Why Do Boat Builders Put In 50 Amp Power Under 50 LOA?

It seems there would be a fire hazard adapting dual 30 amp cords to a 50 amp cord that is plugged into a 50 amp marine receptacle unless there are 30 amp circuit breakers between the two cords. The conductors in the small 30 amp cords are not rated for the higher current.

The reason for the large single cord is for greater amperage service.

A single 50 amp cord rated at 250 volts is two 50 amp 120 volt conductors that are out of phase which gets us the 240 volt at 50 amps; both the 120 volt and 240 volt are 50 amp protected. The total capability at 120 volts is 100 amps or at 240 volts 50 amps.

Dual 30 amp cords rated at 120 volts can be "in Phase" or "out of Phase" at the dock pedestal.

• For "in Phase" power there are simply two 30 amp power sources at 120 volts and no way to obtain 240 volts. The total capability is 120 volts at 60 amps.
• For "out of Phase" power there are two 30 amp power sources at 120 volts which across the two 120 volt cables is measured 240 volts (they are out of phase). The total capability at 120 volts is 60 amps or at 240 volt 30 amps.

The bottom line is anything plugged into a circuit must be suitably rated for that circuit.

 

My 400DA had dual 30 amp 120 volt cords. They were coiled in the transom locker as a pair and manually fed out of the locker as a pair so they didn't get tangled up, usually. It would be a pain when they got tangled; I never let anyone else attempt to feed those cables in or out as they would inevitable get messed up. The interesting thing about that boat is the main breakers on the boat were 50 amp but the cords were 30 amp; I could never figure that out..... There were no 240 volt devices on the boat; it was all 120 volt.
My 52DB has a single 50 amp cord and the main circuit breaker are 50 amps. All of the HVAC and water heater on this boat is 240 volt. The cord is fed out and retracted automatically with a push of a switch. That is not to say there isn't a manual component to ensure it feeds straight and true and the plug doesn't drop in the water.
No doubt for me the arrangement on the 52DB is far better than what was on the 400DA.

 

The previous owner moved the plugs from the port side to the stern and left the originals in place, which are 30's. They look to be original, but could been swapped out by the first owner.

 

They did the same on mine, crazy right? I stopped trying to figure that out and switched it out since the previous owner swapped out the feed to the panel with 6ga wire but kept the 30's, almost as crazy.

 

I don't think it's primarily about boat length; instead, the decision is more likely based on expected energy consumption.

So a 40' boat (perhaps an ACMY, for example?) in a hot/humid climate with maybe 4 ACs... plus cooktop, water heater, washer/dryer, microwave/convection oven, battery chargers, etc etc etc may need the 50A/250V supply total of 100A to operate everything simultaneously.

-Chris