Join Date: Apr 2006
Thanked 173 Times in 170 Posts
Rep Power: 12
Re: 2013/2014 Maintenance Season
"Since the charger circuit is dedicated, I'd like to use the lowest amps possible to allow more current for the other circuits without exceeding 30 amps total. "
It doesn't work that way. You CAN quite safely have 60 amps worth of branch circuit breakers on a panel that is connected to a 30A rated supply.
What you should have is one 30A breaker (or more conservatively, a smaller one) that is the "master breaker" for your AC supply. This can be at the power inlet to the boat, or on the breaker panel, there are arguments both ways. There should be a GFI device at the power inlet in any case. Having an "AC Master Breaker" rated 30A on the AC panel is simply good sense, because it allows you to ensure all AC power is cut, i.e. if you're working someplace in the boat and don't want to take any chance on shock hazards.
Now, if you had breakers totaling 60A in branch runs on the panels, consider what happens if someone comes in, turns everything on, starts coffee and the microwave and the hair drier all at once. Zap, the 30A master breaker still blows, no big deal.
Someone who is more familiar with the boat would know "I can only use this much at once" and simply not turn on more than 25A of loads at once. (Leaving a 5A margin.)
So putting in larger, or more, breakers on the separate circuits is no big deal.
HOWEVER, the breaker on each circuit is designed to protect the WIRE on that circuit. Not the device--but the wire. So if the wiring to that device (or socket. etc.) can safely handle 18A, you might want to put in a 15A breaker, even if the device it runs to only pulls 5A at maximum load. (I say 15A because, again, I like to leave a safety margin, and that's personal decision.)
You certainly can install a breaker that is sized to protect the wire OR at the device's maximum normal draw, whichever is lower. And to me that has always made good sense since anything that is drawing more power than it is supposed to, indicates a problem and I'd rather cut the power when there's a problem. That adds some protection against odd events, and ensures the device gets extra attention.
So for your charger? If it is being hard-wired, the breaker would normally be sized to protect the wiring, based on the wiring gauge's ampacity, which is reduced in engine spaces, etc. But if the maker says it will never draw more than 5A on the AC side, putting a 5A breaker in isn't all crazy either.
Putting in a 10A breaker doesn't 'steal' anything from what is available to the other circuits. It only affects how you are calculating potential situations. No big deal.