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