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Old 02 February 2023, 10:01   #1
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Cable size advice

Hi - I'm not great at electronics.

I'd appreciate advice on the cable size to use from the battery switch up the boat to the helm.

set up will be

2 battery / 1 engine (Honda BF50A - manual says 10A dc output?)
Battery isolation (probably the Blue Sea) switch with ACR also blue sea?
Length from battery to helm and return is no more than 10m overall length

Items at Helm -
Fuse panel connected to switch panel
GPSMap 723xsv - Manual says 36W draw?
Single white light mast
USB charger
Allow for possible future additions like fixed VHF (I use a handheld through choice) / stereo / deck lights

Questions?
What is the AWG cable size should I use for the run from the battery switch/acr to the helm?

Should I incorporate a CB - if so what size Amp?

Apologies if these are the stupidest type of 'lecky questions but no matter how many websites I read I just can't seem to get my head around it

Thanks, Rob
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Old 02 February 2023, 17:56   #2
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Iíve used tinned 6mm/10awg from battery switch to fuse panel although itís not as long a run as yours. 6mm will happily take 50 amp.

I have just wired my console, I have a Fusion stereo, the same chart plotter as you, 2 Yamaha gauges, a vhf, 3 rocker switches for Nav lights and a bilge pump. When all these items are on at full power it still only draws 9 amps.
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Old 02 February 2023, 22:35   #3
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Don't put a circuit breaker on the cable from the battery to the helm - if there's a short on one of the final circuits it could possibly trip the circuit breaker before a downstream fuse blows and then you lose ALL electrics.

For a 10m run with those accessories I would use 10mm≤ cable as it'll give you plenty of headroom for the future. The fixed VHF and possibly the USB outlet (depending on how beefy it is) will want a 10A fuse and probably at least 1.0mm≤ cable, everything else will be fine with a 5A fuse and 0.75mm≤ cable as long as the runs aren't more than a few metres. The longer the cable runs, the thicker the cable needs to be for the same current to minimize voltage drop.
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Old 03 February 2023, 07:28   #4
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Originally Posted by tehguy View Post
Don't put a circuit breaker on the cable from the battery to the helm - if there's a short on one of the final circuits it could possibly trip the circuit breaker before a downstream fuse blows and then you lose ALL electrics.

For a 10m run with those accessories I would use 10mm≤ cable as it'll give you plenty of headroom for the future. The fixed VHF and possibly the USB outlet (depending on how beefy it is) will want a 10A fuse and probably at least 1.0mm≤ cable, everything else will be fine with a 5A fuse and 0.75mm≤ cable as long as the runs aren't more than a few metres. The longer the cable runs, the thicker the cable needs to be for the same current to minimize voltage drop.


It definitely needs some protection as close to the battery as possible. I use fused battery terminals. The last thing you want is an un fused heavy cable wandering around the boat.
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Old 03 February 2023, 12:02   #5
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Thanks for the input guys. I'd have thought that a CB would be to protect the electrics including the fuse panel?

Would a CB be to stop an overload coming from the engine alternator or the battery? Would you have a CB between the engine and battery or just after the battery?
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Old 03 February 2023, 12:48   #6
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Originally Posted by Robmc View Post
Thanks for the input guys. I'd have thought that a CB would be to protect the electrics including the fuse panel?

Would a CB be to stop an overload coming from the engine alternator or the battery? Would you have a CB between the engine and battery or just after the battery?


I wouldnít use a CB, Iíd use a 30A fuse as close as possible to the battery. The fuse is to protect the cable between the battery & fuse box. The fuses in the fuse box protect the outgoing cables & devices.
I have a 250A fused battery terminal on my engine cable.
Avoid any unnecessary mechanical or complicated devices on boats.
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Old 03 February 2023, 16:49   #7
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Thanks for that - good suggestion re fuse v's CB
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Old 03 February 2023, 19:09   #8
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Thanks for that - good suggestion re fuse v's CB


I use these

https://kojaycat.co.uk/Tinned-Cube-F...ttery-Terminal

https://kojaycat.co.uk/fuse-boxes/-h...amic-cube-fuse
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