[c-nsp] Cisco 12012 Power Question

[Paul Stewart]

Thanks guys... Everyone who responded... Appreciate it...

I think we'll fuse at 20 amps per side and then take a meter reading
afterwards to ensure we're good long term etc... It was just the thought
of having 120 amps of power tied up that was kind of scaring me..;)

Take care,

Paul
 

-----Original Message-----
From: Robert E. Seastrom [mailto:rs at valhalla.seastrom.com] On Behalf Of
Robert E. Seastrom
Sent: Monday, July 17, 2006 9:46 AM
To: Paul Stewart
Cc: cisco-nsp
Subject: Re: [c-nsp] Cisco 12012 Power Question


"Paul Stewart" <pstewart at nexicomgroup.net> writes:

> Does anyone know on the 12012 dual DC platform what the *real* power 
> requirements are?  My reason is that the Cisco spec calls for 60 amps 
> per power supply and this seems really huge in my view....
>
> With dual power, would it not be load balanced so the combined draw 
> would not exceed 60 amps?  I realize this is a loaded quesiton 
> depending on what cards are in the chassis - basically 4 single line 
> GE cards, 2 of the 8 port FE cards and dual route processors....

Yep, it would be load-balanced so you have 25 amps or so maximum per
leg.  The whole point of having dual power supplies is that if one fails
you can run on the other one; therefore you can't really run stuff up
more than halfway minus breaker tolerance (the rule for AC at least is
don't load the breaker up to more than 85% of rating; don't remember the
rule for DC offhand but I think it's similar - avoids tolerance stacking
problems and long-term heat issues).

Obviously you need to size your breakers to be able to handle whatever
card complement you put in.  I have a 12012 that I'm looking at right
now which contains a GRP, 2 x engine 1 gige cards, 2 x engine 0 oc12c
cards, full fabric and a redundant CSC...  drawing 11 amps on the -48v
bus.  A similarly complemented 12008 is drawing 10.

The risk to undersizing the breakers is that someone is likely to come
along and plug in more cards, not even thinking about it...  and then
months later when a power supply goes south and the whole load swings
over onto one leg, the breaker pops and the whole box goes down.
Caveat utilitor.

                                        ---rob