I'm confused about wire size for long runs from dc fuse block to LED Floodlights

[Nord]

Hello all,

I have a 40' skoolie. I finally got my solar system installed and operational. I love it.

My question pertains to the correct size of the wires and how I should ground them between the DC fuse block and LED floodlights. Right now my system is grounded to the chassis of the bus from the shunt.

I have 4 LED floodlights that use 1.5amps each. I want them all on one circuit and I have two switches to turn them on/off. One switch is at the entrance to the bus and the other switch is 40' away in the rear of the bus. The electrical components are near the rear of the bus as well. The floodlights are place on the exterior of the bus, one on each corner, so the wire run between all the lights is about 100'. So from the DC fuse block to the switches and then to the lights is at least 160'. I'm looking at the wire gauge chart and it recommends using a really thick wire...like at least 8 awg.

So a couple questions I need answered...

1. Can I ground each light individually to the metal ribs of the bus to cut down on wire length? (Instead of running two wires between each light, I'd just run a + wire between each light, and the ground wire would only need to be a few feet to get to something metal on the bus)

2. What can go wrong if I just use 14awg for this circuit? Will the wire melt? Will the lights be dimmer because of voltage drop? Do I really need to use 8-10awg for a long run like this?

Thanks for your help!

Matt

 

[snoobler]

Any wire suitable for 1.5A current will not melt.

You can use the following calculator for computing voltage drop (power loss) for current, voltage, distance and wire gage:

Voltage Drop Calculator

This free voltage drop calculator estimates the voltage drop of an electrical circuit based on the wire size, distance, and anticipated load current.

www.calculator.net

Lower is better.

1) Probably
2) See calculator

 

[HRTKD]

Even though you use the chassis as the ground/negative, it still counts for ratings. The chassis may provide more resistance than high quality copper wire. (I'm just throwing that out there as something to consider)

 

[Pappion]

4 x 1.5A = 6A. Blue Sea Systems has a good guide. wiring chart
A lot depends on the voltage drop your lights can tolerate. Big difference in gauges for 3% and 10% vdrop.
A 12v relay controlled by the switches could reduce the LED power wire length.

 

[Hedges]

Some LED lights have a minimum voltage spec.
I picked up some driving lights which can work on 12V or 24V, but instructions say to turn off when cranking the engine.
A switching power supply can kill itself drawing extra current at low voltage to deliver the same power.

If IR drop through 14 awg or 12 awg, from minimum battery voltage, still meets requirements of the light, you may be able to ignore the percentage power loss, and the LED driver will just pull extra current to deliver the watts needed.

 

[hankcurt]

Blue Sea Systems has a good guide. wiring chart

I don't understand how that chart can be accurate without taking into account the system DC voltage. I didn't see any indication if it was for a 12, 24, or 48 volt system. I think the Blue Sea Circuit wizard calculator is a better tool.

The real pain in this sort of system is that the voltage drop through the wire is set by the wire resistance times the current. So say you do that calculation and get a 0.6 volt drop in the wiring run. If you have a 12 volt system, that is a 5% voltage drop, where as in a 24 volt system, it is only a 2.5% voltage drop. So, while the voltage drop in absolute terms is the same between the two systems, it is much more significant if the supply voltage is lower.

Another consideration that just occurred to me is that the 6 amp current is not true for the entire run. From the switch to the first light you have 6 amps. Between that light and the second you have 4.5 amps. To the next light you have 3 amps. And to the final light you have 1.5 amps. So if your first light is right by the switch, those amps are only across a very short run. then if you do the adjacent corner across the width of the vehicle, you will only have 3 amps remaining that have to run the length of the vehicle. So you would still need a heavier ga;uge wire on your two way switch runs, but could use lighter gauge going around to the far lights. Or use a wireless remote switch to do the two way switching.

 

[Dzl]

I don't understand how that chart can be accurate without taking into account the system DC voltage. I didn't see any indication if it was for a 12, 24, or 48 volt system. I think the Blue Sea Circuit wizard calculator is a better tool.

Its a 12V chart. I'm pretty sure they don't specify it, or if they do only in the fine print. Which is dumb since voltage drop % depends on voltage.. believe its based on the ABYC chart here:

I agree circuit wizard is better (for standard voltages). For non standard voltage (like PV array voltage, something like the baymarinesupply calculator is better).

 

[hankcurt]

@Dzl , The Bay Marine Supply calculator is much more flexible, and intuitive to use (for me anyway). Thanks for pointing it out.

 

[Zil]

There are different ways to wire lighting circuits. Your plan for a switch at each end of the buss requires a special switching arraignment with three wire between the switches. The switches could be used to control a relay that will permit using smaller wires between the switches. Then heavier wire could be used from relay to lights. Two lights will be closer to the battery and could have smaller wires for 3 amps, by your estimate. The distant lights would have 3 amps with appropriate wire sizes. You could wire each light with its own wire reducing required awg but needing more footage.