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Two Circuits on One Fuse?

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bobair8(at)msn.com
Guest

Posted: Thu May 24, 2007 3:50 pm Post subject: Two Circuits on One Fuse?

<?xml:namespace prefix="v" /><?xml:namespace prefix="o" /><![endif]--> I'm following the AeroElectric Connection philosophy and using fuses.   I have 29 circuits but only 24 tabs on my fuse block.  What is  the "best we know how to do" in this situation?  I'd like to think that  crimping two circuits into one Faston is OK but what size fuse should I use and  should I match the relative current draw or pick a higher current draw circuit  to pair with a lower current draw circuit?  I intend not to pair similar  items such as radio and transponder so that one blown fuse doesn't take down all  my avionics.  

  

 Thanks,

  

 Bob

 GlaStar 

 N248BF

   [quote][b]

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khorton01(at)rogers.com
Guest

Posted: Thu May 24, 2007 4:30 pm Post subject: Two Circuits on One Fuse?

On 24 May 2007, at 19:46, r falstad wrote:



 	  Quote: 	
		   I'm following the AeroElectric Connection philosophy and using  

 fuses.  I have 29 circuits but only 24 tabs on my fuse block.  What  

 is the "best we know how to do" in this situation?  I'd like to  

 think that crimping two circuits into one Faston is OK but what  

 size fuse should I use and should I match the relative current draw  

 or pick a higher current draw circuit to pair with a lower current  

 draw circuit?  I intend not to pair similar items such as radio and  

 transponder so that one blown fuse doesn't take down all my avionics.


	


The fuse needs to be big enough to comfortably handle the worst case  

total load that could be produced by both devices.  Then, all wires  

fed by that fuse need to be big enough to handle the fuse rating  

without overheating.  This is because a wiring fault in a single wire  

could put as much current as the fuse will deliver without blowing  

through that single wire.



Kevin Horton         RV-8 (finishing kit)

Ottawa, Canada

http://www.kilohotel.com/rv8

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klehman(at)albedo.net
Guest

Posted: Thu May 24, 2007 5:10 pm Post subject: Two Circuits on One Fuse?

The fuse should be sized to protect the smallest awg wire that it 

supplies. I have several circuits where a 10 amp fuse feeds two 18 awg 

wires. The total current draw is less than 10 amps. If you pick a larger 

fuse it can allow the wire to overheat and will not provide the intended 

protection.

Ken



r falstad wrote:



 	  Quote: 	
		   I'm following the AeroElectric Connection philosophy and using fuses.  

 I have 29 circuits but only 24 tabs on my fuse block.  What is the 

 "best we know how to do" in this situation?  I'd like to think that 

 crimping two circuits into one Faston is OK but what size fuse should 

 I use and should I match the relative current draw or pick a higher 

 current draw circuit to pair with a lower current draw circuit?  I 

 intend not to pair similar items such as radio and transponder so that 

 one blown fuse doesn't take down all my avionics. 

  

 Thanks,

  

 Bob

 GlaStar

 N248BF

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gmcjetpilot

Joined: 04 Nov 2006
Posts: 170

Posted: Fri May 25, 2007 6:50 am Post subject: Two Circuits on One Fuse?

Dear Bobair8:

   

  Yes you can do it and your house does it all over the place. In 

fact you could gang more than two if careful.

   

  Before Bob's concept of using modern automotive fuses, most 

planes ganged circuits on one protection device, be it fuse or 

circuit breaker all the time. How many small Cessna's have 30 

fuses or CB's. Its a nice luxury to have an individual "fuse" for 

every circuit and has obvious advantages, like when one blows 

you lose one item. 

   

  However from a safety stand point you can "gang" circuits on 

a fuse and it will be as safe and more practical with some 

precautions. (Practical means less fuses than the space 

shuttle.)

  

SIMPLICITY (examples)

   

  You could put in ONLY 10 amp fuses in for all circuits and run 

as many 18 awg wires and circuits you like, 1, 2 or 3 or more, if   

the total is under 10 amps. This is what many planes do. So 

you are just putting in bigger but fewer fuses or CB's. 

  

***********************************************

However it is more than the current draw of the items you must 

look at. You have to look at wire protection, which is the real 

reason for the fuse or CB.

  

 

  You want (must) make sure the fuse does not exceed the 

wires capability.

  ***********************************************

  

EXAMPLE:

   

  Say you have 3 items on one fuse and they are each 3.3 amp 

with a short wire run. So 22 gage wire is enough for each 

individual 3.3 amp run. 

   

  (Note: For electrical, 20 awg is a nominal min gauge just for 

simplicity and durability. If wiring a large aircraft you would 

use 22, 24 and even  26 awg to save weight. For data wiring, 

like a transponder  22 awg is fine, making those busy plugs 

easier to manage. All my humble opinion, pls no hate mail.) 

