Three phase question

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?

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.

Thanks

Jeridiah

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.

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

Jeridiah,

I suspect what you will be getting is 3 single phase transformers wired into a 3-phase delta bank. All your single phase load will come off of one transformer and the other two will only be used for your 3-phase load. And you will have a high leg. Take a look at the transformer diagrams on my web page. If you scroll down the page the first one you come to is a 240V delta secondary like you will be getting.

An open delta bank like Mike says he has will have the same secondary voltages but an open delta is only good for 86.6% of the rating of the 2 units making up the 3-phase bank. They do make 3-phase transformers in both delta and wye secondaries with three and four bushing secondaries. If anyone would like to see wiring diagrams of them, I can post them on my web page. It doesn't make sense for a utility to hang a 3-phase transformer and 2 more to get single phase, kinda defeats the purpose of a 3-phase transformer; having it all in one can.

Some questions you might want to ask the utility; what your rate structure will be, commercial or residential, is it going to be a demand meter, if you go 400A are you going to have to get a panel that will take current coils and a test block?

Don

It makes sense if that's what they have lying in the yard at the time of the install. Utilities will often do odd things to use up what they have on hand rather than having to buy a transformer. Using two small 1 ph transformers to produce a derived neutral is one of those odd things.

Gary

The Southern Building Code won't accept that for a premises that isn't continuously occupied. The main disconnect must be external to the building, and plainly marked. As I said, it is so the fire department can make the building cold without waiting for the power company to show up and pull the pole fuses.

Gary

The main disconnect must be external to the building, and plainly marked. As I said, it is so the fire department can make the building cold without waiting for the power company to show up and pull the pole fuses.

Of course, and here in CA it is not uncommon to find six disconnects on the exterior of a multiple-use building, even though some of the units may not be "continuously occupied".

We don't use pole fuses here, except on the line side of a large customer fed from the utility's subtransmisson system.