Wiring length/gage size help needed

Thread Starter

candymancan

Joined Mar 29, 2021
2
So, i realize this isnt a car forum, but im telling ya a lot of car people know nothing about wiring, ive posted on the specific forums for my vehicle, and others and i cant get any answers as it seems i know alot more than they do and you know how people dont reply to threads they dont know anything about.

Anyway, i have an older Jeep Grand Wagoneer, i wired up 100w Halogen lights to the roof about 8 months ago, I only wanted 4 lights and no more. The lights were wired into pairs for 2 switchs. I ran 12AWG wire 8 feet directly from the battery to a relay and fuse up to the headliner, and from there i split each 12GA wire twice into 16ga wires, ran those through the roof and to each light. So 2x 12ga split 2 times is 4 lights. The 16ga wire only runs 1-3 feet, that can handle well over 100w. The 12ga wire at 8 feet was plenty for 200w.

The issue is this, i installed a new roof rack, and the rack had a wind deflector on it, which i had to cut holes in for the mounting hardware for the lights. I dont know what i was thinking when doing this, but i think it was my GF destracting me when i was working.. I cut three holes in the deflector.. when there should have been only 2, The deflector was in the way of 2 lights the other 2 would have been mounted to the sides of it. Anyway, with 3 holes, i now have 2 lights on the left and two on the right, and a empty spot in the middle..

To me it looks odd.. But i guess its ok, but if i were to add a 5th light, this means the wiring i did would have to accommodate 3 lights on 1 of the 12ga wires. Basically id split a 3rd 16ga wire from one of the 12ga wires and run it the 3rd light. The issue is 12GA wiring is ment for 20amps roughly, it changes for length of wire of course, but 12ga at 7-10 feet is good up to 240w/20amps at least to stay in the 2% power loss margin. The other thing is automative voltage isnt straight 12v, its more like 12.6v for a fully charged battery, and with the alternator its 13.8 to 14.6 depending on the load and charging at the time. So 100 watt lights divded by say 14v is around 7 amps, and 7x3 is 21. But wiring should always go to the next amperage rating at 25amps, And technically the wire is rated for 20A. But this is all relative if i stay in the 2% power loss margin..

Im blabbering now, but what do you guys think ? Would the 12GA wire be fine, or MUST i really re-wire 1 of the wires to 10GA to accommodate the 3rd light better. Doing that would be a pain as i have to remove the headliner, and unwravvle 8 feet of 12ga wires in electrical tape and snake them through tight areas and yada yada
 

Yaakov

Joined Jan 27, 2019
3,464
So, i realize this isnt a car forum, but im telling ya a lot of car people know nothing about wiring, ive posted on the specific forums for my vehicle, and others and i cant get any answers as it seems i know alot more than they do and you know how people dont reply to threads they dont know anything about.

Anyway, i have an older Jeep Grand Wagoneer, i wired up 100w Halogen lights to the roof about 8 months ago, I only wanted 4 lights and no more. The lights were wired into pairs for 2 switchs. I ran 12AWG wire 8 feet directly from the battery to a relay and fuse up to the headliner, and from there i split each 12GA wire twice into 16ga wires, ran those through the roof and to each light. So 2x 12ga split 2 times is 4 lights. The 16ga wire only runs 1-3 feet, that can handle well over 100w. The 12ga wire at 8 feet was plenty for 200w.

The issue is this, i installed a new roof rack, and the rack had a wind deflector on it, which i had to cut holes in for the mounting hardware for the lights. I dont know what i was thinking when doing this, but i think it was my GF destracting me when i was working.. I cut three holes in the deflector.. when there should have been only 2, The deflector was in the way of 2 lights the other 2 would have been mounted to the sides of it. Anyway, with 3 holes, i now have 2 lights on the left and two on the right, and a empty spot in the middle..

To me it looks odd.. But i guess its ok, but if i were to add a 5th light, this means the wiring i did would have to accommodate 3 lights on 1 of the 12ga wires. Basically id split a 3rd 16ga wire from one of the 12ga wires and run it the 3rd light. The issue is 12GA wiring is ment for 20amps roughly, it changes for length of wire of course, but 12ga at 7-10 feet is good up to 240w/20amps at least to stay in the 2% power loss margin. The other thing is automative voltage isnt straight 12v, its more like 12.6v for a fully charged battery, and with the alternator its 13.8 to 14.6 depending on the load and charging at the time. So 100 watt lights divded by say 14v is around 7 amps, and 7x3 is 21. But wiring should always go to the next amperage rating at 25amps, And technically the wire is rated for 20A. But this is all relative if i stay in the 2% power loss margin..

Im blabbering now, but what do you guys think ? Would the 12GA wire be fine, or MUST i really re-wire 1 of the wires to 10GA to accommodate the 3rd light better. Doing that would be a pain as i have to remove the headliner, and unwravvle 8 feet of 12ga wires in electrical tape and snake them through tight areas and yada yada
The size of the wire is only one factor in ampacity. You have to derate for temperature, insulation temperature rating, and bundling as well. Given that a 12GA 200°C rated PTFE insulated cable starts with a rating of 25A, with the various derating requirements of your practical installation, I would say that even 20A could be pushing it.
 

Thread Starter

candymancan

Joined Mar 29, 2021
2
Its silicone insulated wiring i used. Its actually better insulated than normal wiring.

The lights draw 8.3amps at 12v 100w. Thats 16.6A. So the 12ga wire was plenty for the two lights. I run them on a 20amp fuse and standard 30/40 relay.

Anyway i solved my own problem. I was planning on running 4 side led light pods on the roof ( 1 per corner) so light ul the sides of ny vehicle for camping or what not. And those LEDS claim 60w. But knowing the chinese they are probly alot less.

Regardless i was planning on running the wiring for them to the back of the Jeep and come out the washer nozzle hole through the last pillar so save ne from drilling the roof. I was planning on using a 10ga wire for this due to the length.


Instead what im going to do is run the 10ga wire up where the two 12ga wires are. And disconnect the two 100w lights from the 12ga wire. And use that 12ga wire for the led pods. And use the new 10ga wire for the 3 100w lights. When i add the 3rd.

The 12ga wire up there should be plenty for those 4 led pods. They draw 60w.. but again chinese lie and according to reviews they draw maybe 15-20w. So yea.

This solves all my problems. The 10ga should be able to handle 300w at 8 feet just fine. Thats 25amps. But in reality with car voltage being 13.8 min at the battery when on.. itll be more like 21amps. Just round it to 25 though. I think the 10ga can handle up to 30amps
 
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