Fuse size

[David Weber]

Whelen product sheet says Landing light draws 3 amps. So what size fuse do you use? Is there a rule of thumb when they do not tell you what size fuse to use.? My Garmin product info sheets tell me the size fuse to use.

[David Weber]

Well I just read that you "fuse to wire size", so lets say the light uses a 20 g wire, but the light is rated to draw 3 amps. The benchmark for 20 gauge is 3 amps. So this means I should use a 3 amp fuse because of the wire size, but if the light is pulling 3 amps, seems to me that I would be in danger of the light blowing the fuse. Am I understanding this correctly? Logic tells me to use a 4 amp fuse, but my logic might be flat wrong. Any one know how to size correctly?

[Battson]

I would use a 5A fuse, the current it draws might not be steady enough for a 3 or 4A fuse to last through all conditions. Hard to guess. The fuses / CBs are there to protect you from electical fires, yes. That said, I would size them to the expected load in the circuit, not the maximum capacity of the wires. Choose the wires size based on the load - and go generous rather than cutting it fine. Nobody likes electical fires...

[jaredyates]

Your fuses are there to protect the wires from too much current draw, which will in turn overheat the wires and possibly cause a fire. Size the wire based on the guidelines of 43.13 and size the fuses to no more than the size listed there. There's no harm in using a smaller fuse if you are expecting a very low draw, but you obviously don't want to go over on the fuse size. The smallest wire size to use is going to be based on the length of the run and the current, but also don't forget about factors like mechanical strength and cost. For example, while you might have some circuits that only require 24 gauge wire, you would probably be better served to use 22 gauge, since it is much easier to find and much easier to work with. What size wire did you run to the light?

[X'N]

I agree with Jared. The AC43.13 has a nice chart for choosing wire size dependant on current and length of wire.

http://www.aeroelectric.com/Reference_Docs/FAA/AC43.13-1B_Ch11_Electrical.pdf

[Tom W]

David -- What is the part number of the Whelan Landing Light? 3Amp sounds like a 28V landing light.

Pull out your Aeroelectric book. Figure out what wiring diagram you will be using (i.e. Z18) and I'll get you pointed in the right direction.

Incandescent lamps will pull almost 2x the inrush current when your first them on as the filament is cold. So a 7.5A fuse on 18guage wire is right for a 3A 14V lamp.

[David Weber]

I am using Z11 I think. I am shark fishing this week but I think that is number. I have Whelen MB1 tail light / strobe and Aeroflash Signal wing strobes / position lights. Bought Bad Ass Whelen Landing light at KOSH but do not remember number. It is LED and spec's say max draw is 3 amps. That is where I do not understand the logic. If I go by wire size, then 5amp fuse but I am thinking 5 amp will not protect light. That's what is fun I guess, learning all this stuff. I had friend who ditched/crashed twice cuz of electrical fires, so I am being cautious. My Dad was Chief electrical engineer for Mercury, Apollo and Gemini, {MacDonald Douglas} but he died at 62. So, I need to channel him, and read more. I need to understand why a 5 amp fuse (based on wire size) can protect a light that has max draw of 3 amps, or I can not move forward knowing all is safe. Maybe the answer is that a 5 amp surge would not damage a 3 amp light, but I do not know this .

[John Rheney]

David, go back up and reread Jared's post. I think he has it correct in that you should be sizing the fuse to protect the wire from overloading and burning. The wire size is in turn selected for the current draw and length of run primarily.

[jaredyates]

The light is not going to be damaged by too much current. Too much voltage probably would damage it. The light will only take as much current as it needs. If you run too small of a wire, and the light takes enough current overheat the wire, that's when the fuse will come into play. Or, if the wire were to short to the airframe because of something like a routing problem, that very high draw of current would pop the five amp fuse before it starts to burn the wire. Electrical design is certainly something to take seriously, but it would probably be worth the time to read the Aeroelectric book, so that your mitigation efforts are based on science and experience. I'm using an LED landing light in the nose. It would work with 22 ga wire, but I ran the wires to be large enough to feed an incandescent bulb. That way, if the LED turns out to be a flop, I can just drop in an old style bulb without having to rewire. Also, the larger wires will be more durable in the engine compartment, and will only incur a very small weight penalty.

[DRLPatrol]

The fuse is NOT there to protect the light, like Jared and others said, it is there to protect the wire. Think of it this way. In your house the circuit breakers for the outlets are sized for the wire that run to them before anything is even plugged in. They don't care if you plug in a night lite or a big screen TV. What gets people in trouble is when they think they can just change the SB because they want to plug in some big welder or something.
Doug

[David Weber]

OK...I got it now. All is good and I can move on.