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Need advice on fuse/breaker/wire selection

L

Larry DeCamp

Well Known Member
My flap actuator motor is said to draw 5A. It is recommended to supply the motor from a 10A breaker to avoid nuisance fuse faults. If fuses blow faster than breakers, then I need to protect the wire to the breaker with a bigger fuse than the breaker to avoid fuse blowing instead of the breaker. Then I have all this power available to melt the little 20ga wire on the motor that “normally” sees 3-5A. What am I missing ?
 
I used a 10A breaker and 18 AWG wire and haven't had an issue. I believe I pulled those numbers from the builder's manual for my -9.

When deploying the flaps under load, ie. in flight, the flap motor will draw more amperage than when on the ground.
 
Larry DeCamp said:
. . . I need to protect the wire to the breaker with a bigger fuse than the breaker to avoid fuse blowing instead of the breaker. Then I have all this power available to melt the little 20ga wire on the motor that ?normally? sees 3-5A. What am I missing ?
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Use either a fuse or a breaker, but not both. There is no good reason that I am aware of (and I am an electrical engineer) for placing a fuse and a circuit breaker in series. Just use a 10 amp protective device with wire sized for 10 amps.
 
Thanks much Dave

What I am confused about, since I am just "gear head", is what keeps the wire from the buss ( 12V source ) from melting in a chassis short between the buss and the breaker without a fuse ? And then ( with a 10A fuse or breaker ), what keeps the little motor wires from burning ?
 
Your concern is legit. What may be throwing you a curve is that when you use breakers, the 'bus' is typically a fairly heavy brass or copper bar that connects all the 'hot' terminals of the breakers. That 'bus' is 'protected' by the master contactor.


If you're using a fuse block as your bus, fuse adequately in the fuse block & size the wire so the fuse can protect it. Having said that, you're not likely to damage a 20ga tefzel covered wire with a 10A fuse. The wire will handle much more current than that (with a lot of voltage drop), and it's doubtful that tefzel will be stressed by the heat at that load. 20ga does sound pretty small for the flap motor, though, just because of voltage drop.

Charlie
 
A 20 gauge wire, 4 feet long, can handle 10 amps continuous current, in free air. So, the wires from the flap motor are fine at the 10 amp protection devise rating.

An 18 gauge wire, 7 feet long, can handle 10 amps continuous current, in a bundle. So, the harness feeding the flap motor is fine at the 10 amp protection devise rating.

A 20 gauge wire, 3 feet long , can handle 15 amps for 2 minutes (intermittent rating). More than most would imagine.

George Meketa
 
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Thank you all

VAF is just too valuable to express. What a brain trust for the simple price of loyalty and generosity when it is within ones knowlege/ experience to share 👍
 
Charlie is correct, the typical bus is also referred to as a bus bar as it is made up of a copper bar of significant cross section, that is protected by the fuse at the alternator output. As for the flap motor wires, they are sized and designed to limit the motor drive current and thus heat. If there is a short internal to the flap motor wiring, then there will likely be some internal wire melting requiring a new motor (these motors are typically too small and inexpensive to make a motor re-wind economic).

If your flap motor only draws 5 amps, you could probably use a 7.5 amp breaker, so long as the inrush current when the motor is starting isn't too much above 7.5 amps and is fairly short in duration. However, with such low currents, you are typically better off installing slightly larger wire to reduce voltage drop and then protect that larger wire with a larger 10 amp breaker.

Good questions though. Better to ask the questions so you understand than just follow the instructions without understanding why.
 
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just to sum up what has been written in simple terms. the steps to figuring wiring.

1. determine current draw of device, taking into account inrush current. follow manufactures guidelines if they give you one.
2. size wire for that current from chart, taking into account if free wire or bundle and voltage drop.

breakers and fuses protect wires.a dead short will trip a breaker or pop a fuse almost instantly. as long as the wire is sized for the breaker any short should not destroy wires.

as with all things electrical, the aeroelectric web site has a good write up on sizing wire.

bob burns
rv-4 N82RB
 
DRMA said:
Charlie is correct, the typical bus is also referred to as a bus bar as it is made up of a copper bar of significant cross section, that is protected by the fuse at the alternator output. As for the flap motor wires, they are sized and designed to limit the motor drive current and thus heat. If there is a short internal to the flap motor wiring, then there will likely be some internal wire melting requiring a new motor (these motors are typically too small and inexpensive to make a motor re-wind economic).

If your flap motor only draws 5 amps, you could probably use a 7.5 amp breaker, so long as the inrush current when the motor is starting isn't too much above 7.5 amps and is fairly short in duration. However, with such low currents, you are typically better off installing slightly larger wire to reduce voltage drop and then protect that larger wire with a larger 10 amp breaker.

Good questions though. Better to ask the questions so you understand than just follow the instructions without understanding why.
Click to expand...

just remember, if you do upsize the breaker to 10 amps then EVERYTHING on that breaker must be wired with a wire that can handle 10 amps.
 
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