Wire run calculations (gauge) question

IPT

Member
I am running rock lights along the side of the van and rear. Probably 8 lights total at 9 watts a piece. This puts me at 6 amps I think. I'm using the BlueSea wire calculator. My questions is about the length. I'll come off the Aux battery under the hood thru an inline fuse, in the dash, to a switch, and back out. Then I'll grab a negative from the battery creating a switched and fused power source. I'll split that into a driver and passenger side run. When determining wire size, the initial part will carry full load and then it will be split. Do I calculate the first (short part) based on all the lights, and then each "string" off of it based on their individual length and the number of lights? I'm a bit confused.

My current plan is 14 ga to the switch and the long run with more lights and 16ga to the one with less lights.
 
Last edited:

sparkplug

Active member
I would personally put the switch on the low power circuit of a relay and then calculate the wire size required for the two runs of cable.

It's also a good idea to go one size up from what you 'need'.

If you're not familiar with using relays (I wasn't!) then don't worry. They're very straight forward. Plenty of information on YouTube about how to use them and why.

How are you stepping the voltage down to 9v? Is that something built in to the rock lights (sorry, I'm not familiar with them)
 

IPT

Member
I would personally put the switch on the low power circuit of a relay and then calculate the wire size required for the two runs of cable.

It's also a good idea to go one size up from what you 'need'.

If you're not familiar with using relays (I wasn't!) then don't worry. They're very straight forward. Plenty of information on YouTube about how to use them and why.

How are you stepping the voltage down to 9v? Is that something built in to the rock lights (sorry, I'm not familiar with them)

Thanks for the input. Sorry, I miss typed. They are 9w a piece, not 9V. I'll correct in in my original post! I could go one size up from what I need but until I really determine how to calculate the runs I'm not sure what I need :). After I make the split do I have one say 15' run and one 10', or do I calculate it as 25'?


I've read a bit about relays but with the overall draw and total wattage being so low the relay seemed way overprotective. I'm confirming but I am pretty sure the switch can handle twice what I will be putting through it.
 
Last edited:

Midwestdrifter

Engineer In Residence
Calculate wire gauge for voltage drop. Up to 8% for lighting, though I would stick with 5% max. Then fuse to protect the smallest wire in the circuit. You can go lower if your loads allow it. If necessary you can use a smaller second fuse where the wire branches off into smaller circuits.
 

vanski

'05 Box Snow Camper
+1 on wiring in consideration of the voltage drop, especially for lightning.. I installed an Espar D2 for someone who installed lighting with pretty low gauge wire. D2 fired right up but his lights would pulse/sag/blink with every pulse of the d2 fuel pump. He tried to pin it on me until I educated him on voltage drop/sag.

without knowing all the details, I’d go with 12 on the first leg, then 14 on the second.
 
  • Thanks
Reactions: IPT

IPT

Member
Then fuse to protect the smallest wire in the circuit.
How does one determine that? In my limited understanding if I have 6 amps over 6 ft it would be a lot different than 6 amp over 18 ft. According to the calculator 18 gauge is fine for the 6 foot run while 16 gauge is recommended for the 18 feet. It's still 6 amps though and the fuses are based on amps, no?
 

Midwestdrifter

Engineer In Residence
The max fuse size is determined by the smallest wire. The minimum fuse size is determined by the max load current. A 20A fuse on a circuit with an 18 awg branch is not safe, as the wire can catch fire during a short, and not blow the fuse.

Even if that branch has less than 10 A load.
 

IPT

Member
+1 on wiring in consideration of the voltage drop, especially for lightning.. I installed an Espar D2 for someone who installed lighting with pretty low gauge wire. D2 fired right up but his lights would pulse/sag/blink with every pulse of the d2 fuel pump. He tried to pin it on me until I educated him on voltage drop/sag.

without knowing all the details, I’d go with 12 on the first leg, then 14 on the second.
I ran the calculator based on 3% drop.

Whats messing me up is the split with one run to the driver side and one to the passenger side. If I run 20ft x 2 runs, I can use thinner wire on each leg vs 40 feet in one run, yes? So when I run the calculator, am I running three separate inquires? One from power to the split, and then two others for each leg (since the length and number of lights is different)?

Oddly I think if I did that it might say thinner wire for the first leg (higher amps because of all the total lights but a much shorter distance). I'll keep the first leg as thick as the thickest recommended but I am trying to wrap my head around this. My current plan was 14 to power and switch as well as the long leg with most lights and 16 on the other but I might up that (or lose a light or two on each leg).
 

IPT

Member
Yeah, run the voltage drops for each leg and add up. Then fuse appropriately.
I found this chart below that seems to show it visually a little more clear to me than plugging in numbers.

Sorry if this seem rudimentary, I'm just trying to make sure I understand properly.

