- Rack Power Distribution
- AP7XXX, AP7XXXB, AP8XXX series
Circuit breakers are in place to protect the branch circuit and its conductors. With respect to a 30A breaker, all components employed in the circuit (downstream from the circuit breaker) shall be rated for 30A use. In a scenario where a 30A circuit breaker is providing power to a 20A rated outlet, the outlet, load plug, load cord and load are rated for 20A use, thus sized for this rating. In this scenario there is inadequate protection for the outlet and all connected equipment, and potentially (and most harmfully) the conductor that is supplying the outlets themselves.
In North America, Canada, and Europe, PDU's are evaluated under UL/CSA/EN/IEC 60950-1
. This standard requires circuit breaker protection when the rating of an output is less than the rating of the input. A PDU with a 30A input plug and 20A receptacles must have circuit breakers. Section 2.7 of the 60950 standard states: "Protective devices necessary to comply with the requirements of 5.3* must be included as part of the equipment"
*5.3 is the evaluation of abnormal operating and fault conditions.
In North America, the requirement is based on NFPA 70, the National Electrical Code. In general, in the United States, you can have any outlet rated less than 20A protected by a 20A breaker. However, these outlets must have protection rated no less than the outlet rating and no more than 20A. A 30A outlet must be protected by a 30A breaker and may not have anything larger or smaller.
Europe has very similar rules to North America. The main difference is the rating of receptacles and breakers. In Europe, you typically have 10A or 16A outlets protected by a 16A breaker, but the breaker rating can be no more than 16A.
Canada places additional restrictions. In Canada, a 15A outlet must be protected by a 15A breaker. The only exception is the "T-Slot" 20A outlet that accepts either 15A or 20A plugs. This outlet may be protected at 20A.
If a PDU has a 30A plug and 30A receptacles, it will not require and internal circuit breaker. The 30A plug forces it to be plugged into a 30A branch circuit, which will have its own 30A breaker. The PDU can then rely on the branch circuit's breaker rather than an internal breaker.
If a PDU has a 30A plug and 20A receptacles, it must have internal 20A branch rated breakers for the receptacles. The 30A protection of the branch circuit is too high to be safe for the 20A outlets.