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Re: 208v Air Compressors

To: flake@a3115jmf.atl.hp.com
Subject: Re: 208v Air Compressors
From: Donald H Locker <dhl@chelseamsl.com>
Date: Thu, 11 Oct 2001 22:14:14 -0400 (EDT)
Not at all.  I've done a _lot_ of three-phase work.  The only time you
will find 208 phase-to-phase is in a "208-wye-120" system, where there
is indeed three phases of 208 phase-to-phase, but also three
120-to-neutral legs available for lighting circuits.  Usually used as
a way to keep phases balanced in buildings with a predominance of
lighting circuits, rather than a predominance of motor-type loads.

All three hots _are_ equal in a normal 220/230/240 three-phase system.
One of the legs is tapped midway to give two 110/120 phases to
neutral.  A normal house has just one leg of the 220 (230/240) brought
in with a center-tap.  Adding the third phase is a simple matter of
adding another connection to the transformer and bringing it into the
house.  (And then adding a third phase to the power panel, the
metering system, and the branch subsystem.)  Everything else can stay
the same.  Really!  And again, the 208 volts is found from this new
third phase to neutral.  Phase-to-phase (A-B, B-C, C-A) is 240; phases
A-to-neutral and B-to-neutral is 120; C-to-neutral is 208.  If the
actual phase-to-phase voltage is 220, then C-to-neutral will be 190
volts and phases A- and B-to-neutral is 110.

HTH.
Donald.

> From: Joe Flake <flake@a3115jmf.atl.hp.com>
> Date: Thu, 11 Oct 2001 16:59:04 EDT
> 
> 
> Donald said:
> > 208 is
> > actually found as the odd leg to neutral on a three-phase system, with
> > 110/120 to neutral on the other two legs (and 220/230 from
> > phase-to-phase on all three phases.)  If you have two 120v single
> > phase circuits everywhere, then you have 220 single phase too.
> 
> I think 208 is any hot to any other hot in the typical 3-phase voltages,
> with 110 being any hot to ground.  All three hots are equal.
> 
> 220/230/240 is the nominal value for hot to hot in a single phase
> installation.
> 
> Am I showing my age by referring to "110/220"?  Almost everything calls
> it "120/240" now.

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