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

[Keith]

I'll still stick with my original recommendation for two 30's vs. one 50. No 
EE degree needed.

:-)


Keith
_____
Depression is merely anger without enthusiasm.
----- Original Message ----- 
From: "Jim Healy" <gilwellbear at gmail.com>

>
> 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?