Battery cable size?

waynes world

waynes world

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When working out the cable size for the batteries how do i work out the Amps for the cable to carry. I know the Amps=watts/V part, its the knowing how many amps oi will be using, Do i take it if all the system is running together, unlikely, but is that the safe way to work it out.

Thanks

Wayne
 
waynes world said:
When working out the cable size for the batteries how do i work out the Amps for the cable to carry. I know the Amps=watts/V part, its the knowing how many amps oi will be using, Do i take it if all the system is running together, unlikely, but is that the safe way to work it out.

Thanks

Wayne
Click to expand...

The spec for the starter motor should include figures for current when running with no load, when cranking normally and when installed.

but beware of using the power rating to calculate the current from amps = watts / volts. The power quoted for the starter motor is for its mechanical output, not its electrical consumption. That will be significantly greater,


FWIW I have some figures for a 1.1hp/810 watt starter motor :

Unloaded 30-50 amps. Loaded 185-220 amps. Stalled 400-490 amps

In this case I estimate the battery cable to be 50mm² but maybe the wiring diagram in your engine manual will indicate the cable size
 
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As a tip i bought some heavy juty jump leads for the cable. It was cheaper to buy it like this than the cable seperately
 
VicS said:
The spec for the starter motor should include figures for current when running with no load, when cranking normally and when installed.

but beware of using the power rating to calculate the current from amps = watts / volts. The power quoted for the starter motor is for its mechanical output, not its electrical consumption. That will be significantly greater,


FWIW I have some figures for a 1.1hp/810 watt starter motor :

Unloaded 30-50 amps. Loaded 185-220 amps. Stalled 400-490 amps

In this case I estimate the battery cable to be 50mm² but maybe the wiring diagram in your engine manual will indicate the cable size
Click to expand...


There is no inboard engine and the outboard will have its own starter battery.

Tammany said:
As a tip i bought some heavy juty jump leads for the cable. It was cheaper to buy it like this than the cable seperately
Click to expand...

I have a few leads in the shed, well a lot infact. All from welding rigs, jump leads and other boats i have had.
 
Hi 'Wayne', There are several books on wiring, but if you are not counting the starter, as you have an outboard with a seperate battery, the rest is fairly skinny stuff.
I have Charles Wing's book and it has charts for wiring sizes. Anything particular you want to know about?
DW
 
DownWest said:
Hi 'Wayne', There are several books on wiring, but if you are not counting the starter, as you have an outboard with a seperate battery, the rest is fairly skinny stuff.
I have Charles Wing's book and it has charts for wiring sizes. Anything particular you want to know about?
DW
Click to expand...

All i need to know is the minimum thickness for the cable from battery/s to the fuse boxs, this way i can a bit more as i do for the safety aspect, same as i have done with the rest of the loom, i will be having two 110amp batteries. The rest i have used 2mm for the LED lights and 3mm for the USB charge points and horn etc.

PaulRainbow said:
What electrical stuff do you have and how long will the cables be ?
Click to expand...

The battery cable will be 1.5m long.
 
So, if I understand this correctly, you are just asking about cable from the domestic battery to the distribution fuse box. Assuming you're not going to have loads and loads of current-hungry equipment, you might look at using 6mm2 cable. This will cope with reasonable loads, lights, fridge, etc, without introducing undue voltage drop. You should add a fuse in the cable as close as possible to the battery.
 
waynes world said:
All i need to know is the minimum thickness for the cable from battery/s to the fuse boxs, this way i can a bit more as i do for the safety aspect, same as i have done with the rest of the loom, i will be having two 110amp batteries. The rest i have used 2mm for the LED lights and 3mm for the USB charge points and horn etc.



The battery cable will be 1.5m long.
Click to expand...

The minimum thickness will depend upon the maximum current they will be required to supply. They must have a safe current carrying capacity that at least equals that but will have to be slightly oversized to keep voltage drop between the battery and the distribution board within acceptable lengths.

Do a full inventory of all your electrical equipment with the current each item will require. Estimate from those figures what the maximum current is likely to be.

