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com dual PWR/GND on two fuses?

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glastar(at)gmx.net
Guest

Posted: Fri Jun 16, 2017 1:21 pm Post subject: com dual PWR/GND on two fuses?

Hello,



most avionics do have PWR and GND on two pins. My SL30 had even one used 

for the NAV part (3A fused) and one for the COM part (5A fused).



I have to replace it with a new 8.33, but just going for a COM right 

now, that one has as well 2 pins each tied together, PWR onto a 10A fuse 

(8W COM).



Is there any disadvantage, if I use the existing cabling with the 2x 

GND/PWR on extra fuses? Should I use 2x10A fuses (for a 8W com I found 

the 10A a bit high).



Thanks for your advice



Cheers Werner

trigo(at)mail.telepac.pt
Guest

Posted: Fri Jun 16, 2017 2:16 pm Post subject: com dual PWR/GND on two fuses?

Hi Werner



You may use the same cabling, connected to only one 10A fuse.

After installation, just measure the current when transmitting, and then you could use a 7.5A fuse instead of the 10A.



Cheers

Carlos



Enviado do meu iPhone



No dia 16/06/2017, às 22:20, Werner Schneider <glastar(at)gmx.net> escreveu:



 	  Quote: 	
		   

 

 Hello,

 

 most avionics do have PWR and GND on two pins. My SL30 had even one used for the NAV part (3A fused) and one for the COM part (5A fused).

 

 I have to replace it with a new 8.33, but just going for a COM right now, that one has as well 2 pins each tied together, PWR onto a 10A fuse (8W COM).

 

 Is there any disadvantage, if I use the existing cabling with the 2x GND/PWR on extra fuses? Should I use 2x10A fuses (for a 8W com I found the 10A a bit high).

 

 Thanks for your advice

 

 Cheers Werner

Kellym

Joined: 10 Jan 2006
Posts: 1706
Location: Sun Lakes AZ

Posted: Fri Jun 16, 2017 6:19 pm Post subject: com dual PWR/GND on two fuses?

I have my SL-30 (14V) on 5 amp breaker for Com and 3 amp for Nav. Actual 

use, from what I gleaned from the manual is about 3 amp on transmit for 

the Com side.



On 6/16/2017 2:20 PM, Werner Schneider wrote:

 	  Quote: 	
		   

 

 Hello,

 

 most avionics do have PWR and GND on two pins. My SL30 had even one used 

 for the NAV part (3A fused) and one for the COM part (5A fused).

 

 I have to replace it with a new 8.33, but just going for a COM right 

 now, that one has as well 2 pins each tied together, PWR onto a 10A fuse 

 (8W COM).

 

 Is there any disadvantage, if I use the existing cabling with the 2x 

 GND/PWR on extra fuses? Should I use 2x10A fuses (for a 8W com I found 

 the 10A a bit high).

 

 Thanks for your advice

 

 Cheers Werner

_________________
Kelly McMullen
A&P/IA, EAA Tech Counselor # 5286
KCHD

user9253

Joined: 28 Mar 2008
Posts: 1944
Location: Riley TWP Michigan

Posted: Sat Jun 17, 2017 4:15 am Post subject: Re: com dual PWR/GND on two fuses?

The ampacity of a single USB pin is about 5 amps.  Many avionics use dual USB pins for power to divide the current so that USB pins do not get warm.

_________________
Joe Gores

Bob McC

Joined: 09 Jan 2006
Posts: 258
Location: Toronto, ON

Posted: Mon Jun 19, 2017 9:36 am Post subject: com dual PWR/GND on two fuses?

Always bear in mind that the fuses are sized to protect the wire and what is connected at the other end (in this case your radio) has no influence on the fuse size used.   

As long as the wires are adequate for the 10 Amps then 10 Amp fuses are fine and certainly if the existing wiring will supply the new device's requirements then they may be reused/re-purposed.   

Bob McC   [quote]    

--------

_________________
Bob McC
Falco #908
(just starting)

trigo(at)mail.telepac.pt
Guest

Posted: Mon Jun 19, 2017 1:41 pm Post subject: com dual PWR/GND on two fuses?

Hi Bob

 

You are right that the fuse is there to protect the wire, but the size (in A) of the fuse is a function of the current which is drawn by what is connected at the other end of the wire, isn’t it?

 

Otherwise you could put a 1A fuse to protect a #24 wire with a pitot heater at the end, and you would never see that heater working when you wanted it.

 

With due respect

Carlos  

 

De: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] Em nome de Robert McCallum

Enviada: Monday, June 19, 2017 6:35 PM

Para: aeroelectric-list <aeroelectric-list(at)matronics.com>

Assunto: Re: com dual PWR/GND on two fuses?

