For example, if you are installing a switch with maximum consumption of 6786 W, you need three power supplies (6786
W / 3000 W = 2.26 [rounded up to three power supplies]) to operate the switch and its modules.
Step 3 Determine the amount of power that is required from a power source.
The power supplies are rated to have at least 91-percent efficiency.
• To determine the input power (Watts) from the power source to the power supplies, divide the output power of each
power supply (3000 W) by the efficiency of the power supply (0.91) and then multiply the result by the number of
power supplies required to power the switch. For example, if the switch uses 3 power supplies, you can calculate
the amount of power that is required from the power source as follows:
3000 W output / 0.91 efficiency X 3 power supplies = 9891 W
• To determine the number of Amps (A) that are required to power the switch, divide the maximum Watts that is
required for by the voltage that is used as shown in the following examples:
• For 9891 A at 200-volts AC (VAC), use the following formula:
(9891 W)/(200 VAC) = 33 A
• For 9891 A at 277-volts AC (VAC), use the following formula:
(9891 W)/(277 VAC) = 23.8 A
• For 9891 A at 380-volts DC (VDC), use the following formula:
(9891 W)/(380 VDC) = 17.4 A
• To determine the required BTUs, multiply the Watts that is required for the power source by 3.41214163.
For example, for 9891 W, use the following formula:
(9891 W) X (3.41214163 BTU) = 22,496
To size the circuit breaker for the required amperage, you must also divide the required amperage by the percentage. For
example, if the switch requires an input amperage of 33 A and you are able to use up to 80 percent of the capacity of a
circuit breaker, you use the following formula to calculate the minimum amperage that is required of the circuit breaker:
(33 A)/ (80% or 0.80) = 41.25 amps
Step 4 Select one of the following power modes to determine the number of additional power supplies required for reserve
power:
Cisco Nexus 9508 NX-OS Mode Switch Hardware Installation Guide
15
Preparing the Site
Planning for Power Requirements
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