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DC powering for 3845 router

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frnkblk at iname

Sep 11, 2007, 1:05 PM

Post #1 of 8 (281 views)
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DC powering for 3845 router

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.

Kind regards,

Frank

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swmike at swm

Sep 11, 2007, 1:19 PM

Post #2 of 8 (269 views)
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Re: DC powering for 3845 router [In reply to]

On Tue, 11 Sep 2007, 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.

Hmm, as far as I can read from:

http://www.cisco.com/en/US/products/ps5855/products_installation_guide_chapter09186a00802ccf2c.html

"36-60 V, 13 A, positive or negative, single or dual source"

13A at 48VDC is 624W.

Most likely the router will never use more than 13A from any given source
and I'd imagine that getting a 16A fuse per feed for this, two feeds,
shouldn't be a huge problem?

What is it they're "balking" about exactly?

--
Mikael Abrahamsson email: swmike[at]swm.pp.se
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frnkblk at iname

Sep 11, 2007, 2:33 PM

Post #3 of 8 (268 views)
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Re: DC powering for 3845 router [In reply to]

My bad, I wrote watts when I should have written amps, and it looks like 13
A, not 19 A, as you said.

It's just that compared to the other telco equipment they install, these
'data' devices suck a lot more power. It looks like we're going to have to
put in another power bay as this pushes us over our current capacity (we
knew we were close, but didn't plan well enough to know that this would put
us over the limit). Between the 7609-S and two 3845's, it appears to
require 5 feeds per side, and the fuse panel supports just 4 per side. So
that's another fuse panel.

Regards,

Frank

-----Original Message-----
From: Mikael Abrahamsson [mailto:swmike[at]swm.pp.se]
Sent: Tuesday, September 11, 2007 3:20 PM
To: Frank Bulk
Cc: cisco-nsp[at]puck.nether.net
Subject: Re: [c-nsp] DC powering for 3845 router

On Tue, 11 Sep 2007, 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.

Hmm, as far as I can read from:

http://www.cisco.com/en/US/products/ps5855/products_installation_guide_chapt
er09186a00802ccf2c.html

"36-60 V, 13 A, positive or negative, single or dual source"

13A at 48VDC is 624W.

Most likely the router will never use more than 13A from any given source
and I'd imagine that getting a 16A fuse per feed for this, two feeds,
shouldn't be a huge problem?

What is it they're "balking" about exactly?

--
Mikael Abrahamsson email: swmike[at]swm.pp.se

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justin at justinshore

Sep 11, 2007, 4:54 PM

Post #4 of 8 (263 views)
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Re: DC powering for 3845 router [In reply to]

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.

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frnkblk at iname

Sep 11, 2007, 6:29 PM

Post #5 of 8 (255 views)
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Re: DC powering for 3845 router [In reply to]

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.

_______________________________________________
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swmike at swm

Sep 11, 2007, 10:08 PM

Post #6 of 8 (257 views)
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Re: DC powering for 3845 router [In reply to]

On Tue, 11 Sep 2007, Frank Bulk - iNAME wrote:

> 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. =(

Well, a 12008 with a few (engine0 and 1) LCs in it actually draws like
600W which is fairly close to 13A, so personally I wouldn't be too
uncomfortable with 10A per powersupply for a router like the 3845, I'd be
surprised if it ever used more than 200-300W over time (even though I have
never actually tested it), ymmv.

--
Mikael Abrahamsson email: swmike[at]swm.pp.se
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justin at justinshore

Sep 12, 2007, 6:35 AM

Post #7 of 8 (254 views)
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Re: DC powering for 3845 router [In reply to]

Our 3845s are pulling under 2.5a per chassis. On all 3 of our chassis
one PSU pulled approximately 1a and the other was around 1.5a @ -48VDC
(54.36VDC is what our chargers are tuned to). One is loaded (full BGP
feeds, ACLs, 30Mbps of traffic); the other 2 are essentially idle.
We've added an NM-1GE and a 4-port non-PoE HWIC to the loaded 3845. We
are not doing any PoE which would obviously skew our figures.

Our 7613s are only pulling 17.5a per PSU as compared to the 120a max
they could be pulling down.

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. =(

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justin at justinshore

Sep 12, 2007, 6:51 AM

Post #8 of 8 (250 views)
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Re: DC powering for 3845 router [In reply to]

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.
>
>
_______________________________________________
cisco-nsp mailing list cisco-nsp[at]puck.nether.net
https://puck.nether.net/mailman/listinfo/cisco-nsp
archive at http://puck.nether.net/pipermail/cisco-nsp/

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