 

Always bear in mind that the fuses are sized to protect the wire and what is connected at the other end (in this case your radio) has no influence on the fuse size used.

As long as the wires are adequate for the 10 Amps then 10 Amp fuses are fine and certainly if the existing wiring will supply the new device's requirements then they may be reused/re-purposed.

Bob McC[quote]

--------

Kellym

Joined: 10 Jan 2006
Posts: 1706
Location: Sun Lakes AZ

Posted: Mon Jun 19, 2017 3:02 pm Post subject: com dual PWR/GND on two fuses?

Not Bob,

However, the wire has to be sized at a minimum to carry the anticipated 

load, so you would not have the situation you describe.

On the other hand, it is common that relatively small loads, say 1 amp 

are carried on 18-22 gauge wire, and would be fused for what the wire 

can carry, usually more than what the load is. Another common occurrence 

is that wires for lighting on a plane were sized for an incandescent 

load, but the lamps have all been replaced with LEDs that draw 1/5th the 

amount. The circuit breakers would not change, because the wiring did 

not change.



On 6/19/2017 2:38 PM, Carlos Trigo wrote:

[quote] Hi Bob

 

 You are right that the fuse is there to protect the wire, but the size 

 (in A) of the fuse is a function of the current which is drawn by what 

 is connected at the other end of the wire, isn’t it?

 

 Otherwise you could put a 1A fuse to protect a #24 wire with a pitot 

 heater at the end, and you would never see that heater working when you 

 wanted it.

 

 With due respect

 

 Carlos

 

 *De:*owner-aeroelectric-list-server(at)matronics.com 

 [mailto:owner-aeroelectric-list-server(at)matronics.com] *Em nome de 

 *Robert McCallum

 *Enviada:* Monday, June 19, 2017 6:35 PM

 *Para:* aeroelectric-list <aeroelectric-list(at)matronics.com>

 *Assunto:* Re: com dual PWR/GND on two fuses?

 

 Always bear in mind that the fuses are sized to protect the wire and 

 what is connected at the other end (in this case your radio) has no 

 influence on the fuse size used.

 

 As long as the wires are adequate for the 10 Amps then 10 Amp fuses are 

 fine and certainly if the existing wiring will supply the new device's 

 requirements then they may be reused/re-purposed.

 

 Bob McC

 

     --------

_________________
Kelly McMullen
A&P/IA, EAA Tech Counselor # 5286
KCHD

glastar(at)gmx.net
Guest

Posted: Mon Jun 19, 2017 11:32 pm Post subject: com dual PWR/GND on two fuses?

Ok after reading again through the AeroElectric Connection about wires 

and fuses I come to following conclusion.



According the Icom Manual we use 2x AWG20, together they are fine for a 

10A fuse, as I remain with my original SL-30 setup (2 wires with a fuse 

each) I could go for 7.5A each, considering the power of the unit I 

might stay with 2x 5 A out of convienence but will first check how much 

power the Icom draws when transmitting, I bet it will be below 5 . If 

I would go for a dual cable crimped into a single fuse, I would rather 

prefer a 7.5A fuse, so when one wire breaks it will still be adequate, 

however taking into account the relative short wire a 10A might be fine.



Thanks for your ideas.



Cheers Werner

On 19.06.2017 19:35, Robert McCallum wrote:

 	  Quote: 	
		  

 Always bear in mind that the fuses are sized to protect the wire and 

 what is connected at the other end (in this case your radio) has no 

 influence on the fuse size used.



 As long as the wires are adequate for the 10 Amps then 10 Amp fuses 

 are fine and certainly if the existing wiring will supply the new 

 device's requirements then they may be reused/re-purposed.



 Bob McC

user9253

Joined: 28 Mar 2008
Posts: 1944
Location: Riley TWP Michigan

Posted: Tue Jun 20, 2017 4:49 am Post subject: Re: com dual PWR/GND on two fuses?

Some avionics use two pins for positive power and two pins for negative power.  The reason for using two pins is not for redundancy.  The reason is that if only one D-Sub pin carried all of the current, it would be overloaded.  The problem of using two fuses is that if one fuse blows, then all of the current will flow through one D-Sub pin and overload it.

_________________
Joe Gores

Bob McC

Joined: 09 Jan 2006
Posts: 258
Location: Toronto, ON

Posted: Tue Jun 20, 2017 5:25 am Post subject: com dual PWR/GND on two fuses?

Correct. However the assumption must be made that the wire installed is adequate

for the intended task. My point was that if you've installed a 14ga wire you

protect it with the appropriate fuse for that wire, and not necessarily downsize

the fuse because the device you're powering draws only 1 amp. Once adequacy of

both wire and fuse is established then the fuse size chosen becomes a function

of the wire and not the connected load.



