Radio circuit breaker

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Lawson Laslo

Pre-takeoff checklist
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Ive got a bendix king ky97a radio, it’s wired to a 10 amp circuit breaker and was wondering if that was a too big breaker and needed something lower amp, couldn’t find anything in manual
What amp breaker do you guys use?
Thanks
 
I have a Icom A200 which is basicly a Bendix-King clone. The manual for my radio specifies a 10 amp breaker.
 
asicer said:
Says 6A during transmit. Add in some margin and 7.5A breaker ought to do it, but 10A is close enough IMHO.
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The breaker's job is to protect the aircraft's wiring from overheating, not to protect the radio. But if protecting the radio, I'd go with 10-amp anyway. A 7.5 is cutting it a little close, especially as the breaker ages.
 
Well, that's true of house wiring, in an aircraft it's to stop excessive loads. While it may not PROTECT the radio itself, it protects the rest of the aircraft from the radio drawing too much power.
 
Lawson Laslo said:
What amp breaker do you guys use?
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The 97A install manual calls for a 10amp. Normally the equipment mfg determines the CB rating. And as mentioned above the CB is for wire protection vs radio protection. Most equipment that "needs" protection will have it internally installed. But if you ever want to verify CB/wire requirements look to AC 43.13-1B Ch. 11 for determining size and rating--and always round up.;)
https://hangar603.org/wp-content/uploads/2018/02/wacoownman-13-ky97ainstallationmanualocr.pdf
 
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flyingron said:
Well, that's true of house wiring, in an aircraft it's to stop excessive loads. While it may not PROTECT the radio itself, it protects the rest of the aircraft from the radio drawing too much power.
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No, it's to protect the wiring in case of a short somewhere that would cause the wire to get hot. Same as in your house. The rest of the airplane doesn't care what the radio draws as long as the alternator can keep up, which is what electrical load analyses are for.

AC43.13-1B has the charts that specify wire sizes for max current draws. A condensed version of the information, including wire size, corresponding breaker size, and electrical loading, is in Chapter 11 of AC65-15A. See especially page 458.
https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_65-15A.pdf
 
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There's a reason we refer to amps as a draw and not a push. You have a bigger breaker on the circuit than what's drawing on the circuit, whether it is your house or your plane. And you have wire that can handle more amps than what the breaker is. If you had a 6A breaker for a 6A appliance the breaker would pop everytime you used it. Ok maybe not every time, but whenever it was at full power.
 
While the AC (wisely) gives a wire size for the circuit, I stand by my statement. While this is from Part 25, the gist is true for light aircraft as well:

Automatic protective devices must be used to minimize distress to the electrical system and hazard to the airplane in the event of wiring faults or serious malfunction of the system or connected equipment.

Similar wording appears in the normal category rotorcraft. If you read installation manuals for approved avionics, you'll find they usually mandate a breaker size no matter what size wire you use.
 
flyingron said:
While the AC (wisely) gives a wire size for the circuit, I stand by my statement. While this is from Part 25, the gist is true for light aircraft as well:

Automatic protective devices must be used to minimize distress to the electrical system and hazard to the airplane in the event of wiring faults or serious malfunction of the system or connected equipment.

Similar wording appears in the normal category rotorcraft. If you read installation manuals for approved avionics, you'll find they usually mandate a breaker size no matter what size wire you use.
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Sure, a short in the wire to ground could drop the system voltage below what many radios need to stay alive, but the real and imminent hazard is an inflight electrical fire. Fires tend to kill much more surely than a dead navcom, and they usually destroy much of the other wiring along with the overheated wire in the bundle. It can take out a lot of stuff.

The avionics manufacturers specify a breaker size but you have to select a wire size that will safely carry that current. You're not going to use a 10-amp breaker and run a 22-gauge wire between it and the radio.
 
I didn't freaking say that. The AC says you hae to use suitably large wire. However, the wire size is NOT the only determining factor for the breaker.
 
flyingron said:
I didn't freaking say that. The AC says you hae to use suitably large wire. However, the wire size is NOT the only determining factor for the breaker.
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It kind of is. You choose the size of the wire based on the current needs and draw of the device it's supplying and the length of the run if voltage drop is a consideration, and round up as othrs have said. Then, the size of the breaker is chosen to protect the wire, and you round DOWN there..you don't want the breaker rated higher than the wire it's protecting. When appliances specify a breaker rating, they're really telling about the maximum current draw of the appliance...it's not that they require a 10a breaker for instance, but rather a circuit that will supply th appliance with at least 10a. You could supply a radio with 2 gauge wire on a 50a breaker if you wanted to, and it would be perfectly happy. You could also supply it with the same 2 gauge wire and a 10a breaker (assuming the radio specified a 10a circuit breaker). The 2 gauge cable would now be overly protected by the 10a breaker. HOWEVER, you could not supply the radio with the specified 10a breaker and 26 gauge wire... the wire would be undersized for that amount of current and it would melt and become a fire hazard BEFORE the breaker's protection would kick in.

When I started learning about this stuff years ago, the idea of "protecting the wire" seemed stupid...wire is cheap, right? We should be protecting the equipment, right? Well...indirectly, we are...and our plane/car/boat/house...and ourselves... from fires and very bad things... by protecting the wire.

Choose the appropriate wire size based on current to be carried and voltage drop over length of run. Round up. Choose the breaker sized based on that current and round down, or whatever is specified by the appliance, whichever is smaller. If what is specified by the appliance is larger, you chose or calculated your wire incorrectly. Under no circumstances use a breaker rated higher than the amount of current the wire it is protecting can carry safely.
 
No, you choose the wire based on the breaker, but the breaker is determined by the expected load. The breaker is there to prevent excessive currents be it either the wire or the load. The breaker isn't protecting the wire or load directly, but the aircraft from the results of more current than designed. It makes little difference if it's the wire or your radio that bursts into flames.
 
flyingron said:
No, you choose the wire based on the breaker, but the breaker is determined by the expected load. The breaker is there to prevent excessive currents be it either the wire or the load. The breaker isn't protecting the wire or load directly, but the aircraft from the results of more current than designed. It makes little difference if it's the wire or your radio that bursts into flames.
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Semantics. The breaker is protecting the airplane and its occupants by shutting off the circuit before something burns up.
 
flyingron said:
While this is from Part 25, the gist is true for light aircraft as well: Automatic protective devices must be used to minimize distress to the electrical system and hazard to the airplane in the event of wiring faults or serious malfunction of the system or connected equipment.
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flyingron said:
The breaker isn't protecting the wire or load directly,
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FWIW: in a general design description this is true, but in the actual certification requirement it is solely based on the specific circuit protection rating and wire size regardless of aircraft as a whole. So the CB's direct function is to protect the wiring which follows most guidance and Part 25.1717:
§25.1717 Circuit protective devices: EWIS.
Electrical wires and cables must be designed and installed so they are compatible with the circuit protection devices required by §25.1357, so that a fire or smoke hazard cannot be created under temporary or continuous fault conditions.
https://www.ecfr.gov/cgi-bin/text-i...453e25c&mc=true&node=se14.1.25_11717&rgn=div8

 
Ron your wrong. Breakers protect wires. Load analysis during design determines what gets put on the system, and how that system is designed. When designing an aircraft electrical system you decide what systems you need, the power required then design around that.
 
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