wiring

[david nelson]

A  few questions.

 

can anyone direct me to a good guild on car wiring.

I need to wire up my radio (not in the seven) and need to work out the wire rating.

I understand its Watts/volts= amps and this should give you the wire that is needed. seems simple but then lets but a fuse into the wire.

how do you work out that fuse amp is needed?

On my Land Rover Defender 1997 the radio has a max current draw of 15 amps. the fuse is 20 amps. but the wire need extending as previous owner cut it short. The wire look as think as 11 amps, but would this be too low?

 

thanks

 

David

 

[Jonathan Kay]

1. I think it's usually recommended to use wire that has a rating of at least 1.5 x the expected current, but I'm cautious and would use 2x. Have you got current ratings or only gauges or diameter for the wire you're considering? Charles posted a link to a table which has the equivalences in this thread... oh.
2. The fuse should be the lowest that doesn't blow, and always less than the wire rating.
3. Don't know about books or web sites. There's a good introduction in Hillier, which is essential anyway. (There's a Volume 2 on electronics, but I've never looked at that.) Dr Slotter also recommended a book on electrics for racing cars.

Jonathan

[david nelson]

Jonathan

 

thanks. The current permanent live is on a 20amp fuse and also has column switch, trailer pick up, interior lamp unit, clock.

The permanent live to the radio is thin about 11amps by using a gauge. I have looked and can not find the rating for this wire.

 

will have a look at the book

 

thanks again

 

David

[Jonathan Kay]

15A sounds like a lot for a radio: is it very powerful? I'd check that. Jonathan

[david nelson]

Jonathan

 

have tried to copy and past, and page shot but unable to load spec on to forum it states current drain 15 amp max.

looking at the wiring diagram the permanent feed has a 15 amp fuse.

 

 

David

[phil 01]

Try a book by John Dickens, this will really help

Phil

[Jonathan Kay]

Phil, is that this? Thanks. Jonathan

[phil 01]

Hello Jonathan yes that's the book I bought my copy from stoneleigh, I think it was about £6.00 3 years ago but Soft cover A4 size. I'm a spark but 12v car electrics throw different fault situations than 230v and 400v. The reason I bought it was to pick small tricks you learn over time though I does seem quite simplistic to me

Phil

[Jonathan Kay]

Thanks, Phil. Now on my present list.

Jonathan

[Paul Deslandes]

There are two load issues relating to the choice of wire size,

1) current carrying capability i.e. so it doesn't get too hot and melt ( I^2 x R)

and

2) the voltage drop due to the resistance and the current being carried (Ohms law says V= I x R).  If your radio/audio system has a high powered amplifier, the amplifier will draw a high current when you have a sustained loud sound at high volume.  Short transient peaks are normally catered for by large capacitors storing and delivering their charge to cope with the demand spike.

So, even though the average current draw is only a few amps and the wire won't over heat, your audio will be attenuated or distorted if the wire is too thin.

Paul

[Jonathan Kay]

Paul: Do you have a rule of thumb for the factor of wire rating over nominal current?

Thanks

Jonathan

[Paul Deslandes]

Hi Jonathan

I normally rely on the manufacturer's/supplier's specified rating plus about 50% unless voltage drop is critical.  The IEE domestic wiring regs allow 5% voltage drop at max rated current and you're unlikely to see more than that in a car where run lengths are comparatively short.  Cable ratings in general are usually qualified by their environment, i.e. whether they're buried, bundled, in free air etc, detirmining the amount of cooling available.  A 5% voltage drop will result in a power reduction of 11% at the load.

For example, VWP sell Thinwall cable, 11Amp rated 16 x 0.20mm = 0.5mm^2 which has a resistance of ~18mOhm/m.  A 3 metre length (typical maximum length in a car) would drop 0.6V at its rated current, which is 5% of the nominal 12V supply, assuming the return is through the chassis with negligable resistance.  Under the same conditions the wire would be dissipating 6.5Watts which, in free air, would hardly be noticed but, bundled in a cable with other similarly loaded wires, could get quite warm.
 

Paul

[Jonathan Kay]

Thanks

Jonathan

[Bob_Rich]

Hi  Folks

Generally agree with Pauls reasoning but 16/0.2mm  wire is around 35mohm/metre at 20C and at 70C rises 20% and at 100C rises around 40% above  the 20C values.

11A for a PVC 16/0.2 is a bit high. Some "thin" wires are advertised with high current ratings  but be careful.  They may use high temperature insulation.  Apropriate  rules for grouping and ambient temperature will apply. But be careful if such wires are used grouped in with more traditional insulations that cannot run at high temperatures.  Generally in  12V systems a starting point is for a 3% volts drop unless tempertaure rise becomse significant.

For a standard Radio/Audio system  unless a real "blaster"  30/0.2mm  should be fine and a conventional  PVC wire  routed away from hot spots  with a 5A fuse should be fine.  Sometimes speaker wire needs a lower resistance so thicker conductors  for audio peformnace issues and as such current ratings may  not be of great importance.

 

Hpe this helps  

best of luck

Bob

[Paul Deslandes]

quite right Bob - cock up on the table reading front.  1mm^2 is 18mOhm/metre.