wiring 208 heater into 120

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  #1  
Old 11-18-04, 08:11 AM
balatron
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wiring 208 heater into 120

I did not mention that this was an explosion proof heater rated at 208 60hz 17.3 amps single phase that I purchased cheap for $100 (1200 hundred bucks new). I realize it doesn't make sense to spend 100 or 200 bucks on a transformer for a regular heater but it seems like it would in this case. If so can anybody tell me how to and where best to purchase the step transformer to run it off of 240. Thanks for the input and sorry for not being detailed enuff in my first posting.
 

Last edited by balatron; 11-19-04 at 05:48 AM. Reason: missing details
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  #2  
Old 11-18-04, 01:54 PM
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I THINK, that 208V is a three phase voltage, BUT WAIT FOR ONE OF THE PROS TO MAKE SURE PLEASE, but I don't think your heater will work with normal, one phase, house current.
 
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Old 11-18-04, 02:19 PM
sjr
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208V is the phase to ground voltage in a three phase system. However, since you are only using one phase to get 208V, that wouldn't be a problem. Getting a transformer with a high enough power rating to drop 240V to 208V may make it more cost effective to simply buy a new heater that works on 120 or 240V.
 
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Old 11-18-04, 09:29 PM
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Thanks sjr, I knew it was something about 3 phase, but I deal mostly....well ok ENTIRELY with household current, leave the 3phase to you pro's that know how to deal with it correctly.
 
  #5  
Old 11-19-04, 04:40 AM
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208V is the phase to phase voltage of a 120/208 wye system, which is very common in places like office buildings. Each phase provides 120V to neutral for general purpose circuits. The 208V between two phases is often used for larger loads.

208V is the phase to ground voltage in a 240V delta system with a center tapped leg. This system is relatively uncommon, used in places that have lots of 240V motors and a few general purpose circuits. The 208V 'high' leg to ground is never used to supply anything; for low voltage loads you have 120V, and for higher power loads you have 240V, either single or three phase.

I would strongly suggest _not_ using a transformer for this heater. Resistance heaters use lots of current, which means a large, expensive transformer. It would probably be cheaper to replace this heater than to get the transformer.

Double check the dataplate on this heater. It may be useable at different voltages, or have multiple 'taps' for different voltages.

-Jon
 
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Old 11-19-04, 08:24 PM
sjr
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I stand corrected--I'm leaving three phase to the pros from now on too...
 
  #7  
Old 11-19-04, 08:57 PM
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The question clearly states that the heater was 208 SINGLE PHASE. Yes, I have to admit, you usually associate that voltage with a three phase installation, and I've personally worked with it quite a bit, but I suppose it's possible for the heater to be set up for 208 single phase. If that's the case, it MIGHT be possible for that same heater to be operated on single phase 240. I'd do a little investigating to see. Maybe there's jumpers inside for 240, or maybe the mfg has designed the unit to run on 240 without modification, but with a slightly higher rating. Who knows. Maybe the name plate could be checked for confirmation of the 208 SINGLE PHASE part.
 
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Old 11-19-04, 09:24 PM
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Well, I did a quick & dirty investigation of a brand of heater that I've worked with before. All of the ones I've worked on ran on 480 volts three phase, but the same company makes smaller heaters that run on 208, 240, and 277 volts SINGLE PHASE. Many models can run on 208 OR 240 volts single phase. If you want 277 volts you buy a different model. The power ratings were the same for 208 and 240 and the diagrams show a over temp cutout switch in series with the heating coil. In reality you never have an overheating problem with such heaters as long as there is sufficient air flow past the coils to carry the generated heat away to the area to be heated. Assuming that the name plate DOES say 208 volts SINGLE PHASE, I'd hook it 240 single phase without a transformer without giving it a second thought. The 208 voltage is only a nominal value. In reality I've seen the 208 three phase line run a high as 218 to 225. Heating coils really don't much care how many volts you sock to them as long as the insulation will stand it. They won't over heat as long as the air flow is sufficient. The thing that could get hurt the most in this situation is the fan motor, and it won't notice a 15 percent NOMINAL increase in voltage much.
 
  #9  
Old 11-20-04, 10:38 AM
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If you do want to go with the exact voltage for this heater, you could use an autotransformer set up to "buck" the 240V down to 208V. Transformers set up in this way are much smaller and cheaper than a similar kva isolation transformer. The trick is that they are not straight forward to hook up and design.
 
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Old 11-20-04, 10:57 AM
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I looked at doing that, but still the transformer necessary is several hundred bucks. Copper ain't cheap anymore.
 
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