[c-nsp] DC powering for 3845 router

[Justin Shore]

Whoops.  I forget to click send last night.


Without checking the load in person I can say for certain that we aren't 
powering them with my than 15a -48VDC circuits.  Anything bigger than 
15a and we add a set of dedicated breakers on one of the main bus bars 
in the distribution frame.  15a or under and we pull them off the 
distribution panel mounted in each rack.

Shooting from the hip I would say that our's are probably pulling no 
more than 2-3a per PSU.  I'd have to check to be sure though.  I've got 
loaded 3845s and idle 3845s to check too.  I'll get back with you.

It's one thing to budget for the max rating but it's another thing to 
budget for the realistic load.  Each of our 7613s has a pair of 4000w DC 
PSUs in them.  Each PSU has 3 40a leads.  However the last time I 
checked each chassis was pulling no more than 25a across all 6 leads.  I 
should check that again now that we have a little more load on the boxes.

Our plant folks freaked when I gave them the potential max load for our 
2 7613s.  Apparently the potential load of 480a was more than what the 
roomful of old DC0 equipment.  They were much more relaxed when we 
actually powered one of the units online with the Sup and started adding 
modules one by one.   They did have to add 2 frames of batteries and an 
additional charger to cover my additional load plus everything else they 
added to the CO.

My advice would be to always cable for the max load.  Fuse each PSU for 
the realistic load of the entire chassis (assuming 1 PSU failure) unless 
you're dealing with fuses greater than 15a in which case you're probably 
actually talking about breakers in a power frame.  You could overload 
you DC plant if you fuse to the max load if the plant isn't capable of 
handling it, potentially taking down everything and possibly damaging 
your DC plant.  Plan your power capacity based on your utilization 
growth vs your needs for outage duration.  Always check your current 
load prior to adding any additional equipment including service modules 
in the large chassis.

Justin


Frank Bulk - iNAME wrote:
> Thanks.
> 
> The manual also says to power them with 30 A circuits.  Of course, that's
> not actual draw.  
> 
> I was hoping someone was going to say that Cisco has this bit of hidden
> documentation that it's just 13 A per 3845 for 48 VDC fed DC power supplies,
> whether you have one or two of them.  But no one has said so, so we need to
> budget 26 A.  =(
> 
> Frank
> 
> -----Original Message-----
> From: Justin Shore [mailto:justin at justinshore.com] 
> Sent: Tuesday, September 11, 2007 6:55 PM
> To: frnkblk at iname.com
> Cc: cisco-nsp at puck.nether.net
> Subject: Re: [c-nsp] DC powering for 3845 router
> 
> I'll take my DC amp meter to work tomorrow and check our 3845s.  I have
> 3 that are DC powered.
> 
> Justin
> 
> Frank Bulk wrote:
>> Our installers are balking at the 19 W requirements, per DC power supply,
>> for the two 3845 routers we purchased.  Since we have a redundant power
>> supplies, it looks like a total of 4 power supplies totaling 76 W.
>>
>> Are there any shortcuts here?  The documentation, both online and printed,
>> is pretty sparse about the dual-power supply situation.
> 
>