[prkaye]

I'm trying to figure out the best way to get power out from my main bus, through a single SPST switch on the panel, to both wings for my posn lights I have terminal strips under the floor beside both wing roots, with ring terminals (so I can connect more than one wire to the same terminal by stacking the ring terminals). The obvious way seems to me to run the power through the switch out to the terminal strip at one wing, and have a jumper wire going from this same terminal to the terminal strip at the other wing. This has the posn lights wired in parallel with each other, but in series with the SPST switch. So this means if each posn light draws 2 amps, there will be 4 amps total drawn from the breaker, so I could put a 5A breaker there. The diagram below shows my plan. I could run the ground wires back seperately, since they don't need to go through a common switch.
Is this plan OK, or do I need to get a DPST switch to control both posn light circuits?

 

[PaigeHoffart]

Your plan will work fine. Don't forget you actually need to split it three ways, one in each wing, and one in the tail. You didn't mention the wire size, but it should be sized for the total load (i.e. breaker size). As far as the grounds go, if they're just simple position lights, I would ground them locally in the wing tips and tail and save the wire. If they're super high tech LED lights with some sort of switching regulator, you might be better off running the ground leads back to a common ground bus.

Paige

 

[prkaye]

Don't forget you actually need to split it three ways, one in each wing, and one in the tail.

I went with Vans system 6. I believe the tail light is a strobe, powered through the strobe power supply with it's own dedicated shielded wire running back to the light.

You didn't mention the wire size, but it should be sized for the total load

So my understanding here is that for the long wires in each wing, I can size the wire for the current actually drawn by each light (in this case 2 amps - they are wired in parallel, so 2 amps goes off to each wing where the split is, at the terminal block). I'm being super conservative and using 16 gauge wire anyway. It's only for the wires that carry the current from both subcircuits (i.e. the wires between the fuse, switch and my terminal blocks) that would have to handle the full 4 amps. Because these are relatively short runs, I'm thinking 16 guage should work ok here too.

 

[carl nank]

The Aft strobe light also has a nav light built in and needs to be nav switched, so you can turn off the strobes and still have the nav's.

 

[vlittle]

I went with Vans system 6. I believe the tail light is a strobe, powered through the strobe power supply with it's own dedicated shielded wire running back to the light.



So my understanding here is that for the long wires in each wing, I can size the wire for the current actually drawn by each light (in this case 2 amps - they are wired in parallel, so 2 amps goes off to each wing where the split is, at the terminal block). I'm being super conservative and using 16 gauge wire anyway. It's only for the wires that carry the current from both subcircuits (i.e. the wires between the fuse, switch and my terminal blocks) that would have to handle the full 4 amps. Because these are relatively short runs, I'm thinking 16 guage should work ok here too.

Hi Phil. Use the same gauge of wire everywhere in the circuit. In this case, it's not about the amount of load current, it's about the fault current capability due to a short circuit that matters. Don't want a wire to melt and catch fire before the breaker trips.

Vern

 

[prkaye]

Use the same gauge of wire everywhere in the circuit. In this case, it's not about the amount of load current, it's about the fault current capability due to a short circuit that matters

I believe this is what fuses are for. No wire can handle infinite current, which is what a short-circuit would try to push through. Wire size is calculated so as to be able to accommodate the current required by a particular circuit, taking into account loses along the length of the wire run. In the case where you have one wire carrying a larger current that is then branched off to several sub-circuits carrying smaller currents, it seems to me perfectly appropriate to have different sizes of wires appropriate to each circuit branch. In fact, this is precisely what happens at your main fuseblock/bus. You have a large (8AWG) wire feeding a main power distribution bus from the battery/master relay, and this single big wire carries all the current which is then split off onto many smaller wires carrying smaller currents to all the various subcircuits in the airplane. Logically, I'm not sure why what I propose with the position-light circuit is any different... one larger wire (AWG14) carrying the current for all three position lights part of the way, which is then split off into three smaller wires (AWG16) carrying the (smaller by about 1/3) currents to each of the three strobe lights.

 

[Mel]

The fuse should be sized for the smallest wire in the circuit. If something happens to cause excess current, the fuse should blow before burning the smallest wire.
This is primarily why you don't fuse for #14 wire (the total load) then run #16 downstream.

 

[Mel]

I'm not sure why what I propose with the position-light circuit is any different... one larger wire (AWG14) carrying the current for all three position lights part of the way, which is then split off into three smaller wires (AWG16) carrying the (smaller by about 1/3) currents to each of the three strobe lights.

Because in your example from #8 wire to smaller branches, the fuse/breaker is downstream of the #8 wire.

