fuses for the small wires ??

[tonyjohnson]

There are several very small wires in my electrical plan. Those go to LED warning lights, map light, etc. Most of those go through a dimmer module and from there to the device.

My map light, for example, requires a 1 amp fuse for protection. It would seem that you cannot simply attach those small wires to a fuse in the main fuseblock, because the fuseblock has one power input and several fused outputs.

So, how do you fuse those small wires? My question includes both those wires that run through the dimmer module and those that run directly from some other power source such as the low voltage light from the alternator?

Thanks,

Tony

[William Slaughter]

First of all, remember that the fuse is there to protect the wire, not necessarily the device. Further, each wire "segment" does not get it's own protection - each circuit does. A dimmer or switch or whatever between the device and the +14V source does not create the need for an additional fuse, even though the wire is now divided up into two segments, rather than one continuous wire. You absolutely can and should connect these small wires to an appropriately sized fuse right on the fuseblock - that is part of the fuseblock's function to "subdivide" the main power input into the variously fused circuits you are protecting, similar to the function of the bus bar in a circuit breaker array. If you don't have enough fuse slots, get a bigger fuse block, or more than one. You can, however, run more than one light, etc, off of a single fuse, IF the total load of the devices is less than the value of the fuse needed to protect the single smallest wire used, AND IF you're ok with a fault in one circuit taking out both items. If you haven't yet purchased your copy of The Aeroelectic Connection, I'd highly recommend it. I read mine cover to cover when I first got it and still refer to it all the time.

[tonyjohnson]

William,

Thank you for your response. I am still trying to get my head around how I would run the several wires to different small loads from the output pots on my LC 40 dimmer. Each of the outputs from the dimmer may carry a different voltage based upon the pot setting. The only way I can see to do that would to have a seperate fuseblock for each pot output (there are four on the LC 40).

I have a copy of the aeroelectric connection, which I have read twice from cover to cover and keep handy.

[jcoloccia]

The simplest way:

1) Figure out how much the dimmer circuit draws maximum
2) size the wire to the dimmer
3) size the fuse to the wire
4) use the same size wire run to all the lights

You don't have to use a tiny wire just because you can. Larger wires are easier to handle and more robust, too. This is by far the simplest way to do it that guarantees your fuse protection is adequate. Also, the voltage out of the dimmer is not relevant...only the current, and that won't be more coming out of the dimmer than going into it so protection on the "back" side is not nescessary if the wires are the right size.

Hope this helps.

[Jim P]

Take a look at the LC-40 documentation that came with the unit. If I'm not mistaken, I believe there's a limit on each channel and the documentation gives you an overall fuse size to support that circuit. Go with the provided documentation in this case.

Jim

[jcoloccia]

Jim makes an excellent point. I was actually off looking up the documentation and it mentions a 1.5 amp limit per channel but doesn't say if it's internally fused or not. Maybe a call to them would be good. It would stink to have to run 18 or 20 gauge wire for a 1.5amp circuit.

[airguy]

Originally Posted by jcoloccia

Also, the voltage out of the dimmer is not relevant...only the current, and that won't be more coming out of the dimmer than going into it so protection on the "back" side is not nescessary if the wires are the right size.

Hope this helps.

I had to read this a couple times to figure how your thought process was headed before this made sense.

The voltage out of the dimmer is not relevant TO THE FUSE - but the FUNCTION of the dimmer is exactly that - to change the output voltage. The downstream device cares only about the voltage, the fuse cares only about the current.

[gmcjetpilot]

[b]First of all min size wire should be 20 or 22. You start to get into 24 awg or smaller, they tend to be more easily damaged and harder to make connections. The down side or bigger wire is weight. However the small runs you make to those few items with over sized min gage wire is negligible.

here is some data from many sources to help you size the fuse. Regardless of current carried you are protecting the WIRE, that is it, period.

Gage......max.............max.............max..... ......max (AMPS)
............chassis......transmission.....Bundle.. ......Fuse
20............11.............1.5.............1.03. ...........30.1
22.............7.............0.92............0.651 ..........18.9
24............3.5...........0.577..........0.409.. ........11.90

max chassis = short runs, max operation (intermittent, not recommended)
max transmission = long runs based on voltage drop
max Bundle = based on temp rise & cumulative effect of a bundle of wires
max fuse = estimate about when the wire will glow and burn out
Even if your LED is only say 0.022 amps use a 20 awg or 22 awg for durability. Than install a fuse, 1/2 amp, which is plenty for the load and well below the wires safety margin. It covers the load and is way below the working or max amps the wire can handle and no where near fusing. remember the fuse protects the WIRE only not the device.

There are three criteria to picking wire gage:

Voltage Drop (based on operating current and length)
Temp Rise (based on operating current and where and how wire is routed)
Practical (Min gage, easier to handle and less likely to damage)

In the engine compartment I like 18 awg for durability. I don't like anything under 22 awg, 20 awg is nominal. If voltage drop or temp requires it (landing light, heated pitot) than 18 or 16 awg, by all means. In a Boeing 777 they use lots of small gauge wires (24 and less) because they have several 100,000 miles of it. It adds up weight wise.

You could (in theory) go with a blanket wire and fuse of 18 awg wire and 10 amp fuse, it would handle probably every cuircuit in the plane. The extra weight of 18 awg over 20 or 22 adds little weight in a RV, a few pounds. There is no doubt 18 awg wire is easier to handle and stonger. For avionics 20 or 22 awg is required and needed to make connectors for the practical reason of physical wire diameter. You just can't wire a 24 pin connector with larger gage wire.

The crazy thing is Mac trim servos sells that bundle wire harness that goes to the servi, using 24 or 26 awg. I made my own harness with 20 awg. I added a few ounces, but I had little confidence in those tiny wires.

Fuse? That is easy after load of the device and wire size is decided. The fuse has to be big enough to handle max operational load and well below wires fuse.

some secondary info:

Gauge....*Dia.......**Area...***Resistance....**** Weight
AWG.......mils.......sq. in...........ohms(1000ft)......Lbs(1000ft)
18.........40.3.......1.28E-03.......6.395...............4.91
20.........32.0.......8.02E-04.......10.17...............3.09
22.........25.3.......5.05E-04.......16.17...............1.94
24.........20.1.......3.17E-04.......25.71...............1.22

*Diameter of the wire itself in thousandths of an inch. Values shown are for SOLID copper wire. Stranded wire will generally be 10% to 15% thicker.

**Cross sectional area of the wire in square inches. Example, AWG 24 wire has an area of 3.17E-04, which is 0.000317 square inches.

***Resistance, ohms for 1000 feet of wire. To get the resistance per foot, simply divide the listed number by 1000.

****Weight in pounds for 1000 feet of bare wire. To estimate the weight with insulation, add approximately 25%.