T&T: Power Cord

[Gary Bell]

Steve Willett asked: 

...which would be best for a boat with two- 30 amp inputs when a 50 amp  outlet is available on the dock, a splitter dock side and two 30 amp cables to the boat or to run a 50 amp cable to the boat and the Y splitter at the  inputs?

and then he reported the consensus of the other forum:

The discussion is that two 30 amp cables are  cheaper and easier to 
handle than a 50 amp cable.

I agree entirely up to this point.  My 50 foot 30 amp 120 volt cables cost $79 and weigh a mere 15 pounds each. A pair of them will connect two separate 30 amp 125 volt outlets on the dock to the two receptacles onboard.  My power panel provides for separating the 125 volt load into two separate circuits (they still share the same neutral and ground) so I can use a total of 60 amps.  Where only one 30 amp 125 volt outlet is available I use only the one cable, I switch the panel to parallel the two circuits and then have only 30 amps available aboard.  The panel provides an interlock gizmo that prevents paralleling the circuits while both plugs/cables are used -- because one cannot know whether the two 30 amp 120 volt dockside outlets are out of phase with each other, virtually canceling each other -- or not, where a 'ground loop' risk could exist.   
  
My old eighty foot sidewheel paddleboat uses a 50 foot 50 amp 125/250 amp cable which retails for $659 and weighs in at 46 pounds.  I nicknamed that cable "The Anaconda."  I use the term "fifty amp double" to refer to that sort of power service where there is one ground, one common neutral and two separate 50 amp 120 volt 'hot' leads -- each out of phase with the other so potentially (pun) one can have 250 volt service at 50 amps found between the two hot leads, or a total of 100 amps at 125 volts or even a combination of both.  
  
My new power catamaran is presently using a 'Y' connector that joins a single dockside 50 amp 125/250 volt receptacle (fifty double) to two 30 amp 124 volt (thirty single) cables, and ultimately to two separated 125 volt circuits onboard breakered at 30 amps each.  This "Y" adapter lists for $269 and produces two thirty single feeds on 30 amp rated cable with actually 50 amps of current available on each.  My boatside breakers limit that to 30 amps for each separate circuit aboard, although if I had a short in the cable portion it would be protected by the 50 amp breaker. 
  
The other cable adapter I would use at docks with 50 amp 125 volt service (fifty single) retails for $155 and  connects to a single 30 amp cable, which powers my boat at 30 amps.
  
Where dock power is available at less than 30 amps I have adapters to fit my 30 amp cable(s) to the different outlets, and there indeed the breaker on the dock protects and limits my power aboard to whatever it's rating allows.  
  
then Steve reported an alternative opinion from the sailboat forum:

...it is  dangerous to use that set up because of the breaker sizing on the 50 amp outlet  being to high to protect in case of an overload on one of the 30 amp  cables and creating a possible fire hazzard.


While this statement is technically correct, the presence of thirty amp 
breakers on each buss on the boat's  distribution panel will limit the 
current flowing onboard to thirty amps on each circuit.  In that 
instance the only thing the 50amp breaker on the dock does is protect 
the cable from short circuits in the cable/plugs up to the entrance 
breakers onboard.  While a short to neutral or ground is possible there, 
it is uncommon.  It would be pretty spectacular for a moment -- until 
the dock breaker opened.  A dockside breaker of  220 amps would provide 
virtually the same protection.   Otherwise the 30 amp breaker aboard 
limits the current in the entire system (including the cable and 
dockside power pedestal) to 30 amps. 

By far the most common fault in that part of the system (cables, plugs 
and inlet wiring) is a degraded connection from mechanical damage, which 
creates unwanted resistance there, producing heat when the current 
flows, sometimes enough heat to damage the wiring/cable/plug or even 
start a fire.  No circuit breaker anywhere in the system will protect 
from that because the increased resistance causes less current to flow, 
and breakers act on increased current.  The other 'common' fault -- 
reversed polarity -- is not protected by breakers anywhere in the system 
because it causes the hot connection to appear on the ground or neutral 
buss onboard, potentially a dangerous shock risk, but no short circuit.  
This usually is caused by forcing a dockside or boatside plug to fit in 
the wrong orientation, forcing a plug into a mismatched outlet, or least 
likely by a miswired dock outlet or homemade adapter. 
 
I expect your next trawler will indeed have two 30 amp feeds, with the 
appropriate panel wiring, or perhaps if it is a larger boat the 'fifty 
double.'  That seems to be the trend here in the PNW. 
 
Hope this helps...

Gary Bell