Sum Breaker Values >> 70 amp alternator rating?

[gtmize]

Obviously I'm missing something. But I summed values on Essential & Main breaker values ( excepting Batt, Ties, alternator) and get over 100 amps.

The electrical diagrams I can find are at the bus level.

Just wondering

Thanks
Gary

[DaveS1900]

The individual CB's for components are there to protect the wire and items on that circuit. They are the maximum amps that circuit is allowed to take, not what is normally takes. There is always a buffer. Its the same way at your house. If you add up the individual CB's they will be over the main breaker amp rating. For example a radio may take 2amps to run, but the CB is 4amps.
Dave S.

[gtmize]

Sure. However it Would be helpful to see amps By component for larger loads in case u lose the alternator and need to shed load. Sure you can Go to the essential bus but u may want a more grainular approach

[Boatguy]

Yes, you are missing something. The sum of the circuit breakers is as irrelevant as the sum of the tire pressures.

A circuit breaker exists only to protect a wire and represents the maximum safe current for that wire. It literally has nothing to do with anything else. All it represents is the capacity of the wire used to feed the device.

If you want to know the actual consumption of each device, which I agree is useful information, turn them on/off individually and note the change in output of the alternator.

[jast]

Even though it is not completely wrong, it is also not completely right either. Usually you would rate the fuse of a device by knowing the normal power and peak power consumption and adding some buffer for the fuse/circuit braker. Therefore it is fair to assume that some device with a 20A fuse needs more power than a 5A fuse. Exceptions would be something which needs 20A for startup for a few minutes (something with eg a heater) but only 2A to run. The cable thickness is then rated based on the fuse so that it doesn’t create a fire.

Btw: Usually the alternator alone can not provide peak currents. It needs some battery as buffer. (See the famous early DA42NG crash in Speyer/Germany)

[Rich]

Sure. However it Would be helpful to see amps By component for larger loads in case u lose the alternator and need to shed load. Sure you can Go to the essential bus but u may want a more grainular approach

I agree. I'm not entirely sure the selection of what is on the essential bus (and what is not) would be what I would choose to have/lose in a given alternator failure situation. Day, Night, IFR, IMC, VFR all would affect my choices. It's possible I'd leave everything running. Mostly the big loads (fuel pump, pitot heat, landing/taxi lights) are conditional and/or intermittent .

For a lot of the devices you can look up the current draw on the appropriate web site or in the various manuals. For others perhaps test the current draw by turning off the various units. Simply turning off the 430W would probably yield the largest benefit (2 amps? I haven't tested this). Pulling the Attitude Indicator breaker would save roughly another amp. Many of the individual components have minimal draw. For example: The KT74 draws .5 amp. (This is my ADS-B out box so I probably wouldn't even bother)

Over the years I have determined that, for my plane, normal draw in daytime is about 8 amps. I could lose 2-3 of those fairly easily, with minimal effect and not using the essential bus switch.

With 28 volts that 70 amps goes a long way. At 14 volts every device draws twice the amps to operate.

[Rick]

On my DA40 electrical diagram, I have a table of 'typical load' for each CB, which I have attached. I'm not sure where I got the numbers from way back when, and if anyone has better values for items in this table, I'd like to know what they are.

DA40 Electrical Load

Note this is for my 2007-vintage DA40 with a GFC700, and I also just noticed it was from before I replaced my position and strobe lights with LEDs, so those number should be much lower for my bird now... But it is one data point, if anyone is interested.

[gtmize]

Perfect thanks