Re: power delivery question from Liaison report
Thanks for raising the issue of the "48 VDC max" statement in the liaison
report. I assume it is in error (i.e., is not the MAXIMUM but is intended
as a nominal) but we need someone to confirm this. I took a quick look to
see if I could find an ISO/IEC JTC 1 standard that addresses this voltage.
Scanning the ISO/IEC 11801 "Information Technology - Generic Cabling for
Customer Premises", I found no mention of anything but breakdown voltage,
i.e., no safety/fire related voltages and no mention of 48 volts or
anything close to that.
The 10 watt per pair limit and the 175 mA per conductor current limit the
liaison report states, would be compatible with 57.14 volts, which seems a
reasonable upper limit. My view is the upper limit should be about 56
> Rick Brooks wrote:
> I was reading in the Draft Liaison report from ISO/IEC JTC 1/SC 25/WG 3
> to IEEE802.3 on power feeding that was
> handed out at the July Plenary.
> IEEE802.3af had question 4: Info on parameter limits (voltage, current,
> power, source impedance, ...) for world wide standards.
> i.e. what are the restriction beyond SELV.
> The response back was 48 VDC max, 175 ma max per pin.
> My question is:
> Is the 48VDC output from the port really 48VDC max as the response to the
> question indicates?
> If so, my thoughts are the following:
> We would have to spec our power output at the PSE as 48 VDC + 0%, - 8%,
> or something like that,
> so that it never exceeds 48 VDC continuously.
> This will further limit the available load power;
> it would be less than the load power that was discussed at the last
> meeting namely 14.6 watts.
> So, in that case the PD must be designed to draw at most 350 ma, as we
> And the power delivered at 100 meter cable would then be:
> Pwr = [44.2 - (12.5 x 0.35)] 0.35 = 13.9 Watts. (where 44.2 VDC is the
> lowest output voltage to still be in spec)
> For long cable lengths, the current per pin will be balanced, and we
> don't exceed the 175 ma per pin.
> For short cable lengths, we probably need an additional power spec, so
> that neither RJ-45 pin exceeds 175 ma.
> Say that due to connector imbalance, one pin is 175 ma, and the other is
> 20% below that, or 140 ma, which is a total of 315 ma.
> Then the power for a short cable would be (at least) 13.9 watts (44.2 *
> This would say that the PD device should be designed not to draw more
> than 350 ma,
> and at the same time not to draw more than 13.9 watts.
> That way we never exceed 48 VDC nor 175 ma per pin on a continuous basis.
> This puts the burden on the PD end to meet these current and power
> The PSE end would have a max voltage of 48 VDC, but it's current limit
> would be set slightly higher than 350 ma
> by some appropriate margin.
> If, on the other hand, we put the burden at the PSE end, then the
> available power goes down even more, but that may be OK also.
Donald (Don) S. Stewart Phone: 732-817-5495, FAX x4666
Soon to be: Avaya Communication e-mail: dsstewart@xxxxxxxxx
(former Enterprise Networks Group of Lucent Technologies)
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Holmdel, NJ 07733