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

[Jim Healy]

Al,
 
Thanks for sharing your thoughts on 208V derived from a 3-phase wye vs.
115V/230V from a 3-wire, single-phase, mid-point neutral system.  I agree
completely with your observations.  But then...  
 
The difference between 208V and 230V is very significant.  Most compressors
(1/2 HP and up) are designed to work at 230V nominal, no lower than 208V.
If the marina has a nominal 208V line from a wye connected 3-phase source,
and it's on the low side because of a real summer heat wave brownout, that
nominal 208V can be as low as 187V.  That can cause serious compressor start
problems; i.e., the compressor draws lock-rotor current but can't achieve
enough torque to actually start against its load.  Poof!  No A/C?  Spoiled
food?  Equipment damage?  Fire?  Similar over-heating problems in pump
motors.  In a 208V wye, you need the neutral to obtain 120V for control
circuits, just as you said.  If the site has only a 230V delta ring
available, where there is a safety ground but no neutral, the appliance must
have an internal control transformer to develop 115V (properly designed 208V
units should have an internal transformer too, but designs are cost driven,
eh?)  
 
This gets precisely to my point that there are often several ways to do
things, but not all that many right ways to do things.  That was another
point in my thinking yesterday.  There are lots of ways to make things work,
but I encourage all boaters to do things the most safe, if not least
expensive, way.  
 
And of course, what works at the scale of a condo complex does not
necessarily scale to a single owner of a single home (or boat).  This 208V
3-phase wye shortcut works, and saves the cost of a set of transformers, in
a very tightly controlled environment, like a condo complex, where the condo
management office controls what appliances a resident may have and maybe
even who the installer is.  For the condo complex owner, their decision to
use a 208V 3-phase wye saves them capital dollars, but drives up cost and
limits flexibility for the condo residents.  I guess that's OK if it's fully
disclosed.  But, in my view, poor practice.  Because of the potential for
damage to costly equipment, in an environment that's not tightly controlled
or controllable, it virtually can't work successfully.  I'm not sure if it's
OK with the NEC, but I am certain that many local coding authorities would
not allow it.  
 
I would guess that most large marina's do have 3-phase services.  But in a
marina environment, where people come and go and the marina operator has no
idea what appliances might be aboard, I would certainly expect that they
follow the North American standard, and isolate docks from 3-phase yard
applications or mega yacht docks where true 3-phase is needed.  
 
What does your marina tell you to do about your 230V genset?  What do you do
with 208V equipment when you cruise out and stop at a "normal" marina?
 
Thanks
 
Peg and Jim Healy aboard Sanctuary,
currently at Rock Creek, Pasadena, MD