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Subject: Re: TWL: Shorepower system
From: Arild Jensen (elnav@XXX.XXX)
Date: Sat Aug 10 2002 - 00:19:47 EDT
Andy wrote:
> I'm trying to decide on the shorepower system for a new boat. I'm thinking of
> 50A 125/250v shorepower input. I understand this is really two separate 120v
> supplies, plus a neutral and ground. All the appliances inside the boat would be
> 120v, no 240v, so I guess I divide the two 120v legs between the appliances on
> the main breaker panel. My question is this - if I'm at a dock that only has
> available one 30A 120v shore power supply, does this mean that half my ac
> appliances won't be useable because there's no power to one of the legs on the
> breaker panel?
REPLY
Sorry Andy, but the two sides of the supply are not totally separate.
If you have true 240 or 250 Volts available then it comes from a center
tapped neutral transformer.
Any imbalance in the two sides results in what is called unbalanced neutral
current. Under worsts case conditions with all load on one side and nothing on
the other, you have a neutral current equal to the supply current.
But the neutral is also tied to the ground. And your boat is grounded.
In an ideal world this would not affect your boat, but in the real world, you get
all sorts of side effects. Not the least of which is loss of efficiency.
You are also correct about the problems with only having 30 Am 120V plugs
available. One solution called a smart Y is rather expensive; about 3 boats
units.
There are better ways of dsigning the system.
Cheers
Arild
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