Hi,
Known:
50 amp shore power receptacle. 30 amp boat connection. 30 amp isolation
transformer. 4kw generator connected to boatside 110v system. Battery
chargers and 110v receptacles and 13v ac lighting fed from shore
power/isolation transformer.

Required:
Converting this steel boat to live-aboard with occasional trips.
Need to increase electrical system to run: 14amp convection oven, new 10
cu ft fridge and TV and stereo system.

Solution:
I know the simplest, and most EXPENSIVE, solution is to buy and install a
50 amp isolation transformer. BUT..

Could I install a second 30amp boat receptacle, connect it to shore power
using a 50x30x30 'Y' power cord, run second 30amp wiring to new 50amp
breaker thence to receptacles for oven, fridge and TV?
This system would not be connected to generator nor to existing system nor
to hull.

Would this work? My concern is the 'Y' connector on the shore power cord.

Any suggestions most welcome.

David Morton

mortond wrote:
Hi,
Known:
50 amp shore power receptacle. 30 amp boat connection. 30 amp isolation
transformer. 4kw generator connected to boatside 110v system. Battery
chargers and 110v receptacles and 13v ac lighting fed from shore
power/isolation transformer.

Required:
Converting this steel boat to live-aboard with occasional trips.
Need to increase electrical system to run: 14amp convection oven, new 10
cu ft fridge and TV and stereo system.

Solution:
I know the simplest, and most EXPENSIVE, solution is to buy and install a
50 amp isolation transformer. BUT..

Could I install a second 30amp boat receptacle, connect it to shore power
using a 50x30x30 'Y' power cord, run second 30amp wiring to new 50amp
breaker thence to receptacles for oven, fridge and TV?
This system would not be connected to generator nor to existing system nor
to hull.

Would this work? My concern is the 'Y' connector on the shore power cord.

Should be no problem at all. Each transformer's secondary will be fully
isolated from a)shore power; and b)the other transformer's secondary.
Probably the cheapest solution. Just follow ABYC AC wiring practices and
you should be fine.

Chuck


Any suggestions most welcome.

David Morton




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On Wed, 22 Nov 2006 15:42:17 -0500, nospam wrote:

mortond wrote:
Hi,
Known:
50 amp shore power receptacle. 30 amp boat connection. 30 amp isolation
transformer. 4kw generator connected to boatside 110v system. Battery
chargers and 110v receptacles and 13v ac lighting fed from shore
power/isolation transformer.

Required:
Converting this steel boat to live-aboard with occasional trips.
Need to increase electrical system to run: 14amp convection oven, new 10
cu ft fridge and TV and stereo system.

Solution:
I know the simplest, and most EXPENSIVE, solution is to buy and install a
50 amp isolation transformer. BUT..

Could I install a second 30amp boat receptacle, connect it to shore power
using a 50x30x30 'Y' power cord, run second 30amp wiring to new 50amp
breaker thence to receptacles for oven, fridge and TV?
This system would not be connected to generator nor to existing system nor
to hull.

Would this work? My concern is the 'Y' connector on the shore power cord.

Should be no problem at all. Each transformer's secondary will be fully
isolated from a)shore power; and b)the other transformer's secondary.
Probably the cheapest solution. Just follow ABYC AC wiring practices and
you should be fine.

Chuck
Reading the above I see no mention of a new additional 30A isolation
transformer?
Also it seems that you currently have a 30A connector (and probably
30A cabling) fed from a 50A supply, surely unnacceptable.

Richard

Nb "Pound Eater" Parkend G+S

Electricky Dicky wrote:

Reading the above I see no mention of a new additional 30A isolation
transformer?
Also it seems that you currently have a 30A connector (and probably
30A cabling) fed from a 50A supply, surely unnacceptable.

Richard

Nb "Pound Eater" Parkend G+S

Agreed.

Thanks for catching those, Richard.

Chuck

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OK, now that I'm awake, I'll try to be more lucid.

Although it is not exactly what you proposed, the simplest and best way
to provide an additional 24 amps for the oven, fridge, etc., is to add a
second 30A cable, breaker, isolation transformer, and associated wiring.
The isolation transformer is especially appropriate for a steel vessel.
The cost would be greater if you were to install a 50A isolation
transformer and the benefits would be marginal in my opinion. Having two
independent branch circuits offers some redundancy and doesn't require
that you mess with your existing AC wiring.

Connecting the two 30A cables through a Y to a 50A shore power
receptacle would be OK since it is extremely unlikely that all of your
electrical loads would ever be on at the same time. Even if they were,
presumably the combined load would not exceed 54 amps, which, under a
worst case, would trip the shore power 50A breaker. Virtually all
residential branch circuits in the US are sized based on the assumption
that all loads will not be present simultaneously. Of course, if you
know the assumption is false for your boat, that would require a
different approach.

While you could combine the secondaries of the two isolation
transformers and have a single large circuit, I would recommend that you
keep the new wiring separate from the old. Needless to say, combining
the output of the existing isolation transformer with a new, un-isolated
30A circuit would negate the benefits of the existing transformer.

Richard's point about using a 30A cable with a 50A receptacle is
troubling. If that is what you are doing, there is a real risk that the
30A cable could short, cause a fire, etc., without the 50A shore power
breaker tripping! The Y connection would be much safer.

Apologies for the false start.

Chuck





















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On Thu, 23 Nov 2006 15:13:36 -0500, chuck wrote:

OK, now that I'm awake, I'll try to be more lucid.

Snip

Richard's point about using a 30A cable with a 50A receptacle is
troubling. If that is what you are doing, there is a real risk that the
30A cable could short, cause a fire, etc., without the 50A shore power
breaker tripping! The Y connection would be much safer.

Apologies for the false start.

Chuck
My point was that the 30A connector and therefore probably 30A cable
FROM the Y are protected by a 50A breaker. Just had a google and seen
a Marinco AF1 which is I assume is the type of gizmo you are
discussing.
Recipe for disaster IMHO. Though probably thousands in the field (or
shoreside).
Unless each leg is fused? But that is not discussed on the site I saw.

Richard

Nb "Pound Eater" Parkend G+S

Electricky Dicky wrote:

My point was that the 30A connector and therefore probably 30A cable
FROM the Y are protected by a 50A breaker. Just had a google and seen
a Marinco AF1 which is I assume is the type of gizmo you are
discussing.
Recipe for disaster IMHO. Though probably thousands in the field (or
shoreside).
Unless each leg is fused? But that is not discussed on the site I saw.

Richard

I did misunderstand your point. Sorry. Seems I just can't get on the
right wavelength in this thread.

Certainly the NEC would not allow a 50A breaker to protect a 30A branch
circuit. But using 30A cables on 50A receptacles is sort of like using
an extension cord rated at 15 amps on a residential branch circuit
protected with a 30A breaker. Not the smartest thing to do, but probably
just as common as what happens with a lot of shore power connections at
marinas.

I guess UL rates residential extension cords and the NEC jurisdiction
stops at the receptacle. There is some disagreement on whether the NEC
stops at the marina's shore power pedestal but most seem to believe it
does not.

Legalities aside, the 10 ga. wire in typical marine-type 30A cords is
not rated for 50A and really should not be connected to a 50A breaker.

Chuck


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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

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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