Can I use a 20a toggle switch?

J

JC58

Filing Flight Plan
Joined
Nov 24, 2023
Messages
18
Display Name
Display name:
JC58
I am upgrading a PA28R-200 avionics panel. Looking for feedback if anyone has used a 20A rocker switch (power on) connecting the avionics buss via a 20A CB AKA toggle cb which will be powered from the main buss. The current relay that currently is installed is a normal closed (part#150-905) relay. I need to have the switch powered (power on) to connect to the avionics bus. I understand the need for having backup power for Comm, Nav, and lighting (night flt) should the avionics buss fail. Most likely it is not an issue since the G5's have their own battery I carry a headlamp and a portable radio with an external ant port available. If I go the extra security, I could also add a back up switch(battery essential) to power those items to ensure I've covered the alternator/bus fail switch scenario.

The following are on the avionics Buss-
2 G530W (comm1&2- Nav1) 2-10A cb &1 5A cb
2 G5's -ATT& HSI 2-5A cb
1 turn&bank 5A cb
1 xponder& encoder 3A cb
1 PMA audio panel 5A cb
2 stratus USB- 2-4A cb
1 GAD 5A cb
1 G106A nav 2 5A cb
1 KN 64 dme 5A cb
1 Flt Stream 210 1A cb
1 GFC 500 A/P 7.5 cb
total of 15 cb's - 79.5 Amp's/ 60amp alternator
Can 20A CB carry the load when switching on the rocker after the engine starts and Alternator is powering all the electrics?
My switch panel utilizes all 20A rocker switches and for aesthetics not looking to add a toggle/CB of higher Amp rating unless absolutely necessary. Also, am trying to avoid a relay.
Please share your thoughts on any possible solutions/combinations or suggestions you might have completed with your panel upgrade. Thx.
 
JC58 said:
Please share your thoughts on any possible solutions/combinations or suggestions you might have completed with your panel upgrade.
Click to expand...
Will you be signing this alteration off, or will someone else? Anytime the basic electrical system is altered you can get into the gray area of it being a major alteration which is subjective to the person signing the work.

As to your plan, keep in mind circuit protection and circuit control have different requirements. For example, circuit protection is measured by conductor size/length and elec load, where circuit control is measured by start/continuous service load and voltage. And simply adding up the CB ratings on that circuit won’t get you there from a regulatory standpoint.

This is one reason most OEMs use relays to separately control an avionics buss from the main buss. Its also the route I’ve only used… outside of a simple radio/xsponder/GPS system where I would use a conventional switch/CB set up. Also keep in mind, the use of a relay can provide additional control methods and future-proof the system in case you want to add more devices on the buss.
 
If the normal running load plus a downstream overload trips the 20 amp CB switch disconnecting all the equipment at the same time rather than tripping the faulty circuit I'd say nope.

I'm no EE, but to me the avionics bus should be bussed through a CB switch near the rated capacity of the primary bus, and the feeder cabling or bus bars need to be sized to handled an overload without catching fire.
 
Last edited:
Make sure that switch is good for 20A DC. Many switches have a much lower DC rating than AC.
 
All good responses, as the amp, wiring(AWG size appropriate)'s, CBs, and 20A switches are being updated and nothing else is being changed, it would be considered a minor alteration. Originally there wasn't an av master and it was an aftermarket add-on prior to buying the airplane. So I think we will configure a relay and place it on a cb for extra protection.
 
All switches are 20v dc rated
 
disclaimer - not an a&p or avionics tech. Can't speak to the sign off requirements at all. But I will say that from an engineering perspective, a switch should be rated for the actual load being carried, not the rating of the breakers downstream. Depending on the design, breakers and fuses can be designed to protect equipment or wiring, and are often rated at many times the average current of the device. So to know I'd either turn everything on and measure it, or see if you can find the max current for those devices. Highest will likely be radios while transmitting, and you're unlikely to turn off the avionics while transmitting.

The exception to the above is on inductive loads like motors where the interrupt current can be high, but that wouldn't be the case of avionics.

