Battery cable sizing

mrangry

mrangry

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I am upgrading my 12v house battery bank from two to four 110ah leisure batteries. The current battery cabling (including the starter) is a jumbled mess with poor connectors and battery terminals with wing nuts and I want to replace it. The batteries are all together and within a metre of the engine which is a 50hp Yanmar 4 cyl diesel.

I know cables are sized taking account of current draw, voltage,length of run and weak spots may be connections, but looking for some pointers on choosing battery cable size. I have had search but still none the wiser.
 
Impossible question to answer properly without much more information. However, if you don't have an inverter, bowthruster, windlass, etc, then 50mm2 cable would probably be fine.

You might like to offer some more details so that others can take a more considered view...
 
pvb said:
Impossible question to answer properly without much more information. However, if you don't have an inverter, bowthruster, windlass, etc, then 50mm2 cable would probably be fine.

You might like to offer some more details so that others can take a more considered view...
Click to expand...

Good points...I dont have an inverter just now but may look in the future to fit one. At the moment I have no requirement for mains power and I have removed all of the ac wiring that was on the boat as to be honest it was a daisy chain of multi way extension leads wired into sockets. There is a windlass which I think is on the starter battery circuit and for now not looking to replace that cable as from memory it appears sound.
 
This is a useful site
Cable sizing and selection | 12 volt planet

So for example a 2m run - there and back to battery, a 50amp current and 12v you need 10mm to reach the recommended volatge drop of 3 percent. So different cable sizes for starting where much higher amps are needed and for house use unless you have some really heavy draw. Mapping out your highest amps and the round trip distances of each cable is key, or you could just go for huge thick cables.
 
RupertW said:
This is a useful site
Cable sizing and selection | 12 volt planet

So for example a 2m run - there and back to battery, a 50amp current and 12v you need 10mm to reach the recommended volatge drop of 3 percent. So different cable sizes for starting where much higher amps are needed and for house use unless you have some really heavy draw. Mapping out your highest amps and the round trip distances of each cable is key, or you could just go for huge thick cables.
Click to expand...

How would I know the current draw on the engine starter for example? Also, what constitutes thick cables 50mm? I had a wee google and before you know it you have spent hours trawling through stuff and less informed than I was before I started.
 
mrangry said:
How would I know the current draw on the engine starter for example? Also, what constitutes thick cables 50mm? I had a wee google and before you know it you have spent hours trawling through stuff and less informed than I was before I started.
Click to expand...
Sorry, I wasn’t being entirely serious about “thick cables” as you need to know what size you really need for the 3 percent drop before you can decide to oversize them.

The calculator a page or two down the table is what you should mainly look at. Only you will know from your design what the round trip length of each cable is, and for other current draws you should look at your connected equipment for details - e.g. your starter motor model, an inverter, electric windlass etc.
 
The advice above from PVB is good - worth sketching a diagram of the suggested new wiring layout to help planning. Noted you are in Clyde Marina in Ardrossan? I am too (well, when travel restrictions lift) and I have a large cable crimping tool which you will likely need to make good terminal connections, you are welcome to borrow it for your project
 
West Coast said:
The advice above from PVB is good - worth sketching a diagram of the suggested new wiring layout to help planning. Noted you are in Clyde Marina in Ardrossan? I am too (well, when travel restrictions lift) and I have a large cable crimping tool which you will likely need to make good terminal connections, you are welcome to borrow it for your project
Click to expand...
That would be great thanks
 
mrangry said:
I am upgrading my 12v house battery bank from two to four 110ah leisure batteries. The current battery cabling (including the starter) is a jumbled mess with poor connectors and battery terminals with wing nuts and I want to replace it. The batteries are all together and within a metre of the engine which is a 50hp Yanmar 4 cyl diesel.

I know cables are sized taking account of current draw, voltage,length of run and weak spots may be connections, but looking for some pointers on choosing battery cable size. I have had search but still none the wiser.
Click to expand...

If you know those things, you should have revealed them. :)

Length of cables from engine battery to starter ?
 
mrangry said:
I thought I did apart from current draw.
One metre as per initial post:)
Click to expand...

I'll head off to Specsavers first thing in the morning :)

35mm cable will be fine. All negatives should be linked and 35mm, positive from stater battery and domestic bank to isolators, 35mm and obviously parallel link cables on the domestic batteries. The reason all need to be 35mm is for emergency engine starting, either by a 1-2-B switch or separate switches, what switches do you have ?

From the domestic isolator you can step down in cable size, to suit your domestic loads.

You should fuse the cables at the batteries and the domestic supply cable from the isolator if there is any possibility of them shorting.

If you decide to fit an inverter you may need to use heavier cable for inverter, connected on its own supply, with its own isolator, depends on the size of the inverter.
 
Thats great thanks. I have separate isolators. Would i be better using heavier for future proofing say 50mm? I completely forgot about fuses will have to have a look
 
mrangry said:
Thats great thanks. I have separate isolators. Would i be better using heavier for future proofing say 50mm? I completely forgot about fuses will have to have a look
Click to expand...

35mm is rated at 240a, so unless you plan to fit a bigger engine it's more than adequate for domestic needs, you won't be running 35mm all the way to the switch panel anyway.
 
My boat has 4x110 AHr domestic bank, +2 for bow thruster + engine and all inter-battery cable is either 75 or 95mm2. I fitted a VBM712 last year and made 95mm2 patch leads and a bus bar to suit. In addition to the anvil crimping tool a good cable cutter is a worthy investment.
 
Hijacking this thread, one of the post lockdown jobs is to renew all the battery cables and connections in the engine/battery bay. Everything is within a metre of everything else and the starter is 2kw. Is 50mm better than 35mm? The boat is a thousand miles away so I don't have accurate measurements. The stuff is a pain to get locally so I plan to bring the cables, lugs and crimper with me the next time I'm able to go.
 
colind3782 said:
Hijacking this thread, one of the post lockdown jobs is to renew all the battery cables and connections in the engine/battery bay. Everything is within a metre of everything else and the starter is 2kw. Is 50mm better than 35mm? The boat is a thousand miles away so I don't have accurate measurements. The stuff is a pain to get locally so I plan to bring the cables, lugs and crimper with me the next time I'm able to go.
Click to expand...

35mmover 1m has a voltage drop of about 1.4% and is rated at 240a, so it's more than adequate. You can fit 50mm if you want, but it offers no benefit.
 
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