   

  So a +10 amp fuse would be fine for all three, but you might 

go with say the next size up, a 15 amp fuse. Lets say one wire 

shorted and the other items where off or drawing min current. 

   

  A 22 gage wire would get too hot before fuse blowing a 15 amp 

fuse for my comfort (190F?). Now a 22 gauge wire will actually 

fuse at 41 amps, but lets say its a "soft short" and the wire 

is getting hot but the short is not great enough for the fuse 

to blow? You will smell it and 190F might ignite something.

  

Rule of thumb: 22 awg would use 7 amp, 20 awg = 11 amps 

and 18 awg = 16 amp fuses for max chassis wiring. If circuit is 

for continuous voltage transmission over long distance, the 

wire's current ratings are much lower than above "max.

   

  Again  this is all about wire protection than second total current.

   

  The solution is very simple go to 18 awg wire to handle the 16A 

 load, so each wire at fuse rating is safe, not just the individual 

load. Clearly 18 awg is over kill for 3.3 amps over a short length 

 but perfect for 15 amp fuse. Personally I would pick smaller loads 

than 3.3 amps to gang and of course pick non "critical" items.

   

  

It's better to match similar ratings together of course, 

eg, don't match 8 amps with a bunch of 1/2 amp circuits.

   

  There is nothing keeping you from putting 3 or 4 (one) amp items 

on a 5 amp fuse with 20 awg wire. However I would knock it off 

at a gang of 3 my self, but there is no rule, just common sense.

   

  CAUTION: Be sure you can lose all items in one shot from a 

mission critical standpoint. Now simplify!

    

  George ATP/CFI-II-ME/MSME

   

   

  >From: Kevin Horton <khorton01(at)rogers.com (khorton01(at)rogers.com)>

 	  Quote: 	
		  Subject: Re: Two Circuits on One Fuse?



The fuse needs to be big enough to comfortably handle the worst 

case  total load that could be produced by both devices.  

Kevin Horton         RV-8 (finishing kit)


	




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Jekyll

Joined: 22 Feb 2006
Posts: 44

Posted: Fri May 25, 2007 9:17 am Post subject: Re: Two Circuits on One Fuse?

I ganged several of my fuses. I agree with what's been said above so I'll just stay with a systems approach to add something to the topic.



You didn't say if you are using an E-bus or just a main bus. Your architecture bears on the process. First, I wouldn't gang anything I would consider appropriate for an E-bus. Those items are by definition the minimum requirements for safe continued operation. I wouldn't introduce any chance of sympathetic failure to my essential systems. Those circuits should be carefully planned to maximize reliability.



Gang only unrelated items, non essential or those with other backups:

- Gang all your internal lights and panel lights. Carry a flashlight for a backup. I carry several when night flying including a LED on a neck lanyard.

- Gang your flaps with something such as position lights. You can safely fly without either in a pinch.

- Gang intermittent items such as trim, taxi lights.



Just some ideas to ponder.



In the end, you can add to your fuse block without really adding to your fuse block. Run a single fused wire to a terminal block and run individual power runs from there using in-line fuses. Another method is to run a single wire to all CB's you may still use. In my plane, I have fuses but decided to install pullable CBs for pitch and roll trim so I could rapidly locate and turn off the trims in case of a run away condition. I ran 1 fused wire to the daisy-chained CBs. Each CB protects its down stream wires and the power run is fused and sized to feed both CBs.



George, good info.



Jekyll

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N1DeltaWhiskey(at)comcast
Guest

Posted: Fri May 25, 2007 2:37 pm Post subject: Two Circuits on One Fuse?

May as well throw in a few more veggies into the soup here.



I am not a fan of "ganging," but would consider it in some cases.  If one is 

short several fused terminals, I would add another fuse block with a short 

supply lead from the supply of the first block or direct from the firewall .



Since I would tolerate only 2-3 of these cases, I would then look for the 

low current need equipment, 1-2 amps.  Here's the reason.  Minimum 

recommended wire size is 22 ga which would be protected with a 3 amp or less 

fuse.  Combining a couple 1 amp devices on this 3 amp fuse would not 

increase the need to upsize the wire to either device.



If the combined power need of ganged leads approached or exceeded 3 amps, I 

would jump to a 5 amp fuse and change the feed wires to each device ganged 

to this fuse from 22 ga to 20 ga.



Anything more than that, give me another buss.  They're not expensive. 

Ganging larger demand devices may be OK if there is no likelihood they will 

be active at the same time, but the fuse needs to be sized to protect the 

smallest wire and you must be prepared to not miss those devices in case the 

are both powered at the same time and blow a fuse in the process.



Regards, Doug Windhorn



---

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