Tell us that and with the knowledge of the battery cable lengths it will be easy to advise you. Without knowing the current you will drawing from the battery it is impossible to do so
 
If you have two 110 domestics, how are you charging them? OK if on a mooring, from shore power, not so good from an outboard. Solar power?

LEDs & USBs use very little power so your sizes are fine. Horn? just fitted a noisy one with 1.5 mm.
Out of habit I would use 6mm from the batteries to the switch board, but not needed if your stuff is modest.
Just looked at Wing's, 6mm is something like 40amps, so well over what you seem to need.

Edit: Crossed with Vic, who def knows about all this.
 
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VicS said:
The minimum thickness will depend upon the maximum current they will be required to supply. They must have a safe current carrying capacity that at least equals that but will have to be slightly oversized to keep voltage drop between the battery and the distribution board within acceptable lengths.

Do a full inventory of all your electrical equipment with the current each item will require. Estimate from those figures what the maximum current is likely to be.

Tell us that and with the knowledge of the battery cable lengths it will be easy to advise you. Without knowing the current you will drawing from the battery it is impossible to do so
Click to expand...

Ah so as i thought, i need to add up all the current draw from all the items to find the size cable i need.

I think i asked the question all wrong. well infact i did, i have been using a volt drop calc to get the wire size i have used through the boat wiring. and i have gone with max 1.5% volt drop and that was with the horn and bilge pumps.
 
DownWest said:
If you have two 110 domestics, how are you charging them? OK if on a mooring, from shore power, not so good from an outboard. Solar power?

LEDs & USBs use very little power so your sizes are fine. Horn? just fitted a noisy one with 1.5 mm.
Out of habit I would use 6mm from the batteries to the switch board, but not needed if your stuff is modest.
Just looked at Wing's, 6mm is something like 40amps, so well over what you seem to need.

Edit: Crossed with Vic, who def knows about all this.
Click to expand...

Yes two 110amp domestic and will be charged witb a genny and charger.
 
http://www.12voltplanet.co.uk/cable-sizing-selection.html Just over half way down the page is a volt drop calculator
For 6mm2 cable 1.5m from the battery and 50A load volt drop is 0.43V which means you loose 3.56% of your power in the cable.

Plug in some numbers and see what you are happy with. If adding up all the nitty gritty per item is a nuisance then allocate a range of 2A, 5A and 10A circuits, then add up the circuits.
 
waynes world said:
Ah so as i thought, i need to add up all the current draw from all the items to find the size cable i need.

I think i asked the question all wrong. well infact i did, i have been using a volt drop calc to get the wire size i have used through the boat wiring. and i have gone with max 1.5% volt drop and that was with the horn and bilge pumps.
Click to expand...

Yes add up the current of all the items that might be used at the same time.

3% volts drop is the generally accepted maximum figure to work to but you do have to be careful not to end up with 3% drop between battery and distribution board and another 3% drop between the distribution board and the equipment.
Most modern electronics , including LED lights, will operate satisfactorily over a wide voltage range.

The calculator on the 12 volt panet website looks like a good one. Some do not make it clear whether you should enter the "One way" distance or double that.
 
For 1.5m of cable there is no point messing around with calculations, just use 16mm cable and you'll be well covered for now and the future. Fitting smaller cable will only save pence.
 
pvb said:
So, if I understand this correctly, you are just asking about cable from the domestic battery to the distribution fuse box. Assuming you're not going to have loads and loads of current-hungry equipment, you might look at using 6mm2 cable. This will cope with reasonable loads, lights, fridge, etc, without introducing undue voltage drop. You should add a fuse in the cable as close as possible to the battery.
Click to expand...

Mine was wired with 6mm Pete, when the fridge cut in the lights dimmed. I changed to 10mm and it's all but cured.
 
I have gone with 25mm, reason being it is the minimum on the BSS, i should of checked there first, but i still had to know the question. With 25mm and 10 amp draw, VD is 0.25% so well in with 3% max.


Thanks all for the help, as always appreciated :)
 
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