Wire size

quattro

quattro

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I am about to rewire the headlights on Sparky.

Currently (no pun intended :roll: ) he is fitted with 55W halogens which don't always work. I have checked the voltage at the headlights and found it to be between 11.6V and 12.2V, depending on where I test it. I can only assume that there is a voltage drop at some or all of the connections along the way, and that the halogens need a full 12V or more to work.

wiringdiagram.jpg


Now, there are two Brown/Wires coming directly from the shunt to the light switch and the headlamp flash. I am intending to connect the one to the light switch to a heavy duty connector and run wires out to 1/. the switch, 2/. 2 relays behind the O/S light and 3/. 2 relays behind the N/S lights. These (2 & 3) will supply the main power to the relays for all of the lights.

I will then fire the relays with the original wiring, i.e.

Relay 1 will be switched on with fuse 11/12 which originally turned on the inner main beams. It will run inner and outer main beams on the O/S.

Relay 2 will be switched on with fuse 15/16 which originally turned on the O/S dip beam. It will run O/S Dip Beam.

Relay 3 will be switched on with fuse 13/14 which originally turned on the outer main beams. It will run inner and outer main beams on the N/S.

Relay 4 will be switched on with fuse 17/18 which originally turned on the N/S dip beam. It will run N/S Dip Beam.

This way, the main power goes straight to the relay and into the headlights without having to go to the light switch, headlamp flash, dip switch, fuse box or the multitude of connectors on the way. The fuse box no longer gets overheating issues as it is only running the relays now. The relays have their own fuses.

I have bought some 2mm auto cable which will handle 25A or 300W at 12V, but now it has arrived, it looks smaller than the original wires! The original wires are only handing one main beam but this will have to take two of them.

Now, am I missing something with this wire? should I have 3mm or 4mm, or is the 2mm ok?

I don't want to have overheating issues with my cure for the overheating issues :shock:

Richard
 
Richard

I guess the first question is did you get the wire from a reputable source? With the price of copper being so high there is a lot of sub-standard cable out there. If you're happy that the supplier is ok then 2mm cable is good for around 17A. For 25A I'd go with 3mm cable. There's a good guide here.

Always better to er on the thicker side with cable just in case, but it's always a balance between voltage drop, cable length and the ultimate load. If the original wiring looks thicker then it probably is but the only way to tell is to count the number of conductors and compare old with new. Also, when the engine is running, which it generally will be when you have the lights on, the current flowing in the wiring will be lower due to the increased voltage from the alternator - ie 13.5V vs 12.5V.

Dave
 
Cheers Dave

That's a handy article.

While surfing around I was getting conflicting information regarding the ampage capacity of the cable. Some were saying 17A, and others 25A. No one mentioned whether it was maximum or continuous and my school days are a dim and distant past.

This is the cable - http://www.ebay.co.uk/itm/190616750475? ... 681wt_1336. Now I can see that I could run a 55 and 75 from it with an ampage of 9.8 to 10.8 (13.2V to 12V), so it will handle it easily.

Richard
 
The fact it says nominal means their figures are non binding... not much help. Thicker wire has lower resistance per metre length if made of the same material but if their figures and guidelines are correct then it should be fine as you say!

Rich.
 
I bought some automotive flex and it was advertised as being thin but still capable of carrying the same current .
Is it made of some super-conductor ?
Or are things just made to the minimum size these days as you don't see wiring the same diameter on old and modern cars yet the load of a headlamp bulb (etc) must be the same
 
I reckon you wil be OK with the 2mm sq wire Richard.

Assuming the wire has a typical restistance of .009 Ohms/m for 14AWG (2mm sq). Even if you used it to deliver the maximum 300W the ad states, this would give you a current of 300/12 = 25A. Even if you used as much as a full 3m length this works out a voltage drop of something like 25A x 3m x .009Ohms = 0.675Volts.

If I understand what you are doing correctly, assuming each relay has it's own feed, the greatest load any of the wires will have is 55W + 75W = 130W. This works out a voltage drop on 3m length of only 0.3Volts. Also, a 3m length should only dissipate about 1W of heat along it's entrire length for a 130W load which is no big deal.

For safety, you could lower the ratings of the dipped/full beam headlights. These will only be supplying a low current to the relay now. The relays might have a current rating stamped on them that woud tell you. You might even be able to take them down to 1A or less?
 
DaveHerns said:
I bought some automotive flex and it was advertised as being thin but still capable of carrying the same current .
Is it made of some super-conductor ?
Or are things just made to the minimum size these days as you don't see wiring the same diameter on old and modern cars yet the load of a headlamp bulb (etc) must be the same
Click to expand...

Hi, ISTR reading something a number of years ago about a change in the manufacturing
process that did just that i,e produce a type of copper that could handle higher currents
without overheating. Which is why electric motors nowadays are smaller for any given
task. Plus wire sold nowadays has a thinner insulation.

Colin
 
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