What size of wire?

[Overland-Indiana]

I am going to be repairing a friends lights on his roof rack, I am wiring to a panel in the back of his rig and going to wire 4 relays, what size of wire should I run back there to distribute power to the lights? He has 3 6" hella halogen and 2 8" halogen. Any ideas?

 

[jdunk]

The first step in figuring this out is going to be knowing what the draw is on those lights. Then you can determine what safety standard you like best, and then find the related chart.

The gauge changes based on the draw, and length of the run.

 

[Overland-Indiana]

I'll hop up there tonight and see what they are, the run will be around 10ft.

 

[Mad Garden Gnome]

We can calculate draw from wattage rating on the lamps. Also, will all 5 lamps be running on the same circuit? 4 relays says no but it is good to be clear on how many of what draw will be on each circuit.

 

[Overland-Indiana]

I want one heavy gauge wire running to the back where it will be distributed to the relays, then from the relays to the lights, only 2 of the relays will be powering the lights. 3 lights on one and 2 on the other. Since they are halogen bulbs I'm afraid I'll have to run jumper cable sizes lol all iv'e ever wired are LED's and they require a lot less power than halogen. My degree is in Industrial Electricity and Robotic programming so i know a bit about wiring, but I am out of practice since college, that is why I'm here for advice lol "if you don't use it, you lose it"

 

[Mad Garden Gnome]

Good start.
We need to know what total amp draw on each relay OR total amount of watts on each relay. You had stated 10', correct? Is that both from power source to relays and relays to lights? If you give a break down of lengths, we may be able to get your conductor size down to what is necessary plus 10% (aviation background here).

Secondly, are there plans for more upgrades? If we run a heavier than necessary wire from power source to relay panel, this will allow more draw at a later date for mods.

 

[Overland-Indiana]

I think he may eventually upgrade to LED. I'll check the amp draw of the bulbs and the relay capabilities tonight after work and post it back up

 

[Mad Garden Gnome]

Super
Lastly, are these chassis ground lights or two wire lights? If they are two wire and you don't want to or can't ground at the mount point, will you be running a common ground wire all the way back to the relay panel to a ground point, or chassis grounding at the first possible chassis point?

 

[Overland-Indiana]

They are Hella brand, not sure if they are 2-wire. I'll check on that also. If they are 2-wire then I'll run a pretty heavy ground wire and chassis ground it behind the rear lift gate.

 

[Overland-Indiana]

I'll get the wattage one of these days... It has just been too cold to go out and do it. Or, I am just a wimp... lol

 

[Mad Garden Gnome]

In all respect and sincerity to @roamingtimber, don't use that chart. Even in best case single conductor in free air circumstances, it does not even follow max ampacity for a given conductor size. I ran multiple load calculations against it with load, length of conductor, acceptable voltage loss, and max ampacity for a given conductor. It is not safe.

I am asking you, for your safety and the safety of your passengers, don't use that chart. Please.

 

[roamingtimber]

Do you have a better reference?

 

[Mad Garden Gnome]

You could go NEC or FAA Repair Manual AC 43.13-1B. Southwire Co makes an excellent mobile app that goes along the NEC guidelines for constant loads.

 

[roamingtimber]

I will look for those. Thanks for the info.

 

[Mad Garden Gnome]

If anyone needs help with load calcs I am happy to help. I am currently looking for an online load calculator that perhaps can be made into a sticky.

 

[roamingtimber]

@Mad_Garden_Gnome what about this calculator? http://www.wirebarn.com/Wire-Calculator-_ep_41.html

 

[Mad Garden Gnome]

Here is Southwires online calculator. It seams to be one of the only calcs that takes into consideration the max amperage rating of a conductor, regardless of length of run.

 

[Mad Garden Gnome]

@Mad_Garden_Gnome what about this calculator? http://www.wirebarn.com/Wire-Calculator-_ep_41.html

I ran a few load calcs against it. It looks pretty safe. I'd use it. And depend on it to get my family home. I like that it also shows minimum recommended conductor size (the size ok column).

 

[Overland-Indiana]

I used 4ga wire as supply to the distribution block, then 14awg on the rest... All that is getting ran off of the circuit is LED lights on the side and rear of roof rack, the 50in LED bar that is going to go on the top of windshield will be ran separate and on probably heavier wire.

 

[Mad Garden Gnome]

I used 4ga wire as supply to the distribution block, then 14awg on the rest... All that is getting ran off of the circuit is LED lights on the side and rear of roof rack, the 50in LED bar that is going to go on the top of windshield will be ran separate and on probably heavier wire.

That sounds good but without knowing the load on it, an overload is an overload. The 4 AWG feeder looks good. I cruised about teh intrawebs and took a consensus of 50" light bars. They all tend to run in the 20 to 21 amp range. If this light bar is within 5 wire feet of it's distribution you should be alright, as long as that light is the only appliance on that circuit. Any further than 5 feet and you start running into undervoltage and/or overload issues. As to the roof rack lights, it all depends on how many you have per circuit and how long the feeder is.