NFPA 70, the National Electrical Code (NEC), Article 100 defines "Continuous Load" as "A load where the maximum current is expected to continue for 3 hours or more". This is the definition used for determining whether the plug and cord need to be rated for 125% of the maximum current of the product. This is why 1800 W hair driers and microwave ovens are allowed. It is presumed that they will not be on for 3 hours continuously, so they can draw 15 A at 120 V with a 15 A plug and cord. I'm sure there are people who do run their microwave for longer than 3 hours, but this is a rare exception (and I certainly wouldn't want to eat anything that they cook.)
Don't ask me why the 3 hour limit was chosen. The NEC handbook doesn't give the rationale. I believe it goes back to the risk of overheating wiring within the wall. However, 3 hours seems like a fairly random number. I would think that wiring would reach its steady state temperature must faster than 3 hours. If I had to guess, I would expect that it was a political decision. Probably, when the issue was first being discussed for addition to the NEC, one group wanted to keep the time limit as short as possible. Another group probably had equipment that could be on for close to 3 hours and required a lot of current. The latter group may have blocked the adoption of the 125% unless an exception was given for their equipment. This is entirely a guess and I have no idea if it is correct. However, based on my code development experience, it wouldn't surprise me. If somebody does know the history, I would be interested. The NEC limits the Continuous Load on any branch circuit to no more than 80% of the rating of the circuit's overcurrent protector. Even permanently wired equipment would have similar limitations. That being said, the NEC does have exceptions for certain types of equipment. Ted Eckert Compliance Engineer Microsoft Corporation [email protected] The opinions expressed are my own and do not necessarily reflect those of my employer. -----Original Message----- From: John Woodgate [mailto:[email protected]] Sent: Friday, August 8, 2014 11:43 PM To: [email protected] Subject: Re: [PSES] FW: IEC 62368-1 rated AC input connector amperage differs from North America In message <[email protected]>, dated Sat, 9 Aug 2014, "Kunde, Brian" <[email protected]> writes: >Chuck, the rating on the product should be the max steady state >current. The NEC requires the plug to be rated 125% of the max >continuous current, which depending on the product can be significantly >less. You can put both ratings on your product if you want. Where are 'max steady state current' and 'max continuous current' defined? How can these terms be applied to an audio amplifier, for example, where the current is dependent on the input signal *so there is no 'steady state')? -- OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk Quid faciamus nisi sit? John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
