Re: [8023-POEP] Cabling and wire current capacities
Guys,
This
is just a suggestion, but instead of consulting NASA specs, I would ask UL
(Underwriters Lab) or some other group whose opinion will count when it comes
to fire safety. (I spent many years working on the electric power
system for the international space station, and don't have a lot of trust in
specs written by those "rocket scientists" at NASA.)
Here's
what makes me a bit nervous about the ampacity discussions I've been reading in
these emails: One of the great things about PoE is that it's the first
international power standard, but one of the scary things is that it's the first
power standard where most hook-ups will be done by some Joe Blow IT guy instead
of a licensed electrician. There will be people using cables that are not
in the best of condition, hanging them over sharp metal edges, hanging them near
heat sources, tying them into large bundles, doing bad crimp jobs, hooking them
into patch panels with loose screws, etc. There will be plenty of
instances where something is getting a lot hotter than you expect. All it
might take is a few small fires scattered around the planet for
big insurance companies to start putting clauses in their policies for corporate
customers prohibiting the use of PoE in office buildings. That would be
the end of this technology.
I'm
sure that SAE or MIL standards will be useful as guidelines. But when it
comes to safefy in electronics, people look for the UL or CE marks on products,
not NASA, SAE or even the IEEE. I don't know if UL or CE have specs
for wire ampacity, but I'd feel better if I knew they were part of the
discussion.
Anyway, that's my two cents. Sorry if it seems alarmist. I
just think we should be very conservative about ampacity. More
conservative than organizations who might have effective veto power, such as big
insurance companies.
Steve
Robbins
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.
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
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