Select to expand quote
neilmac01 said..
Hi Andrew.
My fridge is a Kings drawer fridge. They recommend powering the fridge direct from the battery. Don't really know why. Maybe a RV thing. I think sometimes I may want to leave the fridge on while away but I would like to have all other "House Power off, Not too worried about it though.
I will be putting a galvanic isolator on the shore power.
I'm at RMYS, a lot of people here have shore power connected 24/7 running dehumidifiers etc. Have not heard of any issues. On my previous boat I even left the charger on to keep the battery topped up. Good Victron gear. They are strict here about how shore power is connected.
Kings probably recommend direct connection to the battery because their own power cable is specified correctly. If the current flows through other circuits, there is the risk that the owner is unaware of what gauges of wire it's passing through and some small wire could heat up and start a fire.
On a more general point, the most helpful thing I ever read about electrical system design was (on this forum I think) from a guy who said he always thought in terms of 'sources' and 'applications'. If the battery is source 1, then a loop of heavy cable out to a +ve bus bar could be considered source 2 - that is, a place to take positive current from for any number of 'applications'.
Each 'application' is a loop of wire that goes out to, say, a light bulb or radio, and returns to a 'common earth' bus bar somewhere else. Whether you have one 'common earth' bus bar, or several, they will be connected with heavy gauge cable back to the negative terminal of the battery.
Once I read that, I could start to read circuit diagrams. Around the boat you might have, say, five 'sources' and from those sources, loops of wire to 'applications'. On boats, nearly every 'application' circuit needs to return to the 'common earth' via some kind of circuit breaker - usually a switch panel, where each switch can break one circuit, or several circuits (e.g. one switch turning off three lights).
To design a wiring system, then, you are really just adding several parallel circuits to the battery to creats those 'source' bus bars - i.e. you are not linking them in series - and from those bus bars, each 'application' returns to earth via a circuit breaker switch somewhere (many applications have their own on/off switches as well, but should still go back through a circuit breaker somewhere before returning to earth).
I don't know if that is helpful, but that's the bit of knowledge that made the whole thing comprehensible to me. From there, you just need to know what gauge of wire to use for each circuit to prevent wasteful voltage drops (and overheating) along each length of cable, what kind of switches to use, and how to use fuses and automatic circuit breakers to keep it all safe.
After redoing the electrics on my boat, I'm hoping to work out a 48V off-grid system for my shed at home (where I live there are power cuts about once every two months). 48V is the highest voltage it is legal to work with without a sparkie's qualification and when stepping up the voltage to 240V through an inverter, it's less of a jump than from 12V to 240V. From what I've read, a lot of live-aboards are doing this on their yachts too, and there are a growing numbre of 48V appliances being sold to cater for them.
neilmac01
> 9 months ago, 29 May 2024