I have 8 lights at 9w each = 72W / 12V for 6 Amps total. One string will have 5 lights, one 3 lights, so 3.75 amps and 2.25 amps respectively. Neither run should be over 18 feet so I should actually have a little buffer room. If I add the distance before the split I might be close or slightly over for 14ga. So I think I'll run 12ga from the power, to the switch and to the split. Then 14 ga to both runs afterwards just to be consistent and safe. I'll put a 7amp fuse inline right after the power source. That wire at those lengths should be able to handle up to that anyway.

I get the min fuse size based on the amps. If it is lower than what you draw it blows. The max fuse is based on the wire size, but that is totally dependent on the length of that wire, no?

Automotive Wire Amperage Capacity Chart
Recommended Length and Amperage for Automotive Wire while maintaining a 2% or less voltage drop at 12 volts
Automotive Wire Size5 Amps10 Amps15 Amps20 Amps25 Amps30 Amps
20 Gauge Wire (AWG) 4.5 ft2.2 ft1.6 ft .
18 Gauge Wire (AWG) 7.3 ft3.7 ft2.4 ft1.8 ft
16 Gauge Wire (AWG) 11.5 ft 5.8 ft 3.8 ft2.9 ft 2.3 ft 1.9 ft
14 Gauge Wire (AWG) 18.4 ft 9.2 ft 6.1 ft4.6 ft 3.7 ft 3.1 ft
12 Gauge Wire (AWG) 29.4 ft 14.7ft 9.8 ft7.4 ft 5.9 ft 4.9 ft
10 Gauge Wire (AWG)46.8 ft 23.4ft 15.6ft11.7 ft 9.4 ft7.8 ft
8 Gauge Wire (AWG) 74.4 ft 37.2ft 24.8ft18.6 ft 14.9ft 12.4ft
 

Midwestdrifter

Engineer In Residence
totally dependent on the length of that wire, no?
Right. Choose wire for Voltage drop. For very short runs you also need to make sure you're under the ampacity limit of the wire. Then fuse within the following parameters. Do not fuse for greater than the wires ampacity for smallest wire in the circuit. And do not fuse lower than the highest load current.

For loads with a surge, like fridges, follow the manufacturers advice for fusing and wire sizing.
 
  • Thanks
Reactions: IPT

Roamers

2020 4X4 170 Crew
Consider running the same wire throughout circuit. Less likely to grab the wrong wire, probably lower cost to buy a bigger spool of heavier wire than several smaller spools of different size wires.
 
  • Thanks
Reactions: IPT

autostaretx

Erratic Member
Here's what you have:

8lights.png
That's a distributed load ... so the wire-thickness-drop table can look at the distances each current level will be carrying.
After the first light on each leg, you're dealing with less current, so less voltage drop per foot on the subsequent legs.
If you have your split "early"(i.e. close to the source), you can drop the two legs by a wire size or two.
I am a fan of "use one size" instead of playing silly games, *and* "look at the loads" ... do i really care if my lights are losing an extra percent or two? 3% of 12v is 0.36 volts. 5% is 0.6 volts. The last LED won't care that it's a half-volt down.
... you're frequently starting with more than 12v (when the engine's running or they're charging, it's over 13v).
... but if you run them down, it may reach 10 v.
For these dinky loads, i have to admit i'd be sorely tempted to use 16 gauge wire from the first lamp onwards.
(that's a gut feeling, i'm not looking it up since i don't know the various segment lengths involved).

Messing up the "fuse for the smallest wire" dictum is that often these LED pucks come with their own last 6 inches of wire ... and it's invariably far thinner than the wires you're running ... so do you fuse for *those*, or for the main run?
(answer: main run, or maximum expected load ... if you're only feeding these LEDs on that circuit, fuse for 10 amps at most.
7.5 amps if you want quicker fuse-pop.)

Let's look at the raw numbers: 16 gauge wire is 0.00409 ohms per foot.
Your longest length is 18 feet, and (i assume) running two wires (one out, one back) instead of using the Sprinter frame.
So that 36 feet of wire, a total of 36*0.00409 = 0.147 ohms.
Assuming the full 6 amp current through that, we'd see a 6* 0.147 = 0.8837 volt loss.
But we're not running that current the full length. We're only running 6 amps to the split.
Then we're running partial currents from then on.
Let's put the split at 6 feet, with 12 feet going to each final LED.
So now it's 12 wire-feet to the split, or 1/3rd the above full loss: 0.8837/3= 0.295 volt loss
Now it's (on the 5-lamp leg) 3.75 amps along the remaining 24 wire-feet: 3.75*24*0.00409= 0.368 v (3% of 12).
Add those two: 0.295+0.368 = 0.6631 volts.
We're very close to the 5% number, and i'm still vastly over-estimating.

Going to 14 gauge lowers the ohms-per-foot to 0.00258 ... one third of the loss is gone.
That would give you about 0.2 volts loss at the split, and a total of 0.44 volt loss over the full 18 feet on the 5-lamp leg.

--dick
 
Last edited:
  • Thanks
Reactions: IPT

Top Bottom