On Tuesday 17 April 2018 12:49:19 Przemek Klosowski wrote: > On Tue, Apr 17, 2018 at 9:00 AM, Gene Heskett <ghesk...@shentel.net> wrote: > > i don't see why not, Lester. 12 or 24 volts would go a long ways > > toward keeping the NEC out of your hair. Insurance folks are > > stickier about such stuff, so I'd ask them if they had any special > > wireing rules for low voltage led lighting. The only thing that > > comes to my pre-coffee mind is the heavier wire the 12 volt stuff > > would need, whereas 24 could run on a gauge smaller, and 48 on > > really small wire. I would fuse it, using fuses of 150 to 200% of > > the leds draw though. Provided that was also under the wires > > textbook ampere capacity. > > > > I don't know if the NEC has addressed that yet. but I'd sure do some > > checking before I built a $100k house with all led lighting. My copy > > is now 20 years old, and that chapter wasn't even a twinkle in > > anybodies eyes then. > > Be careful with DC: the DC circuit arcing is much worse than AC.
That is somewhat subjective. A DC circuit always has to interrupt the full load amps, where as the AC circuit will arc a bit as the switch is opening, but will not re-establish that arc after the first zero crossing extinguishes it. This is of course assuming a resistive load. But adding the inductance of a transformer or a motor to the load circuit, and the AC still functions pretty well, but the DC has to contend with the arc for a much longer and more damaging time, until the switch contacts have separated far enough to extinguish the arc, since the inductance wants to maintain the current flow until the arcing has finally used up the energy stored in the inductance. One trick often used in DC contactor coil controls is to have 2 SCR's, anode coupled by a major fraction of a microfarad, but only one controls the coil. Both SCR's are triggered by the same signal. The one thats on, see's the huge negative pulse coupled from the one thats off turning on, which reverses the voltage on the one that is on long enough for it to turn off. Next pulse reverses the state of the 2 SCR's, turning the relay coil back on. The SCR's are typically able to withstand 600 or more volts, and the inductance's energy is quickly absorbed by the on SCR. The energy that would cause the arc just rings back and forth, with some of the reversed current getting shoved back thru the coil, with a slight improvement of the off time if everything is tuned right. > That's why DC specs for switches and relays are significantly derated > compared to AC. The LED lighting fortunately mitigates that somehow by > using much less current, but if you wire the whole system and push > tens of amps, it would be a mistake to just wing it because "NEC > doesn't cover low voltage". > Agreed, 200%. > ---------------------------------------------------------------------- >-------- Check out the vibrant tech community on one of the world's > most engaging tech sites, Slashdot.org! http://sdm.link/slashdot > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users -- Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
