Rick, I believe you are using the 350 mA as a worst
case number when that was not the intention. Let's look at the words from
the SC25/WG3 Liaison report to us.
Referencing Document ISO/IEC JTC 1/SC 25/WG 3 N 602, page 2,
carefully read both the response and the rationale.
Question 2: Allowable loop dissipation (temp rise in worst case
maximum temperature of cabling behind
current for categories 5 and higher and classes D and higher
exceed 0,175 per conductor (0,35 A per pair) for ambient temperatures
60 °C for solid cables (45 °C for 0,4 mm equivalent stranded
Analysis assumes that conductor diameters of less than 0,4 mm (or
are not used.
Note - No change required to 11801.
Assuming an allowed 40 °C rise for PVC insulating materials and an
temperature of 25 °C, the current capacity of 0,5 mm (24 AWG) and 0,4
mm (26 AWG)
conductors is 2,1 A and 1,3 A respectively. Using the assumption
that the total current in
all conductors of multiple conductor cables may not
exceed 20% of the sum of the
individual ratings of all conductors, the
current capacity for is:
Solid cable: 2,1 A * 0,2 = 420 mA per
Stranded cable: 1,3 A * 0,2 = 260 mA per conductor
current of 420 mA per solid conductor allows for 840 mA per pair based on
ambient temperature of 25 °C. Derating by a factor of 53 % (using the chart
IEC 60603-7 from 25 °C to 60 °C) allows for a steady state current
of 395 mA per pair.
This current assumes that stranded cables are exposed to
ambient temperatures of
less than 45 °C. Connector current capacity of 0,75 A
@ 60 °C per conductor as
specified in IEC 60603-7 is not the limiting
Robert D. Love
President, LAN Connect Consultants
Raleigh, NC 27615
Phone: 919 848-6773
----- Original Message -----
Sent: Wednesday, August 09, 2000 9:01
Subject: current limit on 48 VDC
Welcome back Roger.
Since we are putting in our share of opinions, here
If we assume that 350 ma per pair,
175 ma per pin, is the maximum continuous current that we are allowed to
Then for PTC protection alone, we
would have select a PTC with a lower current value than 350 ma,
since these devices are not particularly sharp,
just as they are not fast.
So, we would
have to choose a PTC with (I'm guessing), say 250 ma limit at low ambient
This choice would drastically
lower our guaranteed output power, especially for higher ambient
We may only be able to get 5
watts to the PD.
Has anyone worked out an example with real
So, I think that we will need some form of
electronic current limit on each port, so we can run close to the 350 ma
If we use PTC's at all, it will be to prevent
fires, in the case of when the electronic current limit fails.
we need more dialog...
anyone for discovery? tennis?