A 20V DC rating sounds kinda light, to me. For avionics probably not a problem because you're probably not going to see voltage spikes on disconnect, but keep in mind a 12V aircraft system will go as high as 15V in normal operation. From memory, most switches I've seen designed for 12v automotive or marine use were designed to be 48V DC. I might be being overly cautious here, though.

Oh, and you'd still want all those downstream cb's or fuses, so you don't end up with a 20A breaker protecting something that wants a 3A fuse. Should go without saying but don't want to assume.

Airplane electrical folks feel free to make fun of anything I've said that doesn't fit with the way airplane electrical works...
 
Pretty sure that Garmin dictates one of the G5s is supposed to be on the main bus (AI, if I recall correctly).
 
Albany Tom said:
disclaimer - not an a&p or avionics tech. Can't speak to the sign off requirements at all. But I will say that from an engineering perspective, a switch should be rated for the actual load being carried, not the rating of the breakers downstream. Depending on the design, breakers and fuses can be designed to protect equipment or wiring, and are often rated at many times the average current of the device. So to know I'd either turn everything on and measure it, or see if you can find the max current for those devices. Highest will likely be radios while transmitting, and you're unlikely to turn off the avionics while transmitting.

The exception to the above is on inductive loads like motors where the interrupt current can be high, but that wouldn't be the case of avionics.

A 20V DC rating sounds kinda light, to me. For avionics probably not a problem because you're probably not going to see voltage spikes on disconnect, but keep in mind a 12V aircraft system will go as high as 15V in normal operation. From memory, most switches I've seen designed for 12v automotive or marine use were designed to be 48V DC. I might be being overly cautious here, though.

Oh, and you'd still want all those downstream cb's or fuses, so you don't end up with a 20A breaker protecting something that wants a 3A fuse. Should go without saying but don't want to assume.

Airplane electrical folks feel free to make fun of anything I've said that doesn't fit with the way airplane electrical works...
Click to expand...
I agree that your insight is accurate and sound. I plan on ensure all the wire, cb, and switches are compatable and protected. Most likely will intall a backup emergency switch. Thanks for the feedback
 
We have the battery backup at the units installed but I'll recheck the install manual and call garmin to confirm , thanks for the tip.
 
I currently have a switch that powers a relay for avionics. It's doing some odd things, and since I'm getting new avionics in a few months, I'm having it swapped out for a breaker. I will go from a switched ground to essentially a switched power, using the breaker as the switch. (This was recommended by the avionics guy).
 
My plane has a relay for the avionics master but it also has a 25A circuit braker that is normally pulled across that in case the relay fails for some reason. You can push it in and get the bus back on line.
 
Just checked install manual , and the g5 will need to be powered from the main buss
flyingron said:
My plane has a relay for the avionics master but it also has a 25A circuit braker that is normally pulled across that in case the relay fails for some reason. You can push it in and get the bus back on line.
Click to expand...
Clever way to back up the buss, I'll most like put in a backup toggle cb switch wired to the essential bus and add on some other accessories
 
Just checked the install manual and you are correct. I'll put the G5 on the main bus.Glad you mentioned it.
 
Albany Tom said:
But I will say that from an engineering perspective,
Click to expand...
All the basics are the same. Its only the specifics that change with aviation electrical systems. For example, materials and components must meet certain standards like Mil-Spec or if you use fuses for circuit protection, how many spare fuses must be stored in the cockpit. I wouldn't think any EE would have a problem with aviation systems especially since the regulatory requirements are spelled out by complexity of aircraft (Part 23 vs 25) and are backed up by various guidance references.
 
Circuit breakers protect wire not equipment. They ideally trip before a wire shorted to ground can become hot enough to become a fire hazard. So the smallest wire directly fed by any breaker is the one that determines the required rating of the circuit breaker. In a well designed system there is a breaker every time the wire size steps down. I have seen circuit breaker confusion over and over. The operation of a circuit breakers is pretty simple. That does not mean your average person is capable of designing a safe power circuit. My recommendation of a reference would be AC 43.13-1B Ch 11. If it's a small plane the 43.13 might be acceptable data.
 
