High beam wire

[M.Bas]

It's all in the voltagedrop you allow.
If I have the energy and willpower to explain it after climbing this evening I'll elaborate, but I'm pretty sure others can explain it as well.

[M.Bas]

Looks like I have more time before climbing then anticipated.
I'll try to keep it as simple as possible, please note that this info is extracted from an excel calculation. There is some magic happening behind the curtains in that sheet so not all units might be correct if you actually use this for calculation. But it's good enough to explain how it works and I don't have the time now to check it.

The formula for calculating cable sizes in a DC system is:
cable size = (conductance * 2*cable length * current * temp correction) / voltage drop
Cable size is in mm²
Conductance is the conductance of copper 0.017241 W·mm²/m, the only time this value changes is if you use cables not made of copper.
You need to take into account twice the cable length measured in meter, although the return cable in a car is often the chassis which would be a pretty big conductor
Current speaks for itself (power / voltage)
Temp correction factor is not often used, but for calculations of power cables for a refinery you need to take it into account.
Voltage drop is the voltage drop in volt that you want. The more voltage drop the warmer the cables get, etc etc. You want to keep this as low as possible.

For now lets make a couple of assumptions.
Cable length is 2 meter and we are going to run cables back and forth to a load.
We are going to power a 55W lamp on a 12V circuit, thus a 4.6A current
Temperature correction factor is 1 since I don't want to deal with this now.
That gives the following formula:
Cable size = (0.017241 * 2 * 2 * 4.6) / voltage drop.
Allowing for a 3% voltage drop (0.36V) mentioned in the cable sizing topic elsewhere on this forum that calculates to a 0.9mm² cable.

So as you can see you only need a 1mm² cable to power the 55W lamp.

As a car manufacturer you want to save as much copper as you can, so you want those cable sizes reduced.
Lets start by not using 12V but 13.5V in the calculation. A car battery that gives you 12V unloaded is pretty much dead, and with the engine running the voltage is 13.8ish I believe.
And since we're at it change the voltage drop to 5% (0.68V) because why not, you're still above 12V at the end of the cable.

And suddenly you only need a 0.4mm² cable and your boss will be very pleased since you just saved him a shitload of money.
But since most of the factors are given the only thing you can change to reduce cable sizes is to allow for more voltage drop.

As long as the cables are still within laws, regulations, standards, specifications, etc, manufacturers use these kind of tricks to save money on the cabling in cars.

[NowForThe5th]

Thanks M.Bas. Good explanation.

I might add that when it comes to lights there is even more "fudge factor" provided since the outputs for any given bulb (in lumens) are quite wide between maximum and minimum while the actual required intensity levels at any given point within a headlight's beam pattern are fairly low and, crazily enough, are specified as maximums, rather than minimums.

Your engineer specifying cable sizes therefore has even more freedom to move with specifications and still have a light that meets the minimum requirements. So, as a manufacturer, what you do is specify a good quality bulb because they're not much dearer than a poor quality bulb and the extra cost is less than the warranty cost for replacement of poor bulbs, while a better quality bulb will maximise the lumen output from your minimum specified wiring. You also require your headlight subcontractor to produce a light that has very high quality reflectors that will get your now much reduced output into the specification level needed to actually comply.

[Interdec]

Just a quick update, I tapped into the high beam wire by opening the plug that connects on to the back of the bulb. There are three small lugs that you can open to open the plug housing. There is plenty of room inside to join onto the blue/yellow wire and there is even enough room in the plug housing to accommodate the additional wire out the back. Neat and no cutting required. Worked a treat for me.

This also worked a treat for me...recommend this...just solder carefully and quickly so as not to melt the plug or the insulation on the existing wire...clean the top of the connector with sandpaper so that the flux/solder will stick immediately. Made a nice neat job since the wire is then neatly clipped to the existing headlight wires and loom.

[Interdec]

Guys, thanks for the calculations on the cable size...appreciate that. I recognise it's all about cable length...I have just been thinking about a 5A cable you can buy from Supercrap Autos, and it's a lot thicker than those wires going into the headlight plugs.

Cheers.

