Cable sizing

[Col_T]

The battery inter-link cables on the boat are 'good-an-thick', but I have no idea what size they really are. So, if I measure length and resistance of each cable, is it possible to determine the CSA sizing of the cable?

 

 

[Alan de Enfield]

2 minutes ago, Col_T said:

The battery inter-link cables on the boat are 'good-an-thick', but I have no idea what size they really are. So, if I measure length and resistance of each cable, is it possible to determine the CSA sizing of the cable?

 

 

 

Not easily or accurately.

 

To get a rough and ready size. Measure the overall diameter of the COPPER (only)

Square the radius (half of the diameter) and multiply by 'pi' (3.142)

 

Take 90% of that answer and you will have an approximate CSA.

 

The correct and accurate method assumes that the ends are not terminated and you can access the individual strands.

Strip back some insulation, measure the area (pi x R squared) of one strand and multiply it by the number of strands = CSA

 

 

However :

The cable CSA SHOULD be marked on the sheath of the cable

[Col_T]

Rather as I suspected, and thanks for the two methods of working it out.

 

I'll have a crack at calculating when I eventually see the boat again, and have a look for any markings on the cable to 'check my workings', as it were!

[nicknorman]

Yes the method you proposed initially would work, but only if you can measure very low resistance accurately, which you can’t! But we’ll done for applying scientific theory, even if it’s not practical!

[BEngo]

Have a look at the crimped on terminals, if fitted.  They usually have 2 numbers on them.  Eg 16 10.  The first is the cable csa the crimp is made to fit and the second is the size of the hole in mm. The BSS says (without any sensible reason being given) that   battery interconnect cables must be at least 25 mm squared csa, so expect a first number that is greater than or equal to 25.

 

N

[Alan de Enfield]

12 minutes ago, BEngo said:

Have a look at the crimped on terminals, if fitted.  They usually have 2 numbers on them.  Eg 16 10.  The first is the cable csa the crimp is made to fit and the second is the size of the hole in mm. The BSS says (without any sensible reason being given) that   battery interconnect cables must be at least 25 mm squared csa, so expect a first number that is greater than or equal to 25.

 

N

All very true, but three 'warnings'

 

 

1) You are assuming a lot from the 'average boater'

2) I have seen oversize terminals used and the terminal hammered / squashed down to fit the smaller cable, even with cold-chisel marks to get the final grip on the wires.

3) I have seen undersized terminals used where some of the conductor wires have been cut off to enable the terminal to fit.

 

Boaters don't always go out and buy what they need if they have something 'in the tool box' that is 'close'.

Edited April 24, 2020 by Alan de Enfield

[ditchcrawler]

1 hour ago, Alan de Enfield said:

 

 

However :

The cable CSA SHOULD be marked on the sheath of the cable

but if they are very short lengths between battery terminals you may miss it

[jonathanA]

Gosh all a bit complicated... find a piece of cable of known size  say 35mm and compare as a starter for ten. Buying one metre of say 25mm won’t be that expensive - a decent place would probably give you a sample for free. Or take one of your interlinks and compare....

[nicknorman]

Just now, jonathanA said:

Gosh all a bit complicated... find a piece of cable of known size  say 35mm and compare as a starter for ten. Buying one metre of say 25mm won’t be that expensive - a decent place would probably give you a sample for free. Or take one of your interlinks and compare....

The only problem with that is there is a lot of variation in insulation thickness. So unless you can see the exposed copper, just looking at and comparing cables from the outside isn’t helpful.

[jonathanA]

And your suggestion is ......

 

 

3 minutes ago, nicknorman said:

The only problem with that is there is a lot of variation in insulation thickness. So unless you can see the exposed copper, just looking at and comparing cables from the outside isn’t helpful.

And your suggestion is ......

[robtheplod]

I'm going to be looking at replacing battery cables once the lockdown has occurred and went through all the calculations, then saw a YouTube video of a boater who just used 95mm2 cable - way over the top, but got me thinking, I know its more expensive but it removes the possibility of using too small a cable and builds in flexibility for the future... this was just for wiring batteries though....

[Alan de Enfield]

8 hours ago, jonathanA said:

And your suggestion is ......

 

 

And your suggestion is ......

 

Don't be a 'bodging boater' do it properly.

[WotEver]

2 hours ago, robtheplod said:

I'm going to be looking at replacing battery cables once the lockdown has occurred and went through all the calculations, then saw a YouTube video of a boater who just used 95mm2 cable - way over the top, but got me thinking, I know its more expensive but it removes the possibility of using too small a cable and builds in flexibility for the future... this was just for wiring batteries though....

Exactly.  A metre or two is going to be pretty cheap even if you bought something hefty like 95mm². If you buy welding cable it's both (relatively) cheap, and very flexible.

[cuthound]

3 hours ago, robtheplod said:

I'm going to be looking at replacing battery cables once the lockdown has occurred and went through all the calculations, then saw a YouTube video of a boater who just used 95mm2 cable - way over the top, but got me thinking, I know its more expensive but it removes the possibility of using too small a cable and builds in flexibility for the future... this was just for wiring batteries though....

 

Err 95mm2 cable isn't that flexible! ? 

 

 

(Unless you use the type used for welding cables )

Edited April 25, 2020 by cuthound
To add the last paragraph

[Detling]

Fit 80 upwards and forget, I have 95mm sq the resistance of the bolt down fastening to the batteries is probably higher than the resistance of the wire.

[nicknorman]

3 minutes ago, Detling said:

Fit 80 upwards and forget, I have 95mm sq the resistance of the bolt down fastening to the batteries is probably higher than the resistance of the wire.

Yes in general, however it does depend on the length of the cable. So for interconnects, you’re right. For a lengthy run from batteries to a 3kw inverter in another room, maybe not.

[cuthound]

2 hours ago, Detling said:

Fit 80 upwards and forget, I have 95mm sq the resistance of the bolt down fastening to the batteries is probably higher than the resistance of the wire.

 

The standard cable sizes around 80mm sq are 70 and 95.

 

Where does the 80 mm sq come from?

 

As Nick says, cable size depends on length of cable, load and acceptable volt drop. The method of containment can also affect which cable size you choose.

Edited April 25, 2020 by cuthound
To unmangle the effects of autocorrect.

[Alan de Enfield]

7 minutes ago, cuthound said:

 

The standard cable sizes around 80mm sq are 70 and 95.

 

Where does the 80 mm sq come from?

 

As Nick says, cable size depends on length of cable, load and acceptable volt drop. The method of containment can also affect which cable size you choose.

 

As I said earlier :

 

6 hours ago, Alan de Enfield said:

 

Don't be a 'bodging boater' do it properly.

 

[Col_T]

As it's been hinted at earlier, why don't people get twitchy about alternator and battery charger cable sizes??

[Alan de Enfield]

1 minute ago, Col_T said:

As it's been hinted at earlier, why don't people get twitchy about alternator and battery charger cable sizes??

Those that understand do.

 

Your battery charger manual will tell you the minimum size cable to use, and for your alternator you can work out the length and the current and use the correct size cable.

 

My 50 amp Victron states 25mm2

 

 

 

[Detling]

5 hours ago, nicknorman said:

Yes in general, however it does depend on the length of the cable. So for interconnects, you’re right. For a lengthy run from batteries to a 3kw inverter in another room, maybe not.

I was refering to the interconnects as they are short and fat, you can basically forget about them.

 

( I couldn't remember the next size down so guessed 80 but was wrong)