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6509 Power

Can someone tell me the total KVA and BTU of a 6509 with two 2500W power supplies?

3 REPLIES
New Member

Re: 6509 Power

Here is the link that has everything you need. Notice that there is something tricky about this though. The "2500 watt" power supply only operates at 1300 watts when plugged into a "low line" (100 to 120 VAC). So if you are plugging this baby in in the US, you need to use the KVA and BTU figures for 1300 (1.8 and 6140 respectively) as opposed to the figures for 2500 (3.6 and 12,000 respectively). Those are for "high line" operation (200 to 240 VAC). I guess this is for folks who run a switch in their kitchen or laundry room.

http://www.cisco.com/univercd/cc/td/doc/pcat/ca6000.htm

New Member

Re: 6509 Power

Is the 6140 BTU total or is it doubled for two supplies? What about the rest of the chassis modules that give off heat. There must be a place on CCO that shows a close total BTU for a fully loaded 6509.

New Member

Re: 6509 Power

OK, here is the deal (as I understand it)... These kind of figures are always approximations at best. But it boils down to something like this:

The formula for BTU/hr is as follows:

BTU/hr = (watts * 3600) / 1054

so, (1300 * 3600) / 1054 = 4440 BTU/hr.

The reason that Cisco lists a higher value is that there is a certain inefficiency to converting AC to DC (i.e. the supply is rated to PROVIDE up to 1300 watts but will also CONSUME some number of watts in the conversion process). It is common for the efficiency to be anywhere from 60 - 85%. In this case it is apparently 72%.

From this, you can see that the BTU/hr figure is actually worst case if the power supply were being stressed to provide the full 1300 watts of RATED power. In order to calculate actual power consumption, you have to know A LOT about all of the internal circuitry in the chassis (you really don't want to get into resistive vs. reactive power, etc. unless you are pursuing EE).

Having said all that, there actually is a page that lists the DC current consumed by each module (again, I'm sure it is worst-case). So you can do some math (P = I * E) to try to estimate how much of the rated 1300 watts are actually being consumed by the modules. Make some assumptions about inefficiencies in the chassis getting the current to the modules. Then redo my calculation above with the total estimated value.

Finally, as for redundant power supplies, things are a little murky here as well. It is reasonable to assume that there is some type of load balancing between the two supplies. However, it is also reasonable to assume that no more than 1300 watts will be drawn (total) from both supplies together, since one is capable of doing the job by itself. Now it is debatable as to whether you need to consider the efficiency issue twice since AC is being converted to DC twice. I suspect that there is some minimal impact to BTU/hr, but probably not worth considering - especially since 6140 is worst-case and unlikely to ever be realized.

So, in closing, again all of this being as I understand it (never finished that EE), I think 6140 is a reasonable estimation regardless of whether you use redundant supplies and regardless of how the chassis is populated. It is somewhat of an "accepted practice" to just use the manufacturer BTU/hr rating for planning purposes.

Scott

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