Bob, There are a few references to the 125% rule (or 80% rule) in the National Electrical Code (NFPA 70):
1. NEC Article 384-16(c) for Panelboards- "Continuous Load. The total load on any overcurrent device located in a panelboard shall not exceed 80 percent of its rating where, in normal operation, the load will continue for three hours or more. Exception: An assembly, including the overcurrent device, shall be permitted to be used for continuous operation at 100 percent of its rating where it is listed for this purpose." 2. NEC Article 210-22 for Branch Circuit - "Other Loads. The rating of the branch-circuit overcurrent device serving continuous loads, such as store lighting and similar loads, shall be not less than the noncontinuous load plus 125 percent of the of the continuous load. The minimum branch-circuit conductor size, without the application of any adjustment or correction factors, shall have an allowable ampacity equal to or greater than the non-continuous load plus 125 percent of the continuous load. Exception: Circuits supplied by an assembly, together with its overcurrent devices, that is listed for continuous operation at 100 percent of its rating." Hope this helps. Tin --------------------- Forwarded message: From: [email protected] (Bob Brister) Sender: [email protected] Reply-to: [email protected] (Bob Brister) To: [email protected] ('[email protected]') List-Post: [email protected] Date: 97-04-25 09:06:53 EDT Hello, Are there requirements or guidelines around what a power supply input breaker should be rated for a given nominal input current assuming the power cord is rated appropriately? Looking through the harmonized 950 standard I did not come across anything. I seem to recall seeing a 125% figure once but don't remember where that was. I would assume that the circuit breaker would be some percent higher than the nominal input current rating to prevent nuisance trips and to take into account surge current or perhaps this is addressed by the trip characteristics of the breaker. To give an example, if the unit has an input current rating of 16amps, is there a rule which states the input circuit breaker must fall within some rating range. Regards, Bob Brister DIGITAL [email protected]
