Battery Cable Sizing

[co256]

I'd like to renew the battery cables but would rather calculate the correct size than simply replace like with like.

The reason being.... the current installation is such a shambles I don't trust what's there to be any guide for what should be there?!

So I'm replacing the cables from the batteries to the 1,2,both,off switch, to the starter motor, back from the alternator and up to the switch panel.

Any advice greatly received.

 

[savageseadog]

Depends on the lengths involved and cranking current. Don't forget that includes the negative return. You also have to include the resistances of connectors and switches to get a full picture. Ultimately, whatever you do a compromise is made and some voltage is always lost and there is a trade off between cable thickness, cost and ease of installation. In my case with a 2GM20 and about 7m round trip I decided on using 35mm sq cable in the starter circuit which has 200-250A cranking current. I can't remember the exact figure but the potential loss was probably less than .25V.

 

[Cliveshep]

Unless your batteries are a long way from the starter I'd look at the same size as from a comparable engine in a car/van/truck. What I would advise strongly is that when you have selected your cables you take yourself off to an electrical wholesaler and buy the all lugs you willl need plus a ratchet crimper that can handle large lugs. These cost around £40 but there is no way you can make any sort of job without one. Lugs by the way are sized by the bolt hole and the cable size they have to fit. So for a starter motor, for example, you might be wanting 8mm x 50mm2 lugs where 8mm is the bolt size.

 

[Dave_Rolfe]

I would recommend going to a battery specialist where they will be able to crimp up the leads to exactly how you want them using quality equipment to ensure a good contact with every one.

 

[boatmike]

For single core multiflex wire the cross sectional area recommendations are related to the amperage as follows

6mm 29amps
10mm 40 amps
16mm 54amps
25mm 71amps
35mm 87 amps
50mm105 amps

Note that these are for PVC covered cable
For Butyl rubber or XLPE cable a slightly higher current is permitted and for long runs always go one cable size higher. As you have indicated these sizes apply to all cables in the charging circuit as well as starter cables.
Hope this is what you were looking for.
Edit:
If you need lighter loads for other wiring it's

1mm 8amps
1.5mm 12amps
2.5mm 17amps
4mm 22 amps

 

[saltwater_gypsy]

Browse through this extract from John Payne's Electrical Bible and you will find all the answers.

wiring data from Payne's Electrical Bible