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Re: [8023-POEP] Cabling and wire current capacities



Derek,
 
Will you be distributing your minutes from the second day of the PoEP interim session (Thursday)?
 
Thanks,

Douglas P. Anderson             Doug.Anderson@xxxxxxxxxx
Micrel Semiconductor
Iowa Design Center                phone: 515-597-7000 x14
402 Campus Drive                  fax: 515-597-7001
Huxley, IA 50124


-----Original Message-----
From: Derek S. Koonce [mailto:dkoonce@xxxxxxxxxxxxx]
Sent: Monday, January 31, 2005 7:20 PM
To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: [8023-POEP] Cabling and wire current capacities

As I stated I would do, here is information I have on the cabling and wire current capacities. I scanned in some documents and, fortunately, found the complete NASA related document. I cannot attach the files to this email because the server limits to 100K of attachments. However, those that would like the files I will be glad to forward upon request.
 
The key document is the MIL-W-5088K. This document has been transferred to SAE control under standard AS50881. The SAE web site has a cost of this document, non-member, of $59. Maybe someone on this list has this copy or can obtain it to share with the list.
 
The NASA TM102179 document discusses the cabling design for space payloads. I feel this would be the best starting point.
    http://snebulos.mit.edu/projects/reference/International-Space-Station/TM102179.pdf
 
Surprenant is a cabling company that has some data on cabling derating. But their ampacity chart starts at 18 AWG and goes up from there.
 
A fusing current document was passed to me, years ago, from a line of engineers and is more of an eye-opener. The fusing current for 26 AWG wire is 20.5 A. Quite a bit to look at, but clearly unreasonable.
 
Looking at the NASA document (referenced above) and running the numbers for 100% wire usage, 50 F temp rise we would get a maximum current loading of 3.588 A per wire. (A 4.46V drop based on 2-pair conduction for 100 m)
 
This is very large and would put a maximum limit on the power. This does not take into account for voltage drop along the cable.
 
A bit more searching brought up the following website
    http://www.powerstream.com/Wire_Size.htm
This states that 26 AWG wire can handle up to 2.2 A for chassis wiring and 0.361 A for power wiring. These currents relate to a voltage drop of 2.74 V and 0.45 V for 2-pair conduction for 100 m.
 
If we look at any baseline for current in the wire. I would say the 0.361 would be a good start. Then add on upping the voltage to 55 V, we can see about 39 W on the PD side.
 
Derek Koonce
Architect, Standard Product Group
JSI Microelectronics
4235 Forcum Ave., Ste. 500
McClellan, CA 95652
916-648-2089 x114