SwimmingDragonfly96 said:
I'll have a 10", a 7", fly LEDs, a GNX375, pitot heat, and seat heaters in a 7A. I currently have a 40A alternator, should I upgrade to a 60A?
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I had a similar question during my build: Do I save money+weight and get a 40A alternator, or go with the 60A? I ended up listing out all of my devices that consumed electric power, came up with a few scenarios, like "taxi" and "takeoff" and "VFR cruise" and simply added up their average current loads.

My configuration is not exactly like yours, but I determined that a 40A would be sufficient, and now that I'm flying, my measured loads are consistently less than what my calculations predicted. For example, my typical day takeoff/climb with fuel pump and lights on but no pitot heat, draws 22 amps (vs. 29 predicted).
 
SwimmingDragonfly96 said:
I'll have a 10", a 7", fly LEDs, a GNX375, pitot heat, and seat heaters in a 7A. I currently have a 40A alternator, should I upgrade to a 60A?
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Since you already have a 40A which should be plenty sufficient for your equipment, especially that seldom all are on at the same time, it might be both cheaper and wiser to get a back up alternator. MKZ is a 30A generator that can be on the standby and will kick in anytime more juice is demanded that the alternator can not support. This way you have the extra Amp as well as a backup just in case. If you are not familiar with it, look up Monkworkz generator.
 
The good people at B&C Specialties recommended to me to size my alternator to carry a maximum of 80% of it rated load. So, a 40 amp alternator should do very well carrying
32 amps all day!

Chuck
RV-6XXL
 
If you turn everything on for a while and the voltage remains steady, then your alternator is sized to handle that load. If the alternator can't keep up, the voltage will steadily decline.

This assumes you're flying now. If you're building: another poster had the right approach which is to make a list of all the loads and their respective current draw, add them up, and use the 80% of alternator capacity rule.
 
A little bit more info may help.
IFR I assume? Single or dual bus?
I have a 2x GDU setup probably not to dissimilar to you. It’s a z14 dual buss with a 60A LX60 and B&C vac pad BC462.
If I turn everything in the plane on at once across both busses I can get to about 48A all told. Most of the time it sits at 37A in the cruise across 2 alternators. 20A/17A

So I can survive either of the alternators failing with no to minimal load shedding.
Mostly the LX60 never sees more than around 20-25A.
I also thought about an L40 at the time but I figured the 1lb and $200 was worth it to have an alternator that never does too much work.
If I had a 40A I'm sure it’d be fine too.
B&C alternators are awesome. 100% reliability - 4 alternators over 3RVs and 20y.
 
Heaters are current eaters so if you plan on using those items, I would recommend the 60.
 
I have seat heaters and an Inogen oxygen concentrator. I went with a 60 amp and happy with my choice. I have a 9A with 0 320 and Catto so a little more weight on the nose helps with CG.
 
CT Brietigam said:
The good people at B&C Specialties recommended to me to size my alternator to carry a maximum of 80% of it rated load. So, a 40 amp alternator should do very well carrying
32 amps all day!

Chuck
RV-6XXL
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Wasn’t part of your conversation obviously but just to be clear, the 80% rated load is for continuous operations. Excursions into this 20% capacity “margin” are fine. If B&C said otherwise, please share.
 
At 14.4V out of the LX60/LR3D, my dual 10"/dual GTN setup will pull ~15A continuous (pitot heat off, radio not transmitting) once the battery is charged, but I've seen alternator current as high as 45A for several minutes at the beginning of flights where the battery (ETX900) starts from a low SoC. I'm glad I went to 60A.

I frequently see charging load on the main battery missed in power analyses for light aircraft. This load can vary considerably depending on the battery state of charge, chemistry, and temperature, and is generally pretty unconstrained since we don't tend to regulate battery current in RVs - the battery will just suck whatever it can from the voltage-regulated alternator output. Depending on how your electrical system limits current out of the alternator (fuse, breaker, or more exotic configurations), there may be no straightforward way to successfully recycle the alternator since your only mechanism to reduce charge current other than to 0 is to lower engine RPM.
 
We run dual g3x, 375, 2 radios, SDS, dual heated seats, etc. Average current draw is right around 20 amps. depending if we have both fuel pumps on with both seat heaters going and all the lights, we'll see up to 30. We don't use the pitot heat much, but I have seen that draw up to 10 amps in bad weather. We have peaked over 50 right after start with a battery that was sitting for a bit. I think a 40 would suffice, but we went 60/30 with our primary and backups.
 
Freemasm said:
Wasn’t part of your conversation obviously but just to be clear, the 80% rated load is for continuous operations. Excursions into this 20% capacity “margin” are fine. If B&C said otherwise, please share.
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I took B&C's recommendation to mean every electrical appliance to be on and operating in a continuously fashion. Battery recharging amperage was never discussed.
 
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