How do I measure electric cable?

[VicS]

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

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I couldn't find how to put the little '2' after mm

[/ QUOTE ] As Lakesailor as already said you copy and paste it from the Character map. Also useful for a number or other symbols including the the degrees symbol, and some fractions.

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it's a Sterling 40amp mains charger

[/ QUOTE ] I'm sure this must already been said but if that is 40 amps total then all you need for a common negative is cable capable of taking 40 amps but if it means 40 amps per output, making 120 amps total, then you will need cable capable of carrying in 120 amps.
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By my calculation 16 sq mm means a radius of 2.25mm (i.e. diam 4.5mm

[/ QUOTE ] Yes that is true for a solid conductor but for flexible cables the nominal area quoted is the total cross sectional area of all the individual strands.

Thus a cable descibed as 50/0.25 means it has a 50 strands each 0.25mm <u>diameter</u> .

Each strand has a cross sectional area of pi x (0.25/2)² = 0.0049 mm² so the nominal cross section of the cable is 50 x 0.0049 = 2.5 mm².

That cable is incidentally rated at 30 amps max.

If you compare those figures with the cable you are talking about then I think you will find that your 16mm cable is in fact more than heavy enough to carry the full return from three 40 amp outlets ie 120 amps total.

I am sure you can find some tables on-line somewhere that will tell you the current rating for different cable sizes but note that flex and solid cables have different ratings and that enclosed cables are rated lower than those not enclosed. Furthermore because volts drop is significant in low voltage installations you will find further recommendations for these applications based on the total length of the cable run (ie there and back) as well as current.

 

[DaveS]

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

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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?



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This seems to be a fairly common misunderstanding, but it does contain a grain of truth. It is known as the skin effect and is frequency dependant. At DC the current distribution through a conductor is almost uniform. With AC the current density is higher towards the outside. As frequency rises the current density distribution becomes ever more biased until at very high frequencies it can be considered to flow almost entirely on the surface. At 50Hz the skin effect can be safely ignored unless very high currents are involved as in, e.g., large scale generation and transmission.

In short, for most practical boaty applications not involving RF, it is the cross sectional area of the conductor that matters, not the length of the perimeter of the cross section.

 

[tugboat]

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

Vic, thanks a lot for taking the trouble to explain all that, I hadn't cottoned on to the difference in cross-section between solid and multistrand so that is a useful lesson learned. It sounds as though 16mm² (hooray, the numpty managed it, thanks chaps!) will do the trick for all supply and return lines. Ooo, my brain hurts! /forums/images/graemlins/grin.gif

 

[2Tizwoz]

Re: Sizing cables

The current carrying capacity of a cable is limited by the temperature which the insulation will withstand. A very thin wire in a light bulb will carry a relatively heavy current but there will be a large temperature rise in the wire (it glows white hot) and there will be a large voltage drop (the operating voltage of the light bulb).

Although a particular size of cable will carry a given current the associated voltage drop which is acceptable varies depending on the voltage available. So that a drop of 2.5volts in a cable operating at 240volts might be acceptable whereas the same voltage drop at 12volts would not be.

As has been suggested above its best to keep your low voltage cables as short as practicable and look into the voltage drop the length of cable run will cause. If the cable is going through a warm area (an airing cupboard for example) it will need to be heavier to limit how hot the insulation gets.

 

[DaveS]

Re: Sizing cables

I don't disagree with any of that (indeed it's a thermal effect that makes the DC current density distribution in a conductor not quite uniform, since the centre runs slightly hotter, with a corespondingly higher resistance, than the outside), and of course voltage drop must be taken into account when choosing cable sizes. I was responding to the specific suggestion that the (DC) current is carried on the conductor's surface rather than through the bulk of the conductive material.