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MTBehnke

Joined: 13 Sep 2007
Posts: 3

Posted: Thu Sep 13, 2007 6:37 pm Post subject: Design Questions

I have a few questions as I'm trying to work through my electrical design.  I've looked through the Aeroelectric book and other sources, but can't seem to find answers to the following:



1.  Is there some design guidelines for minimum fuse sizes to avoid nuisance trips?  I've seen a couple of people mention keeping current below 80% of the fuse size.  I also saw one recommendation on keeping below 50% for radios based on transmitting current draw.



2.  I have the Creativair strobe power supply.  I noticed that the strobe kit came with 22 ga. shielded wire to go from the power supply to the strobes.  The installation instructions give Joules per flash, but nothing about voltage, current, etc.  I don't have any way to validate whether 22 ga. is okay, especially as it's round trip distances.  Should I just assume it's fine?



3.  This may be overly nit-picky, but do you generally use 12 or 14 volts when calculating loads?  For example, I have the Duckworks 100W landing light.  Using 12V gives 8.3A versus 7.2A using 14V.  On top of that, a 5% drop of 12V is 0.6V compared to 0.7V for 14V.  I'm not sure if this would result in any different wire or fuse sizes.



Thanks,

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_________________
Mike Behnke
RV-9A Flying
RV-10 Fuselage
Andover, MN

klehman(at)albedo.net
Guest

Posted: Sat Sep 15, 2007 5:46 am Post subject: Design Questions

MTBehnke wrote:



 	  Quote: 	
		  



I have a few questions as I'm trying to work through my electrical design.  I've looked through the Aeroelectric book and other sources, but can't seem to find answers to the following:



1.  Is there some design guidelines for minimum fuse sizes to avoid nuisance trips?  I've seen a couple of people mention keeping current below 80% of the fuse size.  I also saw one recommendation on keeping below 50% for radios based on transmitting current draw.

  



Rule one is to size the fuse to protect the wire. ie an 18awg wire gets 
	


a 10 amp fuse,  22 awg gets 5 amps etc.  Or use the manufacturers 

recommendation. Some things like landing lights draw momentary high 

current while the filament is heating up which can be a consideration 

but in practice refer to rule one and all should be fine. Some motors 

and solenoids may draw considerable extra current while starting but 

again you fuse to protect the wire from overheating in case the motor 

shorts or seizes. In extreme cases you'd use a circuit breaker or slow 

blow fuse but I think that is rare for us here.  I guess I do have a few 

fuses smaller than rule one would suggest but only because awg22 wire is 

the smallest that I purchased or because the wire was oversized to keep 

the voltage drop low.  Since wire and fuses only come in fixed sizes you 

generally will have adequate margin without any conscious effort. A 

single failed fuse or circuit should not be any cause for great concern 

anyway with a good design.



 	  Quote: 	
		  2.  I have the Creativair strobe power supply.  I noticed that the strobe kit came with 22 ga. shielded wire to go from the power supply to the strobes.  The installation instructions give Joules per flash, but nothing about voltage, current, etc.  I don't have any way to validate whether 22 ga. is okay, especially as it's round trip distances.  Should I just assume it's fine?

  



I'd assume it is fine.  Strobes run on several hundred volts and short 
	


bursts of current so it is intermittant duty (the wire does not 

continuously conduct current).



 	  Quote: 	
		  3.  This may be overly nit-picky, but do you generally use 12 or 14 volts when calculating loads?  For example, I have the Duckworks 100W landing light.  Using 12V gives 8.3A versus 7.2A using 14V.  On top of that, a 5% drop of 12V is 0.6V compared to 0.7V for 14V.  I'm not sure if this would result in any different wire or fuse sizes.



I use 14 volts but if it is close enough to make a difference I would 
	


just upsize the wire anyway in most cases.



Ken



 	  Quote: 	
		  

Thanks,



--------

Mike Behnke

RV-9A Fuselage

Andover, MN





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nuckolls.bob(at)cox.net
Guest

Posted: Sat Sep 15, 2007 6:16 am Post subject: Design Questions

At 07:37 PM 9/13/2007 -0700, you wrote:



 	  Quote: 	
		  



I have a few questions as I'm trying to work through my electrical 

design.  I've looked through the Aeroelectric book and other sources, but 

can't seem to find answers to the following:



1.  Is there some design guidelines for minimum fuse sizes to avoid 

nuisance trips?  I've seen a couple of people mention keeping current 

below 80% of the fuse size.  I also saw one recommendation on keeping 

below 50% for radios based on transmitting current draw.


	


    Ken has already offered some good response to this. I

    would only add that any advise you get for de-rating fuses is not

    based on any real science. Every fuse is designed to carry

    100% of it's rated value indefinitely. See:



http://www.aeroelectric.com/Mfgr_Data/Fuses_and_Current_Limiters/Bussman/ATC_Specs.pdf



    In some cases (for VERY robust fuses called current

    limiters) they'll carry twice their rated current.

    See:



http://www.aeroelectric.com/Mfgr_Data/Fuses_and_Current_Limiters/Bussman/ANL_Specs.pdf



    You see, the manufacturer has already de-rated the

    product.



    Bob . . .

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