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

["Patoka, Martin" <mpatoka@xxxxxx>]

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-----
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/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