T&T: 208v vs. 240v Was: Re: Shore Power and Electricity Concepts (LONG POST - was "Power Cords")

[Al Thomason]

Jim,

Good post and good summary.  One correction with regards to 208v.  It is
common to see 120v feed taken off of different legs of 3 phase power.
Mostly you will see this in commercial and/or larger residential condos.
You are totally correct that the 'right' way to do it would be to
install an additional single phase, center tapped transformer, but this
is rarely done.  When the 120v is taken of a 3-phase feed, there is no
difference to the end application.  But when '240v' is called for,
because of the phase shift between the 3 phase one gets the equivalent
of 208v.  This is why you can purchase 240v equipment as optional 208v.

It is known for larger marinas to bring in 3 phase power, mostly cause
it is cheaper to configure and install.  Except for large yachts and
ships, there is no need for 3-phase power to be brought out to the boat.
But they do it to save money.  What they do is bring in power to the
marina on 3 'hot' lines, and a neutral. (1 hot wire for each 'phase').
Between any hot wire and the neutral, you will measure 120v. So, if you
slip is wired for 120v, they only bring one of those hot wires along
with the neutral to your slip. To keep things balanced, you neighbor
will get his slip wired to a different hot wire (everyone shares the
same neutral), and the one next to him get the last hot wire of the
three. Then it starts repeating. If you have "240v" at a slip, then they
will bring 2 hot wires to your slip, but in this case you only get 208v
across the two hots because of 'phasing' issues.  120v is still not a
problem, it is only the '240'v that is different.  So, they bring
3-phase to your marine, but not all the way to the slip. 

My marina is wired this way, the prior one was not.  And I 1st noticed
this approach many years ago while working on a high rise Condo, where
all the 240v was really two legs of 3-phase, and hence we had special
ordered 208v dryers, heaters, ACs, etc...

Issue is there is no real way to tell how your marine is set up, other
then measuring with a good meter.  If you are using heavy amp 240v with
a 240v motor(ala larger AC's), and they expect 240 but are only getting
208 that can cause issues. 240v heaters do not care if they see 208,
they will just put out less heat.  And anything that uses 120v
(including many smaller to mid sized ACes) will also not care. As Jim
pointed out, even some '240v' items which are actually mixed will not be
a problem.  take a dryer:  Mostly the motors and controls are 120v, only
the heating element is 240v.  So in that case, the dryer will be OK, it
will just dry a little slower running the heater on 208v vs. 240v.  It
is really only a problem for you if you have items which use motors that
are expecting to get a full 240v.  

-al-