 

[roee]

Vern is right

Wire sizing involves several different considerations including: 1) sufficiently low resistance to not cause excessive voltage drop at the normal circuit current given the wire length, 2) sufficient current carrying capacity to not overheat under the normal load current, which has nothing to do with wire length, and 3) sufficient current carrying capacity to not overheat under any current that doesn't cause the circuit protection device to trip, which also has nothing to do with wire length (we usually think about wire gauge and circuit protection the other way around... keep reading). Don't confuse these different considerations with each other.

The purpose of a circuit protection device (fuse or circuit breaker) is to open the circuit in case of an over-current condition (typically a dead short, but not necessarily) that would cause the wiring to overheat and potentially catch fire. That means that for a given circuit, all the wiring downstream of the circuit protection device must be able to sustain any current up to the protection device's trip rating, and preferably beyond with some reasonable margin.

In a simple circuit you usually think of this the other way around -- you size your wires with the load in mind (considerations 1 and 2 above), and then size the breaker to protect that wire size (consideration 3). In this case however, because you have multiple parallel loads, you must size the breaker to support the combined current of all parallel loads (the three nav lights), and then size all the wiring on that circuit based on the breaker size. This is just the long version of what Vern said. I hope that clears it up.

 

[prkaye]

makes sense

Ok thanks guys, now I understand the issues. The calculation gets confusing now though. The books I have seen show how to do the calculation for a simple single circuit.
For my posn light circuit, the run out to each wingtip is about 12 feet (a little shorter for the one back to the tail). For those sections of the circuit (each posn light, in parallel), each wire carries about 2 amps.
The rest of the circuit is maybe another 8 feet total (from main bus through switch, down to the floor and back to the terminal strip at the wing root). This part of the circuit carries up to 6 amps (feeds power to the terminal strip from which I branch out to the three lights).
So how the heck do I calculate the wire size I need in this slightly complicated circuit? One conservative approach would be to pretend all 6 amps travel along a single stetch of 12+8=20 feet of wiring... for a 5% allowed voltage drop this calls for 16AWG, so I guess running that 16AWG for everything in this branched circuit is suitable?

 

[Robert M]

I hope this isn't a hijack.....

What about using a pair of piggy-backs (as shown below) to attach all three wires to the switch and fuse/breaker the circuit for 6 amps on 14 gauge wire?

 

[prkaye]

What about using a pair of piggy-backs (as shown below) to attach all three wires to the switch and fuse/breaker the circuit for 6 amps on 14 gauge wire?

Electrically, that's exactly what I'm doing. Just instead of piggy back terminals at hte switch, I'm branching out with stacked ring terminals at a terminal strip just downstream.
Although my calculation in my previous post gives 16 Gauge wire as suitable for a 6 amp circuit of this size (using the total length including only run of the three parallel runs).
Question remains, does anybody have a suggestion of a better way to do the calculation I described in my previous post?

 

[PaigeHoffart]

I think we're getting a little wrapped around the axle worrying about voltage drop on 20' of wire to some position lights, but none the less here are the calculations:

#16 wire resistance per foot from the aeroelectric book figure 8-3: 4.01 ohms / 1000'

Resistance of 8' feeder wire: 0.03208 ohms

Resistance of 12' wire to wingtip light: 0.04812 ohms

According to ohms law E=IR (voltage = current x resistance),

Voltage drop on 8' feeder wire with 6 amps flowing through it: 0.19248V

Voltage drop on 12' wing wire with 2 amps flowing through it: 0.09624V

Total voltage drop: 0.28872V

Assuming the alternator is putting out 14.4V and neglecting losses through fuses, switches and the main bus, that's 2%.

With these short distances, voltage drop isn't much of an issue, we're mainly worried about the temperature rise on the wire. Figure 8-3 lists the 35 deg C rise for #16 wire as 12.5A. If you went with the smallest wire possible (#20 which is rated for a 35 deg C rise of 7A), the voltage drop would be right at 5%. Having said that, for a continuous load such as nav lights you're probably better off with the #16 wire which will have a temperature rise of around 10 deg C in this application.

Paige

 

[RFazio]

Too Complicated!

Phil, I think your making it a little too comlicated. Since you have 3 lights of 2 amps each, you need wire for 6 amperes. You can not reduce the wire size after the terminal strip. You need 6 amperes worth of wire up to and after the strip, because you need a 6 amp fuse to handle the circuit. As others have said you need to fuse or circuit breaker the wire size. If you put a 6 amp fuse and 6 amp wire up to the terminal strip and then reduce to 2 ampere wire to the wing tips, you would have 2 ampere wire fused at 6 amperes, NO good. The other thing is you should be slightly over sized. I put in a 5 amp circuit breaker for my strobes because it was raited at 3.9 amperes. Guess what, it blew the breaker. I had to go to a 10 amp breaker. Luckily I had large enough wire. One thing to remember also is you are fusing or breaker protecting the WIRE, not the item connected to it. You need to size breakers to the wire even if they are over the amperes of the item connected to it.