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Author Topic: Where Are They Now (Drive-In Heater) Erick Akers Arse Kicker Posts: 201 From: Dallas, TX, USA Registered: May 2001 posted 10-11-2003 02:26 AM                     Drive In A/C... Frostie Man Frostie!!!!  |  IP: Logged Phil Hill I love my cootie bug Posts: 7595 From: Hollywood, CA USA Registered: Mar 2000 posted 10-11-2003 02:44 AM                  I seriously doubt that anyone would be stupid enough to run those high-wattage devices on 120 volts in these times. I’m sure a modern-day “real” engineer would use 240 volts with a local ground-fault interrupter and fuse/breaker protection...as well as a local disconnect required by some codes. Also, NEC and all local codes that I’m aware of would not allow a “floating” isolation transformer secondary...it is unsafe. One leg MUST be grounded for a reference. Besides, that method would be ridiculous cuz of the cost involved. >>> Phil  |  IP: Logged Steve Kraus Film God Posts: 4094 From: Chicago, IL, USA Registered: May 2000 posted 10-11-2003 10:52 AM                    Phil, if you're worried about the current requirements of 120V operation bear in mind that it would be wired as 120/240 3 wire (2 hots and a neutral) with half the posts on one side and half on the other. The maximum current draw will be the same as an all-240 V setup and the neutral will only be carrying the unbalanced current when there happen to be more heaters running on one leg than the other. To the degree there are similar draws on both legs it's as though they're in series across the 240. Thus there is no technical advantage to your wanting to use the higher voltage unless you want to do a 240/480 setup but that would really put you into dangerous territory. As for the isolation transformer idea you're right that with these loads it would be too expensive. I'm not sure why it would be more dangerous. If you're talking about a step-down from a HV primary then yes, if HV somehow found its way onto the secondary then grounding one leg limits the voltage above ground to no more than the secondary value. But if we are going from 240 (ulimately coming from some other transformer) to 240-isolated then a failure in the iso transformer could put no more than 240 above ground on an output line because that is what the input is limited to by virtue of the original step down transformer having one leg of the secondary grounded. I do note there is some limited use (in recording studios etc.) of 120 V transformer which are center-tap grounded meaning that the maximum shock is 60 Vrms but this is for noise isolation as the GFCI's are the best way to manage shock risk.  |  IP: Logged Phil Hill I love my cootie bug Posts: 7595 From: Hollywood, CA USA Registered: Mar 2000 posted 10-11-2003 07:40 PM                  Steve, I was actually more concerned about hardware and the installation costs. Both a 120-volt or a 240 volt system would be inherently safe if installed correctly. But, since there is no way to be certain that the same number of heaters would be on at the same time on both sides of the line in the 120-volt configuration, the big technical advantage of course is the automatic balancing of the load distribution in the 240-volt configuration. This is very important not only from a technical standpoint, but from a power cost consideration. Most power-bill rates for “industrial” installations are based “leg demand” and balance. Power companies don’t want someone to make their life difficult by presenting highly unbalanced loads on their systems. By way of an example, let me put this down so I can understand what I want to say... Assumptions: 1) 120/240 volt single phase "split" system. 2) There are 200 heaters in this Drive-In. 3) Each heater takes 600 watts. 5A@120V. 2.5A@240V. 4) We want the heaters in only 5 rows. Each row has 2 sections that feed from the center at the projection booth. Each section has 10 heaters. 20 "sections" total. The last heater in each section is 100 ft from the booth and the branch-circuit breakers. Total Power for the heater system would be 200htrs x 600watts = 120kW. 1000 amps @ 120V…500 amps @ 240V. While the main panel and feeders would be sized the same there would be one less conductor and bus bars and connections on the 240V system since a neutral is not required. Each hot leg, and neutral, would still have to sized for the 500 amp rating…700CM copper? Each row "section" would have a heater load of 10htrs x 600watts = 6kW. 50A@120V. 25A@240V. (20 sections = 120kW). Again, each branch circuit would still have to have the same size wire for the hot legs but no neutral. Assume no more than a 2% voltage drop at the last heater, the wire size would have to be min #8, #6 AWG copper. Also, since the last heater/post must still be able to trip the branch breaker (40Amp?) in case of a fault, that size wire would have to be carried all the way thru all the branch circuits. That "extra" neutral conductor alone results in a 33% increase in hard costs. Not even taking in consideration of conduit loading/sizing, extra labor costs to install, etc. (Note: there still would have to be a ground conductor in both cases…3 vs. 4 conductors. I like the idea of a 480-volt system, but I think that would be "pushing the envelope" …beside the codes get really weird once you get past to 300 volt mark. Although 480/277 3-phase might be an option. About the grounding of the isolation/distribution transformers. If there is no reference to ground on the secondary, there exists a potential shock/electrocution hazard by way of 2 or more equipment faults such as a conductor "touching" a heater case and other one touching the pole. Some poor patron could be fried if they were to get across that. By grounding a leg, the system has a "reference" with which to draw enough current thru to trip the safety devices including the GFIs. Also it is wise for lightning protection or for insulation leakage currents and pri-sec xmfr failure. The studio example you site is very common for avoiding hum and noise. But note, the center-tap is the ground reference. Now that I have totally confused myself, I hope you can make out the jest of what I'm trying to say! >>> Phil [ 10-12-2003, 08:43 AM: Message edited by: Phil Hill ]  |  IP: Logged

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