Under the hardware installation manual it shows the requirements:

http://www.cisco.com/en/US/docs/switches/datacenter/nexus5000/hw/installation/guide/techspec.html#wp1070378

Depending which Nexus...

http://www.cisco.com/en/US/prod/collateral/switches/ps9441/ps9402/ps9512/Data_Sheet_C78-437761.html

http://www.cisco.com/en/US/docs/switches/datacenter/hw/nexus7000/installation/guide/n7k_installation_psu.html#wp1063424

The 7000 series doc shows this:

Prerequisites for Connecting Power Supply Units to AC Power

Before you connect the power supply units to AC power sources, you must install the power supply units in the chassis, and you must make sure that the appropriate AC power source receptacles are located close enough to the power supply units that the power cords can connect the power supply units to the AC power sources. For 6-kW power supply units, you must have one or two 20-A-rated, 110-V or 220-V AC circuits. For 7.5-kW power supply units, you must have one or two 30-A-rated, 220-V AC circuits.

You should connect each power supply input to a dedicated branch circuit. For North American installations, the 6-kW power supply unit requires one or two 20-A circuits, and the 7.5-kW power supply unit requires one or two 30-A circuits. For international installations, circuits must be sized by local and national codes. If you are using the input source redundancy mode, you must have at least two circuits.