Cisco Catalyst 4500 and UPS recommendations

cardsfan73 used Ask the Experts™
We’re in the process of implementing a Catalyst 4510-R-E switch for a customer, and they are looking for recommendations on sizing a UPS.   Specifically, this is a redundant Supervisor (WS-X45-SUP6-E) in an E-series chassis, and w/ five (5) 48-port PoE line cards (WS-X4648-RJ45V+E).  Running it through the Cisco power calculator, it comes up with a total output power of 4226W, and a “typical” output power of 3380W.  They outfitted the switch with dual PWR-C45-6000W power supplies for redundancy.

The customer is now needing to know exactly how much capacity will really be required in a UPS to handle this.   I’m no power expert, but I’m assuming that even though one of these power supplies seems sufficient to handle the complete load, the 2nd power supply will also be “up and operational”, so that it can take over in case of a primary failure.  So, several questions:

1)      Is the current demanded load split between the two power supplies, so that each would be live and providing about half of what is demanded, or is everything drawn from the primary until it fails or is removed?  

2)      Is how the power supplies are used a configurable option on the Catalyst 4500?

3)      Most importantly, how big a UPS will be required to handle this entire chassis, and keep BOTH power supplies up in whatever state they require?    Will a 6Kv UPS be sufficient?  (I’m asking because even if one power supply was operating at capacity, the 2nd one will still be drawing some additional power.)

Thanks, folks.  

Watch Question

Do more with

Expert Office
EXPERT OFFICE® is a registered trademark of EXPERTS EXCHANGE®
Hi there,

First, I am speaking from a background with the 6513 Chassis, I haven't had direct experience with the 4500 series but I will speak from experience with the 6513.

1. Load depends on the type of power supply and the amount of volts supplied to it. In this case a single 6000 watt power supply on 220v should be able to provide power on its own to a fully loaded system with PoE on half of the ports. You can do a "show tech" which will be able to show the current power load.

Once you go above 49% power load, a single PSU will no longer be sufficient. The devices will begin drawing power from both PSUs at the same time. In the event of a failure, the chassis will begin powering down random modules until it can successfully provide power to the maximum amount of line cards that it can handle on a single PSU.

2. No I don't think this is possible on IOS based devices (maybe you're running CatOS though?)

3. I always plan for 25% extra UPS power above the maximum expected power draw. Normally a device draws maximum power when it is booting, and a UPS will draw an extra 15% when it is charging so plan for a worst case scenario:

Power goes out, Generator fails to start up, UPS runs out of battery. Finally power is restored and you need to boot everything which will be at maximum load and the UPS will begin charging at the same time.

Also make sure you have enough power to feed the UPS, a general domestic socket will normally be contended, as will the Transformer outside of the building. It's best to give the specifications of the switches to an Engineer independent of the installation and ask first how much power is required for a UPS to power that in a worst case scenario, and second can the building manage that much power draw.

Note, an Independent Electrical Engineer is necessary to ensure that they give you an accurate measurement of requirements since they will gain nothing from the actual installation.
OK, here's how electricity works: It doesn't flow unless it has a load. It doesn't matter how much power is available, a given maximum load cannot draw more power than it can draw. A power supply draws power when it has a load attached. Power supplies for computer equipment, called switching power supplies, draw more power than the load they supply with power (they are not 100% efficient). But they're rated based on maximum output power.

Power equipment has a power factor always less than 1.0. The power factor is divided into the maximum output power of the power supply, to give maximum input power. So, if you have a power supply rated at 6,000 W max., and a power factor of 0.85,  when the power supply outputs maximum power, it consumes 7058 W (6000 W / 0.85) = 7058 W.

The redundant power supplies run one at a time. Otherwise they wouldn't be redundant, they'd just be dual. Do they want redundant UPS protection?

If the Cisco calculator gave a total output power, that is the total output power of the power supply that the switch is loaded on. It is not the output power of the switch itself. So, you take (4226 W / PSU power factor) = maximum PSU input power (the maximum draw on possible on the UPS unit.)   If the power factor on the PSU is 0.85, which is typical for switching power supplies of this size, the maximum draw possible on the UPS backup is (4226 W / 0.85) =  ~5000 W.

But you need to verify the power factor, and do the calculation yourself. Power factor is also called percent efficiency. 85 percent efficient = 0.85 power factor. Now the UPS also has a power factor, sometimes down to 0.7. If you're maximum possible load off the protected side of the UPS is 5000 W, (5000 W / 0.7) =  7200 W utility power input for the UPS.  

The rule for UPS power draw is (0.75 x the utility power supply breaker capacity) = maximum OUTPUT CURRENT from the UPS. Liebert UPS units can be run 10% over capacity continuously, without harm to the unit. There's one more thing. VA is not equal to W. (W * 1.2) = VA. A 6000 KVA UPS is officially rated to deliver 5000 W.

All power equipment must conform to standards regarding safety, reliability, fire prevention, and protection of electronic equipment. Therefore, almost every piece of power equipment is able to deliver 20% or more, above maximum load, for a few seconds or more. The ratings are for continuous output power.

It looks from our calculations like you are going to just sneak in with a 6000 KVA UPS backup, exactly

Do more with

Expert Office
Submit tech questions to Ask the Experts™ at any time to receive solutions, advice, and new ideas from leading industry professionals.

Start 7-Day Free Trial