Using One Circuit Breaker (or Fuse) for Multiple Devices

In planning the instrument panel I notice the circuit breakers (or fuses. Please do not comment on the difference between these two. It is not the point here.) consume a large area. A significant portion of the area is occupied by the labels.

In one Remos some devices share one circuit breaker. The grouping is:
1: MasterFuse/EMS; 2: Horizon/EFIS/ELT; 3: FuelPump/PositionLight/AntiCollisionLight; 4: LandingLight/InstrumentLight/RPM; 5: Trim/Flap； 6: Starter/Prop; 7: GPS/Comm2/12V; 8: Directional Gyro; 9: EFIS/Horizon; 10: TurnCoordinator/AP; 11: XPDR/Encoder; 12: Comm1/Intercom.

In another Remos, the grouping is different:
1: Master; 2: Starter Relais; 3: Trim/Flap; 4: AntiCollisionLight; 5: LandingLight/PanelLight; 6: PositionLight; 7: Skyview LH; 8: Skyview RH; 9: AutoPilot; 10: ELT; 11: NAV/COM; 12: Intercom; 13: XPDR; 14: Ext. Power; 15: GPS; 16: Propeller.

It seems that some kind of sharing/grouping is inevitable to avoid devoting too much panel surface to circuit breakers. DA40 does not group devices together, and its spending of premium real estate is lavish. SR22 hides the circuit breakers in a place that a co-pilot can not reach.

Some selection of devices to group together makes sense, such as the grouping of XPDR/Encoder, while others seem random to me. For example, fuel pump operation does not seem to be related to position light or anti-collision light, and landing light is not related to RPM. What are the guidelines/principles in the selection other than total-current-should-not-trip-the-breaker? Should every device have a circuit breaker?

Wantobe;

I'm making the assumption that you're building, since you're posting on Homebuilders and you state that you're planning your panel.

The simple answer, if you are homebuilding, is that anything goes. Not the answer you're wanting, I know....

IMOH, ideally, you'd have a single breaker for every device. That way, if one of the "paired" devices fail, you still have use of the other device(s). However, homebuilding is a compromise. While you'd have better assurance that you'd still have use of the remaining devices, at what cost? As in, not just the weight, but the panel real estate, the wiring complexity, the opportunity for gremlins with the added complexity, greater cost, etc. K.I.S.S has a certain degree of merit here.

Next, aside from the obvious note you made about making sure that the current loads are acceptable for each breaker, give some thought on the pairing. I can see why they might pair a fuel pump with lights. For the most part, you'll want the pump during takeoff, when you TYPICALLY won't be using the lights. Pairing items where their use is one-or-the-other helps reduce risk. Of course, if the fuel pump blew the breaker, and it's night, and the lights now don't work as a result, well, you see the gotcha....

I've built and am flying a day/VFR Sonex. I have landing/strobe/position lights, even though I don't intend to fly at night; I view it as nice to have if I land right at dusk, have to refuel, and then taxi back to a dark hangar. Daytime use of strobes also makes sense. I have all 3 lights on one breaker. I also have both my comm and xpndr on one breaker. I carry a handheld as backup, and, well, if I lose the xpnder, I'm typically low enough to not be of concern to DFW airline traffic. My alternator is on a circuit by itself (I have a crowbar over voltage system), and I have a master. That's it. I have a few fuses for things that don't matter.

Lastly, remember that the purpose of a breaker/fuse is not to protect the device, but to protect the wiring. Gotta keep the smoke inside the insulation.

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Originally Posted by CarlOrton

remember that the purpose of a breaker/fuse is not to protect the device, but to protect the wiring. Gotta keep the smoke inside the insulation.

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I can stress this point of Carl's enough. If you are going to group devices on a single breaker, make sure that they have the same guage wire or the breaker should be sized for the smaller wire. Otherwise you are going to let the smoke out.


If you haven't purchased your materials yet, you may want to consider a Vertical Power VP/X Pro. It certainly solves your panel space and label issue. Although it does require one of the supported EFIS for a display.

It is just poor practice to connect more than one device to a single breaker or fuse. That doesn't mean you can't do it, but it does mean you shouldn't do it. The safest and most trouble-free experimental amateur-built airplanes are built as closely to standard aviation practices as they can be. This is one place where you should not cut corners.

