That's the problem. Nothing/nobody delivers what it/they're "rated"/expected to deliver. I do not consider it acceptable for shysters to shirk their responsibility by citing "engineering practice" as an excuse. Tom
--- In [email protected], "Gerald Pelnar" <[EMAIL PROTECTED]> wrote: > > Eric, > > Nice explanation. Good engineering practice. However, circuits can be loaded > 100% as long as it is not a continuous load (operated for more than 4 > hours). Vacuum cleaners (not likely to run 4 hours at a time) are more > likely limited to 12 amps due to minimum circuit opacity for motor loads > requiring an additional 25% of the load, so as to not overload a 15 amp > circuit. > > I apologize to all the non electricians reading this. Due to the amazingly > confusing way the NEC is written, it's hard for electricians to pass up a > good code argument. :) > > Once again, very good engineering practice, Eric, in spite of the code > details. > > Gerald Pelnar WD0FYF > McPherson, Ks > > > ----- Original Message ----- > From: "Eric Lemmon" <[EMAIL PROTECTED]> > To: <[email protected]> > Sent: Thursday, July 17, 2008 10:47 PM > Subject: RE: RE: [Repeater-Builder] Re: RadioShack Recalls Power Supplies > > > > Ron, > > > > That is a good question. The answer is that one is not supposed to > > connect > > any load greater than 12 amperes to an outlet rated at 15 amperes, that > > is, > > a NEMA 5-15R receptacle. The NEC allows two or more 15-ampere-rated > > outlets > > to be installed on a 20-ampere branch circuit (wired with 12 AWG > > conductors > > and a 20A fuse or circuit breaker), but the limit of 12 amperes on each > > outlet still applies. > > > > Proof of this restriction is evident in the "vacuum cleaner wars" of a > > decade or so ago. Hoover came out with a vacuum cleaner with "7 amperes > > of > > cleaning power." Then Bissel came out with a unit claiming 9 amperes of > > cleaning power. Other vacuum cleaner makers entered the fray until all of > > the brands had units with "12 amperes of cleaning power." The reason that > > nobody offered a unit with 13 amperes of cleaning power is because they > > would then have to equip that unit with a NEMA 5-20P plug and at least a > > 14/3 power cord. Most older homes do not have NEMA 5-20R outlets, so such > > a > > vacuum cleaner could not be plugged in to the outlets in most homes. > > Besides, there is no credibility to a laughable rating of "cleaning power" > > expressed in amperes! That is about as silly as claiming that a mobile > > radio has "13.8 volts of talk power!" > > > > Back to your second question. By definition, a 15-ampere-rated branch > > circuit has circuit conductors of #14 AWG or larger, and is protected by a > > fuse or circuit breaker rated at 15 amperes. The fuse or circuit breaker > > should hold indefinitely at 15 amperes, but the NEC recognizes that > > allowing > > 100% of rated current is never a good idea, since wiring in attics may > > already be in a very hot environment. Therefore, the NEC requires that no > > ordinary branch circuit be permitted to be loaded more than 80% of the > > circuit rating. That's where the 12 and 16 ampere limits come from. > > > > Another issue is voltage drop, which is directly proportional to circuit > > loading. Circuits that are loaded to 100% of rating will probably have > > excessive voltage drop, which leads to inefficient operation. Good > > electrical design dictates that the wire size be increased for long runs, > > to > > keep the voltage drop below 3%. Moreover, an adequate electrical supply > > system should never experience more than 80% loading of any circuit. Very > > heavy single loads should have a dedicated branch circuit of suitable > > capacity, with a single outlet. > > > > 73, Eric Lemmon WB6FLY > > > > >