Last edited:
Good clarification. My new breaker “switch” would feed the individual pop out circuit breakers that are currently installed. I have a breaker for the audio panel, other one for the GNC 355, etc.

This new master breaker would replace the relay I currently have.
 
kontiki said:
Circuit breakers protect wire not equipment. They ideally trip before a wire shorted to ground can become hot enough to become a fire hazard. So the smallest wire directly fed by any breaker is the one that determines the required rating of the circuit breaker. In a well designed system there is a breaker every time the wire size steps down. I have seen circuit breaker confusion over and over. The operation of a circuit breakers is pretty simple. That does not mean your average person is capable of designing a safe power circuit. My recommendation of a reference would be AC 43.13-1B Ch 11. If it's a small plane the 43.13 might be acceptable data.
Click to expand...
That's absolutely true for power distribution systems in the US, from house wiring all the way up to a steel mill. But I didn't know if that applied to aircraft systems, thanks. For good or bad, I have seem some product specifications, not aircraft, where the manufacturer required a specific size and type of fuse or breaker for the supply. To me, that's lousy practice - the device itself should have its own current limiting at the power input, but I've seen it anyway.
 
Checked the install manual and G5 is on the main bus as is JPI 900.
 
Albany Tom said:
That's absolutely true for power distribution systems in the US, from house wiring all the way up to a steel mill. But I didn't know if that applied to aircraft systems, thanks. For good or bad, I have seem some product specifications, not aircraft, where the manufacturer required a specific size and type of fuse or breaker for the supply. To me, that's lousy practice - the device itself should have its own current limiting at the power input, but I've seen it anyway.
Click to expand...

I gave you a reference that was published by the FAA specifically for mechanics working on small airplanes. Here is a second reference written specifically for aviation.

Aerospace Recommended Practice (ARP) ARP1199B, Application, and Inspection of Electric Overcurrent Protective Devices. There are others.​

Go look in the 4313 CH 11, it tells you what aircraft circuit breakers do.​

I have seen STCs that call out a breaker size without specifying a wire size. I've seen STC data with other mistakes. I expect the fault was with the DER (or equivalent) that recommended FAA aproval. If you wish to ignore the physical laws of nature good luck.

Text is only bold because this ipad is a PIA.

​

​

 
Albany Tom said:
where the manufacturer required a specific size and type of fuse or breaker for the supply.
Click to expand...
On the aviation side, one of the main reasons you may see a device/equipment OEM call out a maximum breaker/fuse size or a minimum wire size, or both in some cases, is that they do not control the final installation of the item.

So its up to the installer to ensure those limitations are taken into to consideration for the install. This is especially important when the device/equipment is an upgrade to an existing system where the breaker/wire is not sized properly for the new install requirements.
 
kontiki said:
I gave you a reference that was published by the FAA specifically for mechanics working on small airplanes. Here is a second reference written specifically for aviation.

Aerospace Recommended Practice (ARP) ARP1199B, Application, and Inspection of Electric Overcurrent Protective Devices. There are others.​

Go look in the 4313 CH 11, it tells you what aircraft circuit breakers do.​

I have seen STCs that call out a breaker size without specifying a wire size. I've seen STC data with other mistakes. I expect the fault was with the DER (or equivalent) that recommended FAA aproval. If you wish to ignore the physical laws of nature good luck.

Text is only bold because this ipad is a PIA.

​

​

Click to expand...
solid reference and sound practice as most stc do provide appropriate awg and breaker sizes . However, if its legacy oem equipment or accessories, that information is clearly marked in the maintenance manual. The hard part is being the 3rd or 4th owner and making upgrades only to find somewhere along the way , someone has done some sort of non standard modification and it was never picked up during prebuy inspection. Most of the work completed was a result of a deep dive annual so the panel upgrade resulted in a complete change over of many items to ensure and comply with AC 43. It helps to have a good team to review many different ideas on how to rework, overhaul, and replace items needing a fix but not required because of cost eg service bulletins. The lesson I've learned is everything on the plane can be done to a higher standard, just open up your checkbook because old airplanes need a lot of maintenance to bring them up to a higher safety standard. The standard is at least spelled out throughout AC43 and sometimes not so much with vendors trying to sell cool stuff.
 
