Marine Electrical system design and cable spec

[picardy]

I am in the process of having a 20ft wooden motor boat built. In order to keep the cost down (and make life easy for the one man band builder), he has offered to run the cables for me ready for my electrician to make the final connections.

Can anyone point me to a paper or online resource or a consultant that will help me design the system and specifically which cable sizes are needed. I will be having two batteries and it will include plotter, deck wash, fridge, VHF, AUX and LED deck floor lights.

Specific question I would like answered if that I would prefer to put the 2 x batteries in the bow locker area but that means running the engine starter battery cables around 2.5-3metres to the Yanmar 21hp engine.

1) Is that too far or if not what size cables would be needed?
2) Can I run 6m long cables to the small fridge which I would like fitted in the aft locker - (it will be a Domestic CD30), Rated current is 3A at 12volts.

Many thanks

 

[jwfrary]

Victron have a useful app ‘victron toolkit’ which you can plumb in cable size, volts and amps and get a voltage drop figure.

From what you say though, I expect you can run it all in 2.5mm2 tinned twin and be done with it. Probably by a reel rather than by the meter.

Unless your anxious about weight a little heavier won’t hurt!

 

[Bristolfashion]

Could the builder run trunking which would allow easy push through for whatever cables the sparkie decides & for later additions?

Alternatively, could the electrician advise cable sizes?

Starter cables carry a lot of juice & are therefore usually short. It my be surprising how thick a cable is required over 3m and how much it costs. I'm sure it could be done.

As others have said, there are plenty of online calculators that give cable sizes for length & current. Some devices may give the size cable for certain cable runs - the anchor winch I'm currently fitting does - but watch out as the lengths referred to are sometimes the total run (there & back) and not the distance between battery & device.

 

[picardy]

Thanks all - very helpful advice.

 

[PaulRainbow]

I am in the process of having a 20ft wooden motor boat built. In order to keep the cost down (and make life easy for the one man band builder), he has offered to run the cables for me ready for my electrician to make the final connections.

Can anyone point me to a paper or online resource or a consultant that will help me design the system and specifically which cable sizes are needed. I will be having two batteries and it will include plotter, deck wash, fridge, VHF, AUX and LED deck floor lights.

Specific question I would like answered if that I would prefer to put the 2 x batteries in the bow locker area but that means running the engine starter battery cables around 2.5-3metres to the Yanmar 21hp engine.

1) Is that too far or if not what size cables would be needed?
2) Can I run 6m long cables to the small fridge which I would like fitted in the aft locker - (it will be a Domestic CD30), Rated current is 3A at 12volts.

Many thanks

No problem with 2.5-3.0m starter cables, 35mm will be more than enough.

I'd run 6mm cable to the fridge, current inrush as startup might cause the fridge to cut out if it sees low voltage, so 6mm will keep the voltage drop down.

However, your deck wash will also be at the stern, so it might be an idea to run 10mm cable to a small fuse box at the stern, then drop down to 2.5mm close to the fridge and pump, with 1.5mm for the deck lights.

You could run a 6mm cable to a fuse box at the helm, then 1.5mm cables to the plotter, VHF and lights.

 

[VicS]

I am in the process of having a 20ft wooden motor boat built. In order to keep the cost down (and make life easy for the one man band builder), he has offered to run the cables for me ready for my electrician to make the final connections.

Can anyone point me to a paper or online resource or a consultant that will help me design the system and specifically which cable sizes are needed. I will be having two batteries and it will include plotter, deck wash, fridge, VHF, AUX and LED deck floor lights.

Specific question I would like answered if that I would prefer to put the 2 x batteries in the bow locker area but that means running the engine starter battery cables around 2.5-3metres to the Yanmar 21hp engine.

1) Is that too far or if not what size cables would be needed?
2) Can I run 6m long cables to the small fridge which I would like fitted in the aft locker - (it will be a Domestic CD30), Rated current is 3A at 12volts.

Many thanks

Perhaps you should try to get hold of International Standard ISO 13297-2020 which is the standard for both low voltage single phase AC systems and extra low voltage DC systems in small craft
( Beg , borrow or steal ... its not on line)

For the 250 max loaded starter motor current quoted in the installation manual I think you will need 70mm² cable

For your fridge see this graph in the installation instructions



More useful info on the 12volt planet website at Cable sizing and selection | 12 volt planet

 

[PaulRainbow]

Perhaps you should try to get hold of International Standard ISO 13297-2020 which is the standard for both low voltage single phase AC systems and extra low voltage DC systems in small craft
( Beg , borrow or steal ... its not on line)

I have it

For the 250 max loaded starter motor current quoted in the installation manual I think you will need 70mm² cable

70mm for a 3m run of cable for a 21hp engine ? Seriously ?

Not sure where you get that from, but it's insane. 25mm cable would work fine, i've suggested 35mm to allow some margin.

Actually, i do know where you get that from, you entered 3m cable run, 12v, 250a current draw into the 12v planet voltage drop calculator and it came back with 70mm cable to keep the voltage drop below 4%.

This is one of those situations where reading stuff on the internet without real World experience isn't helpful.

For your fridge see the installation instructions

Or read post #5

More useful info on the 12volt planet website at Cable sizing and selection | 12 volt planet

Indeed, but it's sometimes useful to have an understanding of how things work in the real World.

There probably isn't a 3YM20 in the known galaxy that's wired up with 70mm starter cables.

 

[Aurai]

You can run your cable through this braided protection, which works in boats with so many curves, regular conduit can be limited in its runs.

RS PRO Braided Fibreglass Natural Cable Sleeve, 6mm Diameter, 5m Length | RS Components