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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 capacitiesAs 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.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 websiteThis 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 KoonceArchitect, Standard Product GroupJSI Microelectronics4235 Forcum Ave., Ste. 500McClellan, CA 95652916-648-2089 x114