DIYbanter - View Single Post

wrote:
Can you get 120v single phase by picking off one line of 240v 3 phase
as long as you have a neutral to carry the current back to the box?
Thanks, Steve

Only in a special circumstance, which is a 4-wire, center-tap grounded
delta service. This has one 240 V L-L circuit with a center tap, just
like everybody's home service. Then, it has another transformer that
develops the 3rd phase. In this system, there are only TWO of the three
hot wires that will give 120 V to neutral. (The third will give ~207 V,
so you will definitely be able to tell which is which with a meter.)

This service is pretty rare, at least around where I've lived.
Corner-grounded delta is more common, but you can't get 120 V
directly from that. You need a 240 - 120 step-down transformer.
Corner-grounded delta is most obvious because 2-pole circuit breakers
and disconnects are used. The 3-phase wires are hot, hot and neutral,
and you can wire a 3-phase motor up to those 3 wires. (These are also
labeled hot (A), neutral (B), and hot (C) phases, and therefore
sometimes called grounded B phase.)

If you have 208 V WYE service (sometimes called star) you have three
120 V circuits, from any line to neutral. But, in this system,
you can't get 240 V, without a transformer. Like, the building
I work in, has 208 V Wye for the office section, and we have little
autotransformers to step 208 up to 240 for the window air conditioners.

If you try this on a true 240 V Wye system, which has a neutral, you
will get a rather high voltage of about 138 V. But, 240 V Wye
is pretty rare. If you try this on a real delta system, you might kill
all the lights in the building, as it may trip the ground fault
protection. But, then, a true delta system doesn't have a NEUTRAL,
although sometimes telling the difference between a neutral and a safety
ground can be difficult. Delta transformers have a balancing
transformer in them that makes it look like they are referenced to a
neutral, but if you draw any current from line to neutral that
unbalances it, the transformer should shut down. This normally wouldn't
apply to open delta and corner or center-tap grounded deltas, as they
are expected to feed unbalanced loads.

Is that more than you wanted to know?

Jon