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Please respond to "IEEE 802.3 Power over Ethernet plus"<stds-802-3-poep@xxxxxxxx>
Sent by: owner-stds-802-3-poep@xxxxxxxx
To:
STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
cc:
Subject:
Re: [8023-POEP]
Cabling and wire current capacities
I think that there is a more rational approach here.
Rampant
conservatism is not the road to progress. The point is to determine,
with reasonable judgement and consideration, the actual ampacity based
on factual information whose underlying assumptions are clearly
documented.
I know that the NEC (National Electrical Code) considers
this wiring to
be Class 2 (Inherently limited < 100VA) in article 725. I
cannot find
another NFPA (www.nfpa.org <http://www.nfpa.org/> ) standard that
gives
more detail about a case like this. Does anyone know of one?
I remember that someone was going to see if they could
get actual test
data - and I am not sure if it was Alan Flatman or not.
Regards,
Martin
Martin Patoka
Systems Engineer
Texas Instruments
214-567-5487
mpatoka@xxxxxx
-----Original Message-----
From: owner-stds-802-3-poep@xxxxxxxx
[mailto:owner-stds-802-3-poep@xxxxxxxx] On Behalf Of Steve Robbins
Sent: Monday, January 31, 2005 9:08 PM
To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: 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
-----Original Message-----
From: owner-stds-802-3-poep@xxxxxxxx
[mailto:owner-stds-802-3-poep@xxxxxxxx]On Behalf Of
Derek S. Koonce
Sent: Monday, January 31, 2005 5:20 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/T
M102179.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
|
I think that there is a more rational approach here. Rampant conservatism is not the road to progress. The point is to determine, with reasonable judgement and consideration, the actual ampacity based on factual information whose underlying assumptions are clearly documented.
I know that the NEC (National Electrical Code) considers this wiring to be Class 2 (Inherently limited < 100VA) in article 725. I cannot find another NFPA (www.nfpa.org) standard that gives more detail about a case like this. Does anyone know of one?
I remember that someone was going to see if they could get actual test data – and I am not sure if it was Alan Flatman or not.
Regards, Martin
Martin Patoka Systems Engineer Texas Instruments 214-567-5487 mpatoka@xxxxxx
-----Original Message-----
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
|