T&T: Power Cord

[Ken Bloomfield]

Hi Patrick,

Just thought I would correct one part of your otherwise good posting.

The 50 amp cable does not go to a couple of smaller breakers as you suggest. 
You are close on your description, but each of the two 120 VAC legs (L1 and 
L2) is capable of supplying 50 amps, so the two breakers are each 50 amp 
(albeit tied to trip together as you suggested).  So, as such in a 50 amp 
cord used on a 50 amp boat you have 2 x 50 amps x 120 VAC = 12000 watts or 
12 kW of power useable.  On a dual 30 amp boat with dual 30 amp cable setup, 
you have 2 x 30 amps x 120 VAC = 7200 watts or 7.2 kW.  Note that typically, 
each boat setup usually will have a generator that can equal this demand, 
typically about a 12 kW genny on the dual and an 7.5 to 8.0 kW genny on the 
dual 30.

Ken.

Ken Bloomfield
MTOA# 2062
AGLCA# 3529
M/V Tellico Lady, 50' Marine Trader-Walkaround
Maryville, TN




> If the power inputs on your boat are two 30s, I vote for the two 30A 
> cables
> because it gives you more options; and they are cheaper and easier to
> handle.
>
> Now here is where I may get into trouble, but I know there are plenty of
> knowledgeable folks on this newsletter who will straighten me out.
>
> I don't think you have any problem with ckt breaker protection using a 30A
> splitter out of a 50A source.  Here's why:   The 50A source is actually a
> 240V, 2-phase source wired , in a sense, like your house.  The 240V supply
> is actually two, 120V lines that are 180 degrees out of phase.  If the
> electrical appliance needs 240V (dryers, stoves, etc.) you wire the two
> 120V, 180 degrees out of phase lines across the appliance and they sum to
> give 240V.  If the appliance needs 120V you only connect one of the lines
> and the common ground to the appliance.  This is why if you look in your
> house's electrical load center you will see double breakers for those
> appliances that need 240V (one for each 120V line)  and single breakers 
> for
> the 120V appliances.
>
> Now to the marine 50A, connector.  It also has two 120V lines 180 degrees
> out of phase.  If you have a single 50A cable running to a 50A connector 
> on
> your boat, the two lines will be split at the boat's electrical panel and
> the electrical load will be divided among the two inputs.  If you use a 
> 50A
> to two 30As splitter at the dock supply, this is where the two 120V lines
> are split.
>
> The point of this lengthy, and probably confusing discussion is that the
> 50A ckt breaker on the dock is not a single breaker with a 50A rating.  It
> is actually a double breaker composed of two 25A, or maybe 30A, breakers:
> one for each 120V source.  So in realty, you're getting the same overload
> protection you would get from being connected to two 30A dock connectors.
>
> I hope this helps.
>
> Patrick
> Ft. Myers, FL