Cisco Systems 4500 manual Calculating DC Input Current

Chapter 2 Preparing for Installation

Power Connection Guidelines for DC-Powered Systems

Calculating DC Input Current

Complete power usage tables are in the Catalyst 4500 Series Module Installation Guide. To calculate the DC input current needed for data and inline power applications, use the following steps (the example shows the DC input current requirement in a Catalyst 4503 with a Supervisor Engine II and two WS-X4306-GB modules, and assumes a DC input voltage of -48 VDC):

Step 1 Add the power requirement for each component in your system.

•Catalyst 4503 uses 54 W

•Supervisor Engine II uses 147 W

•WS-X4306-GB uses 2 X 47 = 94 W

•Total DC input power = 295 W

Step 2 After the entries for all components have been added together, divide that number by the DC input voltage to determine the DC input current.

•Input current = 295 W/48 VDC = 6.14 A for data only.

Step 3 to Step 5 are for applications requiring inline power. If your configuration does not include inline power devices, the DC input current is the result of Step 2.

Step 3 If you want to add an inline-capable module (WS-X4148-RJ45V) with 10 inline devices (such as IP Phones) to your system, calculate the DC output power sent to inline devices.

•10 X 6.3 W = 63 W for inline devices.

6.3Watts is correct for a Cisco IP phone. Wattage consumption will depend on the inline device used.

Step 4 Find the DC input power using the DC output power.

• 63/.96 (efficiency) = 65 W of DC input power.

Step 5 Divide the DC input power by the DC voltage input of –48 V to find the DC input current used by inline devices.

•65/48 = 1.4 Amps for inline devices.

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