T&T: Shore Power Cords 30 and 50 Pigtail

[Peter Bennett]

Tuesday, February 20, 2007, 6:22:55 PM, Gil wrote:

GJ> Michael, there may two answers.

GJ> If you're boat is 50A/125V then using two 30A
GJ> cables with 50A "Y" connectors at each end will work.  If you want to use 2
GJ> 30A shore power sources and just use the "Y" at the boat, you may have a
GJ> serious problem.  Again, assuming you need 125V and not 240V, the two
GJ> unrelated 30A shore power sources may be out of phase and therefore, can not
GJ> be bonded together through the 50A "Y". If you want to do this without risk
GJ> measure the voltage between the 2 hot leads from the 2 30A sources.  If you
GJ> read 0 volts beween them you're okay.

Paralleling two 120V 30A outlets into a single 120V 50A inlet would be
extremely hazardous and SHOULD NOT BE ATTEMPTED!!!  Adjacent 30 amp
outlets are almost certain to be on different phases, so attempting to
parallel them would result in a short circuit. (I haven't looked for
50 amp outlets of any flavour, but I expect that a 120v 50A outlet is
rare - most will be 120/240V.)

GJ> If you need 50A/240A then your two 30A
GJ> cables don't have the capacity to support 50A.  With 50A services circuit
GJ> breaker protected to 50A, using a 30A cable cord is dangerous.  As described
GJ> above, if you want to use 2 different 30A shore power sources, you need to
GJ> measure 240V between the 2 hot wires from the two 30A shore power sources.
GJ> Again, because of the danger in using a 30A cable on a 50A protected circuit
GJ> this is not recommended.

Well, the 30 Amp outlets should be protected by 30 amp circuit
breakers in the marina, so there should be no concern about overloading
your 30 amp power cords.  However, since your boat expects to be able
to draw 50 amps from each hot wire in the 120/240V 50A service, you
will have to be careful about how much power you use - otherwise you
may trip the marina's circuit breakers.



-- 
Peter Bennett, VE7CEI    Vancouver, B.C., Canada
Lien Hwa 28 (AKA Polaris 30) "Sea Spray"
GPS and NMEA info: http://vancouver-webpages.com/peter 
Vancouver Power Squadron: http://vancouver.powersquadron.ca