Respectfully



Bob McC

[quote] --------

_________________
Bob McC
Falco #908
(just starting)

glastar(at)gmx.net
Guest

Posted: Tue Jun 20, 2017 6:43 am Post subject: com dual PWR/GND on two fuses?

Thanks Joe for that warning, that is why I fuse for 5A only I still 

wonder the Icom uses a 10 A fuse for a 8W com and the SL30 was using a 

5A for a 12 W COM.



I believe the d-subs are rated 5A and if you break one cable it would be 

the same issue at the pin side.



At the end I will see what it draws (the KX-125 I had at the begin on 

Xmit was drawing 3.8A) and leave another 30% as margin. it is just 

convenient to leave it as it is.



Cheers Werner



On 20.06.2017 14:49, user9253 wrote:

 	  Quote: 	
		   

 

 Some avionics use two pins for positive power and two pins for negative power.  The reason for using two pins is not for redundancy.  The reason is that if only one D-Sub pin carried all of the current, it would be overloaded.  The problem of using two fuses is that if one fuse blows, then all of the current will flow through one D-Sub pin and overload it.

alec(at)alecmyers.com
Guest

Posted: Tue Jun 20, 2017 8:14 am Post subject: com dual PWR/GND on two fuses?

It would be dumb to put a 20A breaker on a 2A load, regardless of what wire you used.



On Jun 20, 2017, at 09:23, Robert McCallum <robert.mccallum2(at)sympatico.ca> wrote:



Correct. However the assumption must be made that the wire installed is adequate for the intended task. My point was that if you've installed a 14ga wire you protect it with the appropriate fuse for that wire, and not necessarily downsize the fuse because the device you're powering draws only 1 amp. Once adequacy of both wire and fuse is established then the fuse size chosen becomes a function of the wire and not the connected load.



Respectfully



Bob McC



[quote] --------

ceengland7(at)gmail.com
Guest

Posted: Tue Jun 20, 2017 12:12 pm Post subject: com dual PWR/GND on two fuses?

Unless, of course, you have a limited       number of fuse locations, and therefore, multiple non-critical 2A       loads on one circuit that has wiring sized to accept a 20A fuse.

       

       The fuse protects the wire. (There's a period at the end of that       sentence...) 

       

       On 6/20/2017 11:13 AM, Alec Myers wrote:

     

     [quote]              It would be dumb to put a 20A breaker on a 2A load,         regardless of what wire you used.

       

         On Jun 20, 2017, at 09:23, Robert McCallum <robert.mccallum2(at)sympatico.ca (robert.mccallum2(at)sympatico.ca)>         wrote:

         

       

                         

Correct. However the assumption must be made that the wire           installed is adequate for the intended task. My point was that           if you've installed a 14ga wire you protect it with the           appropriate fuse for that wire, and not necessarily downsize           the fuse because the device you're powering draws only 1 amp.           Once adequacy of both wire and fuse is established then the           fuse size chosen becomes a function of the wire and not the           connected load.

                  

Respectfully

                  

Bob McC

                  



                  [quote]            -->                       -->           --------

alec(at)alecmyers.com
Guest

Posted: Tue Jun 20, 2017 1:02 pm Post subject: com dual PWR/GND on two fuses?

It's not an absolute requirement that, for the fuse to do its job and protect the wire, it has exactly to match the listed maximum current capacity of the wire. A fuse can protect the wire just as well (and some who live on the edge might say even better) if the fuse is chosen to be appropriate to the load. However you want to punctuate it.

On Jun 20, 2017, at 16:15, Charlie England <ceengland7(at)gmail.com (ceengland7(at)gmail.com)> wrote:

                   Unless, of course, you have a limited       number of fuse locations, and therefore, multiple non-critical 2A       loads on one circuit that has wiring sized to accept a 20A fuse.

       

       The fuse protects the wire. (There's a period at the end of that       sentence...) 

       

       On 6/20/2017 11:13 AM, Alec Myers wrote:

     

     [quote]              It would be dumb to put a 20A breaker on a 2A load,         regardless of what wire you used.

       

         On Jun 20, 2017, at 09:23, Robert McCallum <robert.mccallum2(at)sympatico.ca (robert.mccallum2(at)sympatico.ca)>         wrote:

         

       

                         

Correct. However the assumption must be made that the wire           installed is adequate for the intended task. My point was that           if you've installed a 14ga wire you protect it with the           appropriate fuse for that wire, and not necessarily downsize           the fuse because the device you're powering draws only 1 amp.           Once adequacy of both wire and fuse is established then the           fuse size chosen becomes a function of the wire and not the           connected load.

                  

Respectfully

                  

Bob McC

                  



                  [quote]            -->                       -->           --------

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