JC58 said:
The hard part is being the 3rd or 4th owner and making upgrades only to find somewhere along the way , someone has done some sort of non standard modification and it was never picked up during prebuy inspection.
Click to expand...
I recently posted an opinon on prebuy inspections in another thread. It's too late after you bought it.

With wiring, I see it as acceptable or unsat.

I had similar issues when I bought 13 years ago. The Gen field breaker was in wrong hole, 3 breakers had no load. The power side terminal on those breakers was used as an attachment point for wires that went elsewhere because there was no real power busbar.. One of those wires went to an avionics master switch that was installed when aircraft was owned by seller. All of that wiring was unsat. I know the seller lied about the master because the switch was mounted in a hole vacated by a mod he had records for. There was a breaker that drew it's power from the amp meter! There was a lot of wiring with cracked insulation. Some shops don't consider that a problem. I think it is. The power bus was part strip of dead soft aluminum, part large double staked wires. I just got a piece of copperand made a bus, it's not rocket science.

I knew how or could figure out how to address all the issues. The airplane had 4 or 5 STCs incorporated. There were log entries for them. About 3 had no installation data, and no record documents in the FAA cd. I did find the guy that did the work and got missing 337s filed. I never wanted a project airplane, I just wanted to fly. I refuse to give any more money to the shop that seemed to have no ability do see the problems. It was a shop associated with a flight school. It's gone now anyway. I have a very low opinion of the seller who I consider dishonest.

The root cause was in reality the fact that I did not handle the purchase well. Annual inspection requirements are wery well documented. Good Pre-buy inspection requirements are not described at all really. If I ever buy again, everyone will have a detailed inspection plan I write. Good luck.
 
Last edited:
I've got a fairly low avionics current draw and monitor my current closely. It will sometimes go as high as 20 amps. When I did what you are looking to do, I felt more comfortable with a 40 amp switch. To tie the two busses together, I used a SPST Longacre 45423, 40 amp toggle and protected it with an MS2522 Switchguard Cover. It is off to the right (avionics bypass). Schematic follows.

bp.jpg

ab.jpeg
 
JohnAJohnson said:
I've got a fairly low avionics current draw and monitor my current closely. It will sometimes go as high as 20 amps. When I did what you are looking to do, I felt more comfortable with a 40 amp switch. To tie the two busses together, I used a SPST Longacre 45423, 40 amp toggle and protected it with an MS2522 Switchguard Cover. It is off to the right (avionics bypass). Schematic follows.

View attachment 133713

View attachment 133712
Click to expand...
I like this backup technique, I was planning on using a toggle cb as an essential bus connect, so your diagram is really helpful.
Thank you for sharing it.
 
JohnAJohnson said:
I've got a fairly low avionics current draw and monitor my current closely. It will sometimes go as high as 20 amps. When I did what you are looking to do, I felt more comfortable with a 40 amp switch. To tie the two busses together, I used a SPST Longacre 45423, 40 amp toggle and protected it with an MS2522 Switchguard Cover. It is off to the right (avionics bypass). Schematic follows.

View attachment 133713

View attachment 133712
Click to expand...
I like this design, but...maybe I'm missing something. What keeps the feed from the diode when the external receptacle is plugged in from flowing to the main bus after the ammeter, if the avionics switch is in the off position, other than the voltage drop of the diode?
 
Albany Tom said:
I like this design, but...maybe I'm missing something. What keeps the feed from the diode when the external receptacle is plugged in from flowing to the main bus after the ammeter, if the avionics switch is in the off position, other than the voltage drop of the diode?
Click to expand...
Tom, you're right. The Avionics Switch in the OFF position will allow two paths to the Primary Bus. The first will be directly from the External Source and whatever the shunt drops, and the second will be through the diode and Avionics Switch. So Kirchhoff’s rule of parallel resistance (overall resistance will be lower) would apply.

I've got my Dynon monitoring voltage at the shunt and at the Avionics Bus. I'll have a look at the differences with the Avionics Switch in both positions and let you know what I find.
 
You must log in or register to reply here.
Back
Top