On 28 Aug 2003 14:03:50 -0700, (Jeridiah) wrote:
Perhaps I need to clarify....

They are putting in a 240V Delta xfrmr, with 2 piggy backed off from
it to provide the single phase service.

My understanding is this - all power will go through the 3 ph xfrmr.
The single phase will be tapped off from the 2 piggy backed xfrmrs.
Can't quite figure out why there are 2 "secondary" xfrmrs. Shouldn't
it be a single 240 single phase and just center tap it to give me my
120 service? Or is it a balancing issue?

Exactly. The two piggy back transformers are providing a "derived
neutral". The only currents flowing through these transformers is
neutral return currents for 1 ph 120 volt loads. These transformers
will probably be smaller than the 3 ph delta transformer. This is
just a way of providing a center tap on one of the 3 ph windings
when the main transformer doesn't have one.

As for the main panel... Talking to a electrical supplier, their
recommendation for service cable for 400A was to use dual 3/0. Can I
split this to 2 panels and be legal as far as code? Reason for doing
this would be that I can get 200A 3 ph panels for free. Not sure
about a 400A. I will have to check with my source on that.

You'll have to check with an electrician familiar with your local Code,
but the rule today for residential installations in this area is that there
must be an externally accessable positive disconnect for power
entering the premises. The purpose of this is so the fire department
can make the premises cold without having to wait for the power
company to show up.

This disconnect can be a switch rather than a breaker. That'll have to be
400 amp. But you can then split the feed in that switch box and feed two
separate 200 amp panels via metallic conduit. Each of those boxes must
have its own main breakers, but they only have to be 200 amp. The idea
here is that you're only extending and splitting the service drop before it
reaches the breaker boxes.

Gary