Dave Prizio

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Originally Posted by wantobe

What are the guidelines/principles in the selection other than total-current-should-not-trip-the-breaker? Should every device have a circuit breaker?

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For certificated airplanes (probably true for consensus designs as well):

Protective devices are not required in the main starter motor circuit or any circuit where lack of protection would create a hazard.

Each circuit for an essential device requires individual circuit protection. However, each circuit on an essential source does not require individual protection.

So if you're making your electrical system as simple as possible; One bus for power distribution. Essential for flight safety circuits have individual circuit protection. The frills can share a circuit protection device. Even something like trim and flaps can be connected to the bus through a single circuit protection device as neither are 'essential' to flight safety.

The protection device has to be located as close as possible to the power source. So install a bus bar to connect all your protection devices. This means CPD's will all be located next to each other so find a place that allows this but remember the essential CPD's have to be accessible in flight.

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The safest and most trouble-free experimental amateur-built airplanes are built as closely to standard aviation practices as they can be. This is one place where you should not cut corners.

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This should be repeated until it becomes like a mantra. It's kind of the aviation equivalent of "Sic vis pacem, para bellum" (if you wish for peace, prepare for war). Maybe it should be, "If you wish to fly peacefully, treat your design and build process like a war against things that will ruin your flying"?

The first thing to remember here is why you have a breaker. It's not there to protect your devices hooked up to it. It's there to protect your wiring from overheating and causing a fire. You can group a few things on a breaker. However, all devices grouped together should not exceed the capacity of the breaker. And all the wiring down stream of the breaker must be heavy enough to take the rated amperage of the breaker without overheating. If you are going to group devices, you want to group things that make sense to be grouped together, by both priority and utilization. For instance, it wouldn't make sense to put your strobes on the same breaker with your comm radio due to noise considerations, and the fact that should you have a problem with your strobes, you wouldn't want it to also take out your ability to communicate. By contrast, it may make sense to have your landing light and taxi light on the same breaker provided that all wiring downstream of the breaker is heavy enough to carry the rated amperage of the breaker and the breaker is rated to carry the load of both lights.

Often times where homebuilders will get themselves into trouble is adding on devices at a later date. Let's say they built their plane with just a comm radio and transponder and have them both on a 7.5 amp breaker with the correct wiring for that loading. Then at a later date they add a new whiz bang portable GPS. But heck, that GPS only draws .25 Amps, so can be run on 24 gauge wiring with no problems. Since the GPS is a kind of avionic and they want it to come on with the avionics switch they pick up the hot lead after the avionics switch which is on a 7.5 amp breaker. Should the plug short on the GPS, they have just created a fire starter as the 24 gauge wire will overheat and burn without tripping the breaker.

In general, it is easiest to use discrete breakers for individual devices and that will likely keep you out of trouble. However, I think you'll find that even mother Cessna grouped things like landing and taxi lights onto a common breaker.

-CubBuilder

Thanks for all these discussions. Very enlightening. Seems the best solution is to have a circuit breaker for each device, and put the circuit breaker array on the ceiling!

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Seems the best solution is to have a circuit breaker for each device, and put the circuit breaker array on the ceiling!

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Honestly, I'd rather just have one of the Vertical Power systems on my panel because installing things on the ceiling might be more difficult because of less space to work with behind the panel. To each and to their own, but I would also prefer an extra inch or so of headroom if that was the alternative to an overhead breaker panel.

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Originally Posted by wantobe

Thanks for all these discussions. Very enlightening. Seems the best solution is to have a circuit breaker for each device, and put the circuit breaker array on the ceiling!

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Aargh! Overhead CBs led to the invention of tri-focal eyeglasses. I had to locate and pull a pair of CBs on three separate occasions. They were located behind and above my head. Why two CBs? Some systems on certified aircraft are reduntantly powered. Such as by both #1 DC primary and essentail busses. If there are items in the Emergency checklist that require pulling a CB, it should have an identifiying collar in red, yellow, green or white. ACS sells these.

Bob

All the devices have their own significance and while pairing devices together you have to see that you group only those which have almost the same current rating and are not used together during flight, landing or take off. It’s ideal that you don’t group any devices together and but if you have to group them together, make sure that they are not used together, which will obviously break the circuit and also make sure that they are of the same current rating so that you use the correct circuit breaker.