How do I measure electric cable?

[tugboat]

I'm about to connect up my smart charger and the recommended cable size is 16mm(squared).
I'm feeding 3 battery banks, so do I need to make the common earth return cable bigger cross section by a factor of 3?

 

[Norman_E]

The cable size should have been calculated to cover the maximum current output of the charger, therefore you do not need to multiply up for the common return if all you need to consider is charging current.

 

[steve_cronin]

16mm squared is WRONG WRONG WRONG!!!!!

... because "16mm squared" is 256 square millimetres and I don't know ANY charger that needs that sort of cable since a welder only needs 50 sq mm. It would be the thickness of a bicycle tyre!

Sadly cables tend to get labelled "XXmm squared" rather than "XX sq mm" because they seem to get manufactured exclusively in The People's Republic of China these days who care rather less for accuracy than income. (Before you ask,Yes, I DO resent China's taking over our previous manufacturing base in just about everything these days)

If you need 16 sq mm for each bank then yes you do need three times that in the negative but that does NOT mean three times the thickness*. Good news is that , since the negatives will be linked by something substantial if you have common alternator charging and battery engine starting, you will only need this from the charger negative to the point where the -ves are linked,

*For information, if you need 16 sq mm ( the cross sectional area being directly proportional to the current being carried), then since the formula for the cross sectional area is Pie ( 3.1412...) times the radius ( twice the diameter) squared, the cable diameter required is roughly 2.2mm.

For the negative return for three battery banks, three times the c/s area, would be 48 sq mm which would be a diameter of (again roughly) slightly over 4 sq mm

So, since commercially available cable can be had at 2.5mm diameter and 6mm diameter, use these; the former for each of your positives and the latter for the common negative return. If you're going to have a seperate negative return for each battery bank, then use 2.5mm for each of these.

Steve Cronin

 

[roly_voya]

Just checked that cable size, are you sure you read it right because 16sqmm is rated at around 110a which would be a mighty charger and would be designed for a bank of 600a/h +

There is a good selection of cable available from Index marine www.marineelectrics.co.uk including formula for calc the size cable you need.

Most multibank charger are rated for total output ie a 40a charger charging 2 banks will supply 20a to each bank so return cable only needs to be rated to 40a

 

[Birdseye]

The cable size will depend on the distance run. For any given level of current there will be a voltage drop along the cable, and the longer the cable the bigger the drop. Since most smart chargers sense the voltage at the charger itself, you dont want that voltage drop to be too big. For example, you dont want to end up with the charger thinking it is sat there "soaking" the batteries at 13.4 volts when the actual voltage the batteries are seeing is 12.5 because there is a 0.9v drop in the cable.

Presumably the supplier of the charger will have given you advice on this ie different wire diameters for different run lengths. If they havent, then its best to ask them what they recommend for your run length. Since the drop happens in both feed and return so you need to add both lengths together.

The charger rating will be the total amps and I would expect their recommended wire size to reflect the possibility that all these amps end up going to one battery. So whilst the feed to each battery needs to be (say) 16 sq mm the common return only needs to be the same size.

You can get 50m reels of this sort of wire from the auto electrical store in most big cities. It wont be tinned (does that matter?) but it will be cheaper than a specialist marine outfit.

Finally, carefully try the hand test when wired up and operating. Do any of the joints or the wire to the batteries get warm? If they do, then you have found a source of higher resistance which needs sorting out.

 

[jonathankent]

Re: 16mm squared is WRONG WRONG WRONG!!!!!

[ QUOTE ]
since the formula for the cross sectional area is Pie ( 3.1412...) times the radius ( twice the diameter) squared

[/ QUOTE ]
Wrong..... pi x (radius squared) .... where the radius is HALF the diameter

 

[Norman_E]

Re: 16mm squared is WRONG WRONG WRONG!!!!!

Quote "If you need 16 sq mm for each bank then yes you do need three times that in the negative but that does NOT mean three times the thickness*."
I disagree, if 16 sq mm will take the output from the charger, it will also take the return current. If the charger has three outputs and only one return, the cable will be sized for each output to be capable of taking the full current, because this is what would happen if two batteries were disconnected, hence the single return only needs to be the same size. I also cannot imagine that the charger is big enough to require more than 16mm unless the run is very long.

 

[fluffc]

Re: 16mm squared is WRONG WRONG WRONG!!!!!

Cable is ALWAYS sold in mm2 (sq mm) sizes - not by diameter. Go into a store and buy "2.5mm" cable, and you can be sure that you will be given 2.5mm2 cable - Not cable of a 2.5mm diameter. 16mm2 cable is fairly large - current carrying capacity is about 70Amps!

 

[VicS]

Re: 16mm² is RIGHT RIGHT RIGHT!!!!!

What is more
[ QUOTE ]
... because "16mm squared" is 256 square millimetres and I don't know ANY charger that needs that sort of cable since a welder only needs 50 sq mm. It would be the thickness of a bicycle tyre!

Sadly cables tend to get labelled "XXmm squared" rather than "XX sq mm"

[/ QUOTE ] What a load of 8/3 pi r cubed

The trouble is that it is difficult to input the correct abbreviation for the units.

16 sq mm may have been the correct abbreviation in distant times but these days the correct one is 16 mm².
That put into words is sixteen millimetres squared which is what tugboat was quite correctly saying.

Its got nothing to do with the Chinese. It all down to having to fiddle about with the Character map to get the ² and other people not knowing the modern way of expressing the units.

