Is there some kind of transformer action this would cause?

I sketched this out to make what I'm speaking of much clearer...but I can't figure out how to make an attachment with the new "Start a New Thread" input screen. Anyone know how to do this?

Here is the sketch I made for the original question. There are three sheets. Two options and a picture.

• http://files.engineering.com/getfile.aspx?folder=d571c201-57d3-42e5-8a27-a7

The main (possible) problem I see with this arrangement is with any 1 switch closed the other 3 switches will have hot knife blades.

Looks like your building a switchyard not "some equipment over 600 volts".
I would at least look at the NESC.
Check out the BurRec substaion manual- it's good and it free.

The other 3 switches are hot, but it's like this on all of the other similar installations at the plant. The similar installations have worked with no issues for many decades. The switches are 30' up in the air.

I did improve the design some by reversing the phase sequence on one pair of the switches. See the attached.

• http://files.engineering.com/getfile.aspx?folder=def64285-eb29-4311-90ad-6b

I don't like the last one with its inconsistent phasing. Might cause confusion in the future. Hot blades is always a possibility with this type of switch. Make sure you have phase to phase clearance from open blade to nearby bus.