For example, if you are installing a switch with a maximum consumption of 6786 W, you need three power supplies
(6786 W / 3000 W = 2.26 [rounded up to 3 power supplies]) to operate the switch and its modules.
Step 3
Determine the amount of power 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 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) required to power the switch, divide the maximum Watts required for by
the voltage 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 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 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:
• Combined power—Do not add any power supplies to the number of power supplies calculated for the available
power in Step 2. This power mode does not provide power redundancy, so no extra power supplies are needed.
• n+1 redundancy—Add one power supply (reserve power supply) that can output as much power as the most
powerful power supply used for active power. This form of power redundancy provides a reserve power supply
that can replace any active power supply that goes offline.
• n+n redundancy—Add enough power supplies (reserve power supplies) to at least equal the total output of the
active power supplies (the number of power supplies calculated in Step 2). Typically, you double the number of
power supplies. You must plan for a second power source for the reserve power supplies. For example, if you
calculate that you need two 3-kW power supplies for 6 kW of available power, you need another two 3-kW power
supplies for 6 kW of reserve power (for a total of four 3-kW power supplies used for available and reserve power).
Step 5
Be sure that the power source circuits are dedicated to the switch and not to other electrical equipment.
Cisco Nexus 9508 NX-OS Mode Switch Hardware Installation Guide
13
Preparing the Site
Planning for Power Requirements
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