Sadly although you can get ² and ³ you cannot get the Greek letter pi so the best I can do for the volume of two spheres is 8/3.pi.r³.

 

[fluffc]

Re: 16mm² is RIGHT RIGHT RIGHT!!!!!

Where'd cubed come into it? Volume got nothing to do with cable sizing.

The calculation between cable diameter and cross-sectional-area (CSA) is

Area = pi x (diameter/2) x (diameter/2) [or, A=pi.r²]

Area is measured these days in mm².

 

[cliff]

Re: 16mm² is RIGHT RIGHT RIGHT!!!!!

Bølløx! as in the volume of two spheres, or to put it another way - balls!
--------------------

"Artificial intelligence is no match for natural stupidity"
sailroom <span style="color:red">The place to auction your previously loved boatie bits</span>

 

[rogerthebodger]

Re: 16mm² is RIGHT RIGHT RIGHT!!!!!

Yes but of cause the relationship between specified area of electrical cable and its diameter is only valid for solid cable and not stranded which what should be used on boats.

Electrical cable area refers to the area of the copper and to be correct is the area of each strand calculated as diameter for each strand squared multiplied by pi divided by 4 multiplied by the number of strands in the cable.

 

[VicS]

Re: 16mm² is RIGHT RIGHT RIGHT!!!!!

Thanks Cliff.

 

[fluffc]

Re: 16mm² is RIGHT RIGHT RIGHT!!!!!

Yes, and who is going to get their Vernier out to start measuring individual strands of wire!

Academic really, cable being specified and sold in terms of CSA...

 

[cliff]

Re: 16mm² is RIGHT RIGHT RIGHT!!!!!

And I always had problems with unit analysis in my long haired, pot smokin, beer drinking student days /forums/images/graemlins/smirk.gif /forums/images/graemlins/wink.gif /forums/images/graemlins/grin.gif
--------------------

"Artificial intelligence is no match for natural stupidity"
sailroom <span style="color:red">The place to auction your previously loved boatie bits</span>

 

[rogerthebodger]

Re: 16mm² is RIGHT RIGHT RIGHT!!!!!

Yes you could say it is academic but the problem is that the less experanced may measure the overall diameter of the cable apply the formula and get the wrong answer and be confused or worse use too small a cable as applying the calculation results in the larger area that it really is.

 

[2Tizwoz]

Re: 16mm² is RIGHT RIGHT RIGHT!!!!!

A 16mm² cable diameter will vary depending on the type of copper strands used, lots of fine ones giving a flexible cable or stiffer fewer thicker ones, and the insulation thickness but will be from roughly 8mm thick.

Welding cable will be thicker - this one's 11.5mm

 

[tugboat]

Re: 16mm squared is WRONG WRONG WRONG!!!!!

OK, I had a senior moment! I put (squared) cos I couldn't find how to put the little '2' after mm. P'raps VicS or 2Tizwoz will tell me where to get that info.
Anyway it's a Sterling 40amp mains charger and I'll be charging 3 banks totalling 550AH. I've got the fancy goldplated fuses etc for feed and return and had intended to use proper tinned cable. At roughly a tenner per metre I didn't want to get it wrong!
From charger I need about one metre of cable (per bank) to where I link into heavy duty battery cables that are convenient to charger location. Sterling recommend 16 sq mm for up to 40 amps, and 25 sq mm for 50 amps. By my calculation 16 sq mm means a radius of 2.25mm (i.e. diam 4.5mm). The charger has a negative return wire built in ready to connect to the fuse holder which goes in the negative return line. This is a pretty chunky stiff piece of cable, but it is well insulated so I can't see how thick it is. I 'll check with Sterling next week, but I wanted to jury rig the charger this weekend to test run using some cable I already have, and don't intend to leave it running unattended. Just had a doubt about sizing the return cable and wanted to get opinion from people more into the electrickery. Thanks for all the replies so far.

 

[jonathankent]

Re: 16mm squared is WRONG WRONG WRONG!!!!!

You can get the "little '2'" by using either the character map (start>programs>accessories>system tools) or by using word and insert>symbol

Theres thousands to choose from....

 

[ChasB]

Re: 16mm squared is WRONG WRONG WRONG!!!!!

I've never totally understood the conventions your discussing here.

I always thought that electricity ran along the the surface of a cable, so a multi-stranded cable would surely have more surface area and so less resistance than a solid piece of copper that calculated out to the same mm square thingy?

In fact, there's some research these days into conductors that have intentionally rough surfaces ('super-rough'?) to increase the surface area and so reduce the resistance....

And aren't there different grades of copper? I can't believe they're all standardised.

Check the manual Tugboat on your charger. Mine has three outputs, but will only charge one battery at max output at a time (though I think it will trickle charge the others simultaneously) so my earth return only needs to be the same thickness as the output to any one of the chargers, and not three times.

Consider moving the charger to somewhere with a shorter cable run.

Take meter readings at each end of your cable run to see what the voltage drop is.

My charger had a noisy fan, so I replaced it with an 'Acoustifan' at £17 from Maplin. Virtually silent. The motor is housed in a sealed unit so no dust nor moisture can get in. It comes with a standard red, black and white lead for a PC mother board. I ignored the white lead since this is only a return to the PC's motherboard to let it know the fan is running. Silent night. Well, apart from the ropes, sails, water, wildlife...