Jim

[Twitch94]

I just bought some angel eyes from prestige offroad, and it is all plug and play, but when i turn my high beam on the low beam turns off,

Is there a way to tap into the high beam and make the low beam turn on when i got the high beam on?

Thanks heaps by the way

[Cowboy Dave]

I suspect that might not be legal? Not sure but even stock the low beam drops out when the high beam goes on.

[NowForThe5th]

I just bought some angel eyes from prestige offroad, and it is all plug and play, but when i turn my high beam on the low beam turns off,

Is there a way to tap into the high beam and make the low beam turn on when i got the high beam on?

Thanks heaps by the way

Yes, this is the way that the standard H4 bulb works. You can change it with a little elec-trickery or just buy a harness that includes a control unit that will do it for you. The harness is the same that is used on H4 HID conversions. Needs addition of a relay and change of plug fittings at the light.

[Twitch94]

I just bought some angel eyes from prestige offroad, and it is all plug and play, but when i turn my high beam on the low beam turns off,

Is there a way to tap into the high beam and make the low beam turn on when i got the high beam on?

Thanks heaps by the way

Yes, this is the way that the standard H4 bulb works. You can change it with a little elec-trickery or just buy a harness that includes a control unit that will do it for you. The harness is the same that is used on H4 HID conversions. Needs addition of a relay and change of plug fittings at the light.

Oh wow, thanks for the reply, msybe not worth it then, was hopeing for an easy fix haha like splice two wires or something

Im getting a LED bar soon anyways so not too much point anyways haha
Cheers bro and cowboy dave

[Moogun]

Wired up Cree lightbar today including connecting to high beam on headlight. Terrific outcome. Thanks to everyone for comments above as made it a simple process.

[Tomotriton]

Found an easy wire under the dash but don't know how to post a picture

[tony73]

Hi all,

Awesome info here, I've just got one that hasn't been answered,

Wiring loom that was included with my light bar has two power cables to battery, one off cabin switch and one off relay! Their diagram doesn't show which one to splice into high beam just shows both to battery! A mate told me to splice the switch power wire into high beam

do people here concur?

Cheers

[Naff]

98% sure that would be correct.

[Cowboy Dave]

It sounds a bit like your diagram isn't contemplating the need for high beams to be on first. Usually the relay would have two power sources. One from the battery is the one that actually powers the light, and one from the switch which is what triggers the relay to supply the other power feed to the lights.

It sounds like you've got it sussed but if you're worried about it scan or photograph the diagram and post it up here.

[tony73]

All good used the switch lead to high beam works a treat!

Just burnt myself 3 times with the soldering iron, not bad for a fat fingered firefighter

[markhood01]

All good used the switch lead to high beam works a treat!

Just burnt myself 3 times with the soldering iron, not bad for a fat fingered firefighter

Hey Tony, did your cabin switch have a negative wire as well? Did you take that back to the battery? Cheers

[NowForThe5th]

Hey guys whats better to wire hid spotlights to the high beam or to have them on their own switch in the truck?

Driving lights must be wired so that:
1. They only come on with the high beam; and,
2. They must have a separate switch so that they can be turned off.

As noted above, "scotch locks" are literally hit or miss. If you get it wrong you can miss the very thin wire inside the insulation or you can cut it or break enough strands to cause a hot spot with resultant fire risk. Solder is better although this has it's own problems, creating a stress point at the end of the solder. Best is to solder the tips and then slide a crimp up the line, followed by heat shrink (the waterproofing one is best) and then some self amalgamating tape to seal everything over the top.

Scotch locks often work loose with vibration, don't use them. An elec mate used them on my headlights and they all gradually worked loose and I lost lighting, not good on a dark and stormy night...

[pappas001]

Is it legal on nsw roads to wire up a led light bar to a seperate switch without tapping into the high beams?

[GadgetMN]

Is it legal on nsw roads to wire up a led light bar to a seperate switch without tapping into the high beams?

The ACT regulations are that "All driving lamps must be wired into the main beam (high beam) circuit"... I imagine most of the states are pretty similar in that respect.

Check out these... Related to the new LED Light bars and changes to legislation to work with them, viewtopic.php?f=85&t=18720&p=489019&hilit=led+light#p489019