Today' Project

[greg g]

I am going to run an alternative wire to power my brake lights. I currently have 165 watts of halogen lights going when I step on the brake. If I need to keep my foot on the brake pedal for an extended period, say at a traffic light with a grade at the intersection, That load is cooking the 30 AMP fuse on the light switch.

The idea is to power the light switch from a new wire with an iline fuse directly from the battery (hot post on the solenoid).

I want to be in complience with Ohms law so I will be running either 12 gaoug or 10 gauge wire and either an inline 30 amp or a 30amp resetting circuit breaker. But since I am not a math major, and have forgotten the tenants of OHMS Laws, I want to make sure that is going to be sufficiet for the load.

Any Ohms lawyers out there who can run the numbers, I appreciate it.

Each light in the three that light up for the brake lamps puts out 55 Watts.

[1950 Special Deluxe]

Watts is equal to Volts x amps, so amps is equal to Watts divided by volts.

165 watts divided by 6 volts equals 27.5 amps, not factoring any losses for the resistance of the wire. I would suggest #10 wire.

If you are using 12 volts it would be 13.75 amps and #12 would be fine.

[Plymouthy Adams]

55 watts per lamp equals a lot of candle power...are you also using these elements for the turn signals? I would suggest using relays when and where possible to keep this current off electrical componets other than the lamp itself

Edited May 25, 2010 by Tim Adams

[greg g]

No the signals are fired individually through normal 6 V bulbs. the running lights are 35 watt though, so ther are 70 watts before the brake light come on. MAybe I will change the center one back to to incandescent.

[Don Coatney]

I am not a Barrister but as has been mentioned volts times amps equals watts. My question is are you going to run the new wire through the headliner? I need to do a re-wire job on my car possibly this coming fall or winter. I am considering an alternative path for the new wire other than through the headliner.

[DutchEdwin]

Greg, I run 1.5 square on indicators and 2.5 square on head lights (= AWG12 and AWG10 on all the lighting wires, but have to check for sure tomorrow to make the conversion from EU standards to AWG). This includes the ground wires I have running from the front to the rear of the car to every electric consumer. This illuminates the rusty joints.

For safety I have an old fuse box from an early 1980's car that has the plastic cigar like fuses. Up to 25 amps.

In the lighting circuit (high beam, tail and brake lights, interior light and dash light combined) I measured 28 amps, without the engine running.

Measuring the voltage, I measured 6.4V at the battery and 5.1V at the fuse box. I placed the fuse box about 6 feet from the battery with 4 connection point between the battery post and the fuse box. So there are some losses

For the headlights I use halogen bulbs. Brake light is standard 21/3 Watt bulb.

For the formulas: Volts=Amps*Ohms. Watt=Volts*Amps

For the headlights I measured 17 amps. It is advised to use a fuse 20% to 50% higher then the max amps going through the fuse.

I made that mistake by letting 26 amps through a 25 amps fuse, didn't know that until I measured it. The fuse stayed alive for 10 minutes, almost starting a fire, as a result the hole in the cap of the fuse box. I'm going to measure the amps with running engine this weekend but I do not expect other readings.

The amps I measured for the battery post with everything on light radio etc. reads 55 amps.

[greg g]

My wires are still all in good shape. New wires ware run for the rear signals, they run under the floormate next to the sill plates.

Nice work Edwin. Mine was not blowing the fuse on the headlamp switch but rather similar to yours over heating it and melting the solder inside. The fues look fine but will not pass current. So I guess they worked as designed, but since there wasn't a dead short to ground they melted out rather than blew out.

I think removing them from the headlamp circuit, and fusing them at 30 amps also will do the trick. Plus the headlamps probably won't brown out when the brakes go on. If that doesn't do it I will go to a circuit breaker and relay. I like the idea of the self reseting relay as after it cools down, you still have lights rather than having to dig around and replace the fuse.

Needles to say between two doctors appts and temps in the high 80's, this has now become tomorrow's project after another appointment.

I'll grab wire fitments, ad fuse holder on the way home.

[Plymouthy Adams]

Don..the only wires I am running through the headliner is the actual overhead lamp wires and the power to the insdie rear view mirror..I have routed my new harness with the upgrades to run along the chassis and pass through watertight rubber grommets out the cowl and into the rear of the vehicle. The inside rear harness I am running through the rocker panel itself as there is a open path from the kickpanel to the rear quarter trim panel. I would have run them along the threshold except these are flat as a fritter and the rear cubby pockets are metal and set on the floor..would be hard to route the wires through all that.

[Captain Neon]

Greg, I went the opposite route for brightness and installed LED bulbs which have less of a power draw but brighter than incandescent.

[greg g]

Call me, I will provide an escort. seriously if and when you're gonna be in town, give me a shout.

[DutchEdwin]

I promised to deliver some math.

You can calculate the losses in the wire you installed. Be aware, this does not include the losses at all connectors you have in between but will give you a good start.

Sorry if this is in metric, but the methode is the same in inches.

Copper has a specific resistance, Rho=17xExp-9 Ohm-meter.

resistance R=Rho* (length/ cross section of the wire)

The wire I used for the head light is 2.5mm2 (is about AWG 12).

Make the equation for a wire length of 4.5 meters (= 13.6 foot)

R=17xExp-9*(4.5/0.0000025)=0.031 Ohm resistance in the wire.

To calculate the loss in Voltage: V=ampsxR

In my case I measured 17 amps for the headlights high beam.

V=17*0.031=0.53V loss. I measured 1.3Vols of loss. This means 0.8Volts on all connection points. There are 4 = 0.2V per connection point.

Do the math with AWG10 (= 5.2mm2), the result is 0.015 ohm resistance, result in 0.3V loss in the wire. AWG10 can give you problems in the connection to the lamp socket. So if you will have to make more connection points, this will lead to more loss in voltage then running a little bit smaller wire that can connect directly to the lamp socket.

Hope this helps on the decision

[greg g]

Thanks Edwin. In checking the situation I will actually be shortening the overall wire lenght by about 2 feet.

[Don Coatney]

Old coat shrunk. (

My A$$

[Captain Neon]

I hear ya about the deer. My wife hit a deer with our '92 LeBaron convertible early Monday morning. She didn't get a scratch, but the car is a total loss. I am thankful that I had the foresight to have an airbag switch installed in the LeBaron as well as her Acclaim. If not killed, my 5' 1" wife likely would have been seriously injured had the airbag been deployed.

I think the time has come that for at least one year, it be declared open season on white tail deer and declare them equivalent to feral hogs. IMHO, professional deer harvests w/ legal retail sale of wild venison should have been implemented 20 years ago. I stopped counting how many close calls I have had my self with deer on highways. Almost hit a 10-pointer on Kentucky's I-57 last fall. They are a menace to publick safety!

[Captain Neon]

My A$$

I don't think it was just that.