I'm adding two 100-watt driving lights to my 2011 CC SRW F350, and intend to run them off the first upfitter switch. As I understand it, these have relays, and the first two switches are rated to run 25A each.

By my calculations, two lights of 100W each (wired together to a single feed), divided by 12 volts, should work out to around 16.6 amps, so are theoretically well within the rated capacity for upfitter switch #1. However this amperage calls for 10- or 12-guage wiring, and when I looked at the upfitter pigtails under the dash they look more like 14- or 16-gauge wire.

Can I safely just connect my 10-gauge wire to the end of the pigtail, or do I need to run it right to the relay?

Secondary question: Has someone found an easy way to get to the high-beam wire, without having to dismantle the entire front end? Thinking of installing a secondary relay, linked to high beams.

Thanks for any advice/assistance!

 

Personally, I would use a relay to power the lights and one of the switches to control the relay. Be safe not worried.

 

I agree with Larry. I think the upfitter wiring is smaller than I'd want to use for a big draw, thus out of the 3 upfitters I've used so far two just control another relay. Those relays are fused off the batteries and power my lightbar and airbag compressor. The only one wired direct is for high idle.


I have the lightbar tied into the fog light circuit with another relay so it will come on with high beams. Decided not to mess with the highbeam wire as they are pulse width modulated by a transistor. Fogs are powered by a relay in the BCM. I made the connection in the cab in the r/s kickpanel.

 

You can use separate relays if you feel better about it....
However, the wire gauges of Upfitters are perfectly fine for the currents that they are spec'd for.

Automotive wiring is covered with a high temperature insulation (I need to go look it up, but I think it's rated for at least 150 Degrees C.). So, the current carrying capacity (Ampacity) of automotive wiring is actually higher than household wire. (Temp rated at ~75 Degrees C.) So, automotive wire can safely carry more current in a smaller wire diameter than household wire.

Ford fuses the Upfitter switches to protect the wire sizes & lengths that it has provided. They also calculate the maximum temperatures the wires see when they're are in a tight bundle So, the Ford wiring is fine for it's particular Upfitter application current rating.

The only concern is the wire that you tie on downstream of the Upfitter switch:
If you have a high current circuit, you need to understand the voltage drop that occurs over the length of the added wire going to your load. If you've got a long wire run, the resistance of your wire starts to add up over distance. (If the 100 Watt lights are on the back of your truck, then you could easily have a 20 foot long wire run.)

Example: (Assuming 12 Volt DC operation)
10 Gauge Wire: If you use 10 AWG wire to your 16.6 Amp load.
Its Resistivity is 1.2 milliohms per foot.
So, for a 20 foot wire run = 0.0012 Ohms x 20 feet = 24 milliohms.

The voltage drop: 16.6 Amps * 0.024 Milliohms = 0.39 Volts
So your lights see: 12 V - 0.39 V = 11.61 Volts => BRIGHT LIGHTS

16 Gauge Wire: If you use 16 AWG wire for your 16.6 Amp load.
Its Resistivity is higher => 4.5 milliohms per foot.
A 20 foot wire run: 0.0045 Ohms * 20 feet = 90 milliohms.

The voltage drop: 16.6 Amps * 0.090 Ohms = 1.5 Volts
So your lights see 12 V - 1.5V = 10.5 Volts => DIMMER LIGHTS

So I my opinion, you don't need an Upfitter switch to run another relay. The circuit is perfectly fine and has been designed to run the rated Load directly. Only time you'd need a separate relay circuit, is if you have a higher current draw than the upfitter circuit rating.

 

Many thanks for the responses, gentlemen! I think I'll go with 10-gauge from the upfitter pigtail to the lights, which is probably 7-9ft, so voltage drop should be negligible.

Thanks again for your assistance!

 

Wow - they PWM the High Beams? Where'd you find that out?

Also WHY?
Pulse-Width-Modulation is usually used for Dimming - especially for LED lighting.


Reason I asked is that I tied a relay into one of the High Beam wires to turn on my driving lights. It works fine - but now you've piqued my curiosity/

 

Both high and low beam lights/circuitas are fed from the BCM and use a FET (field effect transistor) to regulate them.

 

It is 125 degrees C.

 

I have a pair of 100w hellas on my brush guard. I used a #12 from each one to the firewall where I spliced to the pass through wires and tied it right to upfitter #1. No need for extra relays and such.

 

Trying to remember my electronic engineering training, but I believe if you wire those lights in parallel, the current will be double what you expect because the resistance will be half. (and the lights will not burn full brightness because they are current starved thru your wiring) Ohm's Law.

 

Apologies if I'm misunderstanding this, but if I run a single live to the "+" on both lights, and the "-" on both to ground, then they are in parallel. Resistance in this case is (1/R = 1/r + 1/r), so if r = (say) 2 ohms, then R=1 ohm.
The alternative would be to run them in series, in which case the live would run to "+" on light 1, then the "-" on light 1 would connect to the "+" on light 2, and the "-" on light 2 to ground. in this case resistance would be (R=r + r), so with r=2 ohms, R = 4 ohms.
Higher resistance would mean less current, so less light. Or am I missing something?

 

Incandescent lamps act differently than regular resistors - when they first turn on, the impedance of the filament is low. As it heats up, the impedance increases by a factor of ten or more. The result on turn on is a big in-rush current for about 10 to 20 milliseconds or so until the filament heats up. It may draw 10 times (or more) of its the steady state current. Once the filament reaches its operating temperature, the impedance rises and stabilizes, then the lamp runs at its rated power.

You actually want to wire up a pair of lights in parallel. That way they both see the same voltage. If you wire them in series, they will still work, but there will be voltage drop from the first light to the second light. If you put them side by side, the 2nd light will be a bit dimmer than the first. You won't see it with a 1/8 Watt Christmas tree light running at 110V, but you wiill with a 100 W lamp at 12 VDC.

The circuit does see a lower overall load impedance with two lights in parallel versus one light in the path...but as long as the circuit can source the required current of two lights without dropping the voltage, you're not going to see a difference in the light output.

 

Yes, parallel results in less resistance, higher current, and brighter lights. You just have to make sure you calculate the resistance of the lights as half of one light (when there are two parallel branches) when calculating your expected current so you don't blow fuses or worse.

Impedance is irrelevant, as this is all DC.

If you wire two lights up that have the same resistance in series they will also see identical voltage drop, it will just be half of the source.

Its all coming back to me now. LOL

 

I agree this is the best way to handle it.

 

Adding complexity and parts that are not needed?


As far as the parallel vs serial, that is way over thinking this