Sizing Circuit Breakers

[Bartman]

In moving my project along I'm trying to get the details of the new panel figured out. Some of the pieces for the panel such as the Garmin USB ports and the AV-30 attitude indicator have circuit breaker requirements in their installation manuals. Others don't.

So, in going to AC 43-13 , the circuit breaker sizing guidance says;

Circuit protection devices must be sized to supply open circuit capability. A circuit breaker must be rated so that it will open before the current rating of the wire attached to it is exceeded, or before the cumulative rating of all loads connected to it are exceeded, whichever is lowest. A circuit breaker must always open before any component downstream can overheat and generate smoke or fire. Wires must be sized to carry continuous current in excess of the circuit protective device rating, including its time-current characteristics, and to avoid excessive voltage drop. Refer to section 5 for wire rating methods.

The Garmin GTX 345 Comm lists the electrical power requirements but doesn't specifically list the circuit breaker that should protect the unit. Here's the power table and the area I think I will need to follow is highlighted.


So there needs to be a CB for this device but what should the CB be rated for? Less than the 14V max current of 1.43 or the next larger breaker of 1.5A or something else? This is assuming the wire won't be the limiting factor determining what the breaker should be.

Thanks!
Bart

 

[kubark42]

So there needs to be a CB for this device but what should the CB be rated for? Less than the 14V max current of 1.43 or the next larger breaker of 1.5A or something else?

Definitely needs to be more than the 1.43A. Two things to consider: 1) at 9V, which is the minimum operating voltage, the input current will be closer to 2.25A. Secondly, the purpose of the circuit-breaker is to prevent bad short circuits, and the device typically going over the rated current by a little bit is not a "bad short circuit". Their quote of 20W maximum power has some fudge factor, and is intended as a "typical" maximum power and not a "only occurs once every thousand years".

I feel 3A should be a very solid protection whereby you cannot pull so much power from the 12V rail that you risk a fire or other electrical system damage, but at the same time you give enough headroom to allow for deviations from the published spec, both for the Garmin and for the CB itself.

 

[Bartman]

ok, thanks for the reply and the explanation.

 

[Hiperbiper]

3 amp CB. Wire sized for 5 amps @ whatever distance per 43.13.
Chris

 

[Bartman]

Is it safe to say that 22 ga wire is about the smallest you'd want to run through the fuselage or wings for small stuff like nav/strobe lights? 24 ga seems too thin/fragile for that purpose.

Thanks

 

[kubark42]

24ga doesn't strike me as too small for an LED, and if the insulation is strong then the wire will be fine.

What do they use in cars? The automotive industry would be my gold standard for longevity.

 

[Hiperbiper]

The wire graph in AC43.13 will tell you how much amperage a given wire size will allow over a given length. It has mostly to do with line loss and temperature rise all in a handy chart...For your purposes use the line corresponding to open air (unbundled) wires in continuous service should be used.
So figure all the end device power requirements for your circuit. Now, measure the distance between the power supply (CB or fuse) and the device(s) on that line. Leave room for twists, turns and service loops in your final length. A little longer and looped is better than too tight and chafing.
So using the chart, find your voltage (14 vdc), length of the wire run and amps to be available to the device. See where the lines cross to indicate wire size required. If it's close to the max rating it's best to go up a wire size up.
Next, using the power requirements of your radios, intercom, etc. determine the rating of the CB or fuse you need. Say your radio pulls 2 amps during transmission. A 3 amp would work. A 5 amp might be better to deal with a low voltage condition should it occur (as long as the wire you chose is of adequate size).
CB's are always sized to be capable of LESS AMPERAGE than the wires they protect.
The circuit breaker protects the wire not the device itself. The idea is to fault the circuit if it goes directly to ground. If a radio shorts out the CB is there to keep the power wire from overheating and melting.
As a general rule I use 16ga. Tefzel on runs less than 10 feet and 10 amps or less. Even if it's slightly overkill size-wise It's easy to work with and easy to terminate. Smaller gage wire is too easy to nick and lose strands...
Hope this helps.

Chris

 

[kubark42]

Hiperbiper has the right answer, although 16ga doesn't sound like only "slightly" overkill for a 0.25A LED.

According to the Table 11-9 at https://www.steinair.com/wp-content/uploads/2016/03/AC-43.13-1B-Chapter-11-ELECTRICAL-1.pdf, 16ft of 24ga should be a half ohm. This would support up to 1A before reaching the allowable 0.5V drop for continuous operation (Table 11-6).

Be sure you consider your ohmic loss for the ground wire as well. Although if the airframe is ground the return resistance becomes effectively 0.

(Are airframes ever used as ground? Is there a risk for galvanic corrosion if so?)

My feeling for wiring is that it adds surprising amounts of total weight to the plane, and overspec'ed wiring doesn't provide any advantages for the lighting equipment. The LEDs are almost certainly current controlled so they won't run any better or brighter at 14V than they do at 10V, so voltage losses in the wires aren't critical.

However, overspec'ed wire might provide non-obvious benefits, such as better ability to withstand aerobatic forces or easier handling. I've only done wiring for drones, boats, and cars, so any approach born of my experience is only somewhat applicable here.

P.S. No matter what, if you don't already have a good ratcheting crimper, this would probably be the best bang for your buck. Easier and better results. A good used crimper can always be bought and sold eBay, working out to pennies per crimp. https://www.mattmillman.com/info/crimpconnectors/ has suggestions for good crimpers at reasonable prices.

 

[Bob Turner]

Good advice. Next, what about wire strippers? All the fancy ones I have used have a release mode that crumples the newly stripped copper strands.

 

[Hiperbiper]

Actually LED's will vary output with voltage. The one's I linked to can either be dimmed by varying the input voltage with a regular panel reostat (a little notchy but it works) or with a Pulse Width Modulator (PWM) type of dimmer.
On metal (framed) planes the easiest grounds are indeed the frame itself as long as EVERYTHING is commonly grounded...alternator to engine, engine to airframe, airframe to panel, panel to instruments/radios. You always need as much ground potential as you have power potential to each device. By using the airframe as a common ground point you can avoid ground loops that can generate static and all sorts of electrical gremlins.
On Composite airframes (as well as Corvettes) keeping everything electrically level can be a challenge...



A Velocity XL panel I did a few years back...

Chris