Cable size/rating confusion

[smashweld]

This cable is rated 2*16.5 amps. 32*0.2mm strands....1mm(sq)area

http://www.12voltplanet.co.uk/2-core-thin-wall-auto-cable-2-x-16-5-amp.html

The cable I have here, also flat twin is rated 2*8.75amps ...14*0.3mm strands, also 1mm(sq) area


Why is one rated almost double the other?

Half thinking there's more gaps between the strands in the 14/0.3 stuff but then that doesn't seem right, area is area..?

 

[angellonewolf]

@brightspark might be able to help

poss insulation is not as high grade so will not carry current without braking down ?

 

[smashweld]

I was thinking that's a possibility but its such a big difference.
The stuff on 12v planet is thin walled but not sure what this is....seems flexible so maybe not thin walled but I dunno..

 

[ratherbewindsurfing]

The simple answer as to why the nominal current ratings are different is that they are different spec cables.

Thin walled auto cable does have a high current rating as the max conductor temperature of 105 Deg C, the other cable may have a max conductor temp as low as 65 Deg C. We also don,t know what assumptions have been made about how the cables are installed in arriving at the nominal ratings.

A couple of things make me think the thin walled rating is optimistic -
1. 16.5 Amp is a lot of current for 1mm2, typically a lighting circuit in a house would have a 5A MCB on a lighting circuit using 1mm2 T&E
2. They quote the same rating for single core as twin, that can't be correct.

Hope this helps
Martin

 

[tlovatt]

You can also factor in the greater number of conductors, electricity only really flows along the surface of the copper, not through the centre and so a greater number of smaller strands can have a higher current rating, plus as others have siaod the different temperature classes of the cable, plus different manufacturers will claim different ratings.

 

[awemawson]

You can also factor in the greater number of conductors, electricity only really flows along the surface of the copper, not through the centre and so a greater number of smaller strands can have a higher current rating, plus as others have siaod the different temperature classes of the cable, plus different manufacturers will claim different ratings.

Surface conduction is only true at elevated frequencies. At 50 Hz current will flow in the bulk of the material. You need to get up the HF for the skin effect to kick in.

 

[smashweld]

so...2 cables with the same area of conductor can have a different current rating

would the voltage drop be the same on those 2 cables?

 

[angellonewolf]

2 cables with the same area of conductor can have a different current rating

would the voltage drop be the same on those 2 cables?

would depend on the ristance of the cable

 

[dannyp]

resitance of cabel is probaly same, rateing diffrence it probaly due to the melting point of the insulation

 

[smashweld]

Guess i'll just err on the side of caution with ratings!

On a related note....

Is there any disadvantage to running an earth/negative wire from each light right to the earth point vs connecting them in series like you'd usually find on clusters etc?

 

[angellonewolf]

if run in series the load on the first cable increases the more load you put on it so 4 lights eg 11 watt lamp will produce

11/240 ish .046 amp run 4 lights the first cable will 1.5 amp the 2nd 1.1 the third so on so on

but using haligen lamps are 50 watt

50/240 2 amp each so 8amp in the first cable

if using lamp holders that will take the highter wattage lamps eg you can get led lamps that dont have a replacable lamp bulb you just replace the lamp and holder all in one if you use the holders that will take the haligen bulbs you must factor in it can be replaced with such and use wire to take the higher amps

this is house wiring car car as @pdg has put but the idea is the same

 

[smashweld]

Cheers both.

Yupp its DC 12v....I'll just run a separate wire from each light to the earth point...it actually makes it easier in this case....was just having a bit of a 'why is it always done the other way' moment. But its looks like it's just to save time/money.

Cheers

 

[dannyp]

you have to be carefull in some situations with running neatives or returns eg in a car, if you run a negative wire back frm say a light to the battery you must ensure that the negative is isolated from the body at the light end, ad a friend rebuild his s3 landrover and ran negatives back from all the lights worked great till a bad connection beveloped in hs starter motor ground strap

 

[smashweld]

What happened? The starter try to use the light wires as an earth and fried them somehow?

The lights I'm fitting are in a GRP/alu sided Luton box van...my plan was to have the earth point in the cab roof space (top of door frame area) to keep it clean and dry...cable from there to the back (all the flasher/brake etc feeds come from inside the cab along the roof edges anyway) then connect all the lights at the back....had thought it might be an idea to have them all connected to part of the steel floor/chassis at the back too but that sounds like exactly what you're warning of?