On Fri, 24 Nov 2006 21:43:55 -0500, "mortond"
wrote:

Hi,

Sorry, I guess I wasn't as clear as I thought!

The marina only has a 50 amp receptacle.
I wanted to avoid the cost of buying a second 30 amp or a single 50 amp
isolation transformer.
Basically, I wnted to hook-up the convection oven and fridge to the 50 amp
shore power receptacle, like an extension cord, except, the extension cord
would be routed via a 30amp breaker on the boat (revised that from a 50
amp) to one leg of the 'Y' branch of the shore power cord.
The appliance grounds would not be connected to the boat at all. I assume
the shore power ground would be isolated from the boat by the existing
isolation transformer on one leg of the "Y" and because there is no boat
connection on the other "Y" leg.

Thanks for the interest.

David

David
Either do it right - 1 or 2 isolation transformers, or do it wrong as
above.
Any system design must consider how it handles any fault condition not
just the run-time conditions. The above scenario is a no-no. Just
sketch your circuit, think about what wires are ground referenced and
apply some faults. (Shorts to hull etc).
Tip. - The hull going live or people dying should force a rethink!

Richard

Nb "Pound Eater" Parkend G+S

mortond wrote:
Hi,

Sorry, I guess I wasn't as clear as I thought!

The marina only has a 50 amp receptacle.
I wanted to avoid the cost of buying a second 30 amp or a single 50 amp
isolation transformer.
Basically, I wnted to hook-up the convection oven and fridge to the 50 amp
shore power receptacle, like an extension cord, except, the extension cord
would be routed via a 30amp breaker on the boat (revised that from a 50
amp) to one leg of the 'Y' branch of the shore power cord.
The appliance grounds would not be connected to the boat at all. I assume
the shore power ground would be isolated from the boat by the existing
isolation transformer on one leg of the "Y" and because there is no boat
connection on the other "Y" leg.

It is true that with what you are considering, the shore power ground
would be isolated from the boat. Partly true, anyway. Problem is that
unless you electrically insulate your steel boat from the water, it will
make a connection of sorts through the water back to the shore power
ground. At least near the dock, which is where your shore power
connection will be.

Here's the problem. Suppose you have a conventional (i.e., NEC/ABYC
approved) system and there is a short from the hot wire to the fridge
case (which is most probably connected to your hull). Normally, such a
short will trip a breaker instantly, giving protection as well as notice
that something potentially serious is amiss.

If the shore power ground is not attached to the hull, as you are
considering, that same short will probably not trip the breaker. The
path through the water will limit the current through the short, but not
enough to avoid possible electrocution. If the fridge case is not
grounded to the hull (as you are also considering) it is now at 120
volts relative to the hull!

If the boat is at the dock in sal****er, you have a potentially lethal
situation. It will work electrically, but unfortunately will introduce
serious hazards that include fire and electric shock.

The safe solutions are 1) isolation transformer(s), properly installed;
2) shore power ground connected to the hull (hopefully with a galvanic
isolator); and 3) a plastic boat. I don't recommend #3 as a way around
#1 or #2.

Hope that helps.

Chuck


Thanks for the interest.

David



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On 2006-11-25 13:43:55 +1100, "mortond" said:

Hi,

Sorry, I guess I wasn't as clear as I thought!

The marina only has a 50 amp receptacle.
I wanted to avoid the cost of buying a second 30 amp or a single 50 amp
isolation transformer.
Basically, I wnted to hook-up the convection oven and fridge to the 50 amp
shore power receptacle, like an extension cord, except, the extension cord
would be routed via a 30amp breaker on the boat (revised that from a 50
amp) to one leg of the 'Y' branch of the shore power cord.
The appliance grounds would not be connected to the boat at all. I assume
the shore power ground would be isolated from the boat by the existing
isolation transformer on one leg of the "Y" and because there is no boat
connection on the other "Y" leg.

Thanks for the interest.

David

Here in Australia a major manufacturer of fibreglass cruisers has a
dual inlet system. This is because virtuall all marinas supply shore
power at 240VAC 16A single phase. Some marinas do supply a limited
number of berths with 32 A single phase or 32A 3-phase power. The said
manufacturer supplies these dual inlets predominently on 40' or larger
and factory fitted air conditioned boats. The two feeds are brought to
the AC switch panel and there is generally a facility to parallel the
loads so that a single feed can be used subject to shore power delivery
capability.

I have observed the same system being used on Benneteau power cruisers
which have factory fitted air conditioning.

Your boat being steel requires that you do it right! If you need
separate feeds to accommodate the demand then put in two feeds and two
transformers. To do anything less is both foolish and shortsighted. The
life you save may be your own!

P.S. I am a boat owner and an EE with an Electrical Contracting L/C.

--
Regards,
John

Well, I guess I sort of knew what the responses would be. Hoped I could
save a chunk of change!

So...does anyone know of a good source for (inexpensive) 110 volt 50 amp
isolation transformers?
I live 30 miles East of San Francisco.

Thanks for keeping me on the straight and narrow :-(

David

mortond wrote:
Well, I guess I sort of knew what the responses would be. Hoped I could
save a chunk of change!

So...does anyone know of a good source for (inexpensive) 110 volt 50 amp
isolation transformers?
I live 30 miles East of San Francisco.

Thanks for keeping me on the straight and narrow :-(

David


As you know, the transformers are not cheap. But you can get a 30A
Charles for $400, which isn't too bad.

I see you're headed toward a 50A transformer. These are likely to be
three times the price of the 30A models. Also, you may want to consider
that 120V 30A service may be easier to find than 120V 50A. So two 30A
transformers may be more convenient.

Depending on the transformers, it may be possible to wire the
secondaries of two 30A transformers in parallel, giving you effectively
60A of transformer capacity (they'll run cooler). I'm reluctant to
recommend that unless the two transformers are compatible. You should
definitely check with the manufacturer before attempting that.

Whether or not you parallel the secondaries, you always have the option
of paralleling the primaries! If you choose to go the 50A service route,
go ahead and put the two 30A transformer primaries in parallel. You can
do this with any isolation transformers. If the secondaries are not
paralleled, you simply have two independent branches: the old and the new.

Just some additional stuff to consider.

Good luck!

Chuck

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JohnDW wrote in
:

If you can get center tapped (55-0-55v) secondary version,
which are (were-it's changing...) common for construction
sites in Europe, then the maximum voltage you'll have with
respect to "earth" is 55v, which is safer. However, in the
US, I suppose you'll have to conform to your wiring standards



US Navy ships have 115VAC outlets that are center tapped to ground for
safety like that. Not sure if that practice continues. My ship was built
in 1952...(c;

Larry
--
Guess what I want to do with the Little Drummer Boy's drum
by Christmas Eve....rrrrump..pa-pum...pum...up his bum....