Chipper 3 phase power with single phase backup?

90cummins · Jul 10, 2012

I have 120/208 3 phase power in my garage for my 10hp 3 phase compressor and a 3 phase drill press.
All other equipment is single phase 120/208.
I have a 25KW single phase 120/240 PTO generator and a couple small portables that I would like to use as backup during an outage.
I can do without the 3 phase equipment but would like to move all other circuits to incoming lines 1&2 so I can power the entire shop with the exception of my 3 phase tools.
Does anyone see any issues with this?
Does this affect the electrical incoming meter as far as accurately measuring my electrical consumption when on the grid with line 3 basically unused?

Tim

spruce232 · Jul 10, 2012

if the branch circuits are sharing the neutral between the 3 phases, you will have problems

90cummins · Jul 10, 2012

spruce232 said:
if the branch circuits are sharing the neutral between the 3 phases, you will have problems

Could you explain a bit more.

Paul N · Jul 10, 2012

What you need is a transfer switch. They're available at Northern Tool here in Minnesota. This will completely isolate each circuit from the electric utility when using your backup generator. I think they come in sizes of 6, 8, or 10 circuits, and they're kind of pricey. $300 - 700 depending on size and brand. You can do a Google search for "Transfer Switch".

Rock knocker · Jul 11, 2012

spruce232 said:
if the branch circuits are sharing the neutral between the 3 phases, you will have problems

90cummins said:
Could you explain a bit more.

What he is refering to is the common practice, especially in three phase systems, of using a multi wire branch circuit (MWBC). In this, in a three phase system, three hots ( X,Y, and Z) can safely share one neutral. If you start jinking with things, and try to convert a full boat (MWBC) to a single phase , you can mistakenly end up with a system where you may have three hot ( A, A, abd B) sharing one neutral. The neutral is now overloaded with double its amperage, and is also not protected by a overcurrent device.

Best to consult a sparky

wyeguy · Jul 11, 2012

I know from what I've seen that 240V motors operated at 208V will burn up. Can't remember if the motors were single phase or three phase, but i know the utility service was 3-phase 208V wye connected. : ) There's no reason not to use every hot leg for single phase loads on a wye connected service. You only have the third "wild" leg on a delta connected service, like 120/240V 3-phase.

I am the modern man...my smart phone smells of diesel & grease.