heater hell??

temporary) pump Why not do it the right way in the first place and avoid all doubt? Go down to your local electrical wholesaler and pick up a 480 V unit heater and avoid mickey-mousing a solution. Life is short, too short to spend time explaining to the fire department why they got called out to a fire because you save $10 on heaters.
Series-connected heaters aren't usual, and I conjecture they aren't tested and rated for that configuration. I'm not sure even if I found a line thermostat rated 480 V that I'd trust it.
Bill

Well, yes, right off I can think of one. The thermostat switch contacts are rated for 240VAC and whatever current the heaters draw. Your arrangement, the current will be right, but the voltage will be double.
If the contacts 'weld' some cold night, you may have a fire in the pump house.
You might want to reconsider...
daestrom

I'm sure that the 240 volt heaters are not "tested" at 480 volts, but the heater manufacture's engineer was the one that suggested the above setup. Also the 480 heater unit is not $10 more. They are like $400 for one VS $25 each for the 240 volt baseboards
William...

I read through the complaints and the serious one is the thermostat issue. Just switch them with a relay rated at 480v and use 600v wiring methods. As a sanity check, measure the voltage drop across each heater under load to insure they are fairly well matched. A good set of labels explaining what is going on will help the next guy figure put what you are doing. It is still not "legal"

If I put the thermostat between the heaters it will only "see" the 240 volts right??
William...

stepdown transformer AKA "dry pac"

when there is no current (heater "off" condition) the entire voltage will be placed across the switch contact of the thermostat.

----------------------------
---------- Wrong.
As was said by "no one" the full voltage will be across the open thermostat contacts. The problem is not when the contacts are closed, but when they open and (try to)interrupt the current. If rated for 240V they may not open as desired but sit there and arc, producing local heating which can lead to fire. This could be very expensive.
Don't fool around with what you don't know. It isn't worth it. If you heaters are well balanced and you have your thermostat contacts operate a relay rated for 480V at the current level needed, no problem. If you use a 1000 watt heater in series with a 2000 watt heater, you will have problems regardless.

Good point! I did not think about the off voltage, just its operation under the load.

Yes, I agree I wasn't thinking about the off voltage just the operating.
I was going to use 2 of the same units ( I would hope that they would be better matched than 1000w/2000w :-) The relay/contactor idea will fix up the thermostat problem. The whole system is only a temporary setup, as soon as I finish my house the well tank/water softener will be in the basement and not need the small heater.
William....

Hummmmm.... the transformer "idea" gave me another (Bad??) idea:-)
Some background...... The "system" is at the new home site about 400 feet from my shop. The well pump is another 300' from the home site. The pump is 208 3 phase, as will be the house when it's built. I didn't want to have the nasty voltage drop from sending 208 all that way, and I have 480 in the shop. So I ran the 480 to the temp "well tank/storage shed" that is sitting over where the utility room of the house will be. I have a 35kva 480 to 208 transformer in the shed that so far has powered the pump and is costing me an arm and a leg to run 24/7/365 for the little 1.5hp pump ( that maybe runs for 15 min a day) ( it does seem to keep the shed warm in the winter though!!) So now I'm going to kill/overkill the transformer "problem" with a AB powerflex VFD ( I know, a spendy way to make 208v, but it will also soft start the pump and is useful for other things after I no longer need if for the pump) and only run the transformer when I need 208/120v power at the job site.
The idea..... Put a AB #2 contactor switched by a thermostat in front of the "heater" AKA transformer and switch it.... I know it's not going to be the most efficient heater but I assume that most of the idle current that it takes to run is converted into heat?? I also know it's probably not a great idea to switch it on and off allot but will it damage it? It is very dry here in the winter so I don't think that the insulation will get wet from humidity and cause problems that way, but the documentation I read about dry transformers mentioned that if they get wet it could be a problem. Total cost for this project even less than the heaters as I have all the needed parts other than the thermostat.
William...

"heater" AKA You'd buy a variable frequency drive but you WON'T buy a 480 Volt heater? Why not run the heaters on the 208 V supply? *That* is a documented, tested configuration and if you're really uptight about voltage drop you could interlock the heaters with the pump motor so that only one load is on at a time.
Bill

As above, the drive will have some use after the job is done, on another machine, and the heater ( about $400 BTW 2X what I paid for the drive) would be useless for me after the shed is removed.
The requirement for the heater is only needed because I don't want to run the $$$ transformer! The transformer (35kva) has no problem with any load from the pump or a rather large heater running at once ( around here the power co will run 4-5 homes off a 50kva transformer). It just burns about 4 amps per leg to idle it, and that adds up fast when it is on all the time.
William....

think about heat tape

4A at idle is mostly magnetizing current, you do not pay for that.
Let' say the 35KVA xfm is 95% efficient at full load, that is 1.75KW loss. Let's say that half the loss is in the copper and half in the iron. At very low load you only have ~900watt loss in the iron that is not exactly negligible but is not an arm and a leg either.
Rather than looking at the current look at the meter.
Just thought to point this out.
MG

Ok so it's not an arm and a leg maybe just part of an arm :-) it seems that it kicks about $35 per month to the power bill when I leave it on all the time so about $420 per year just to keep it running seems like a lot for the actual required run time of the pump. That's why I got the VFD even though I paid about $200 for it new, it will cost almost nothing to have it idle when the pump is off. Also the submersible pump manufacture seems to indicate that the life of the motor will be much longer if the pump is ramped up with the smooth start instead of slammed on with the contactor, they say it makes the thrust bearings on the motor last a lot longer I suppose that a ramp down might be good to keep the water hammer down too. I can also give it a little more voltage to take care of any line loss in the 300' of wire from the VFD to the pump in the well. I guess I could even keepthe VFD running after I finish the house as I will still have the 480v line running from the shop up there...
Yes wish I could do that, but the power co's meter has 200 amp CT's and ticks off 100kw as 1 meter unit so watching it is not a good way to see the use. The power meter I have in the switchboard has 800 amp CT's and when the current is low ( like trying to just run the one transformer ) I don't get saturation of the CT's and I get a lower kw than the power co's meter records ( at least that's the best explanation they can think of for the LARGE difference in their meter's reading and mine :-(
Thanks William.....

If you have 4 wire 480 (three hots and a nutural) why not run it off one hot and a nutural giving it 277 not 240 but that should be close enough to do the trick....

Well, this doesn't DIRECTLY apply to the problem BUT the "air handler" for my home heat pump has a label that NO conductor may be more than 150 volts above ground. The unit only requires 240 2-wire power. It's clear that the manufacturer didn't want it used on any 3 phase service (aside from "dogleg" 120/120/240.) Maybe Dave Lennox knows something you don't know.
With good reason.
Temporary or not, if it's running on 480 3p hase then there is some serious money already put into the project. Spend a little more to do the heater "right." Either get a heater that is rated for the application (including voltage above "ground") or buy a transformer and it's protection.
But before you spend much money, you might want to check to see if those nice folks who make "pump house heaters" for 120 and 240 have a 480 volt model.
IF you have 4 wire rather than 3 wire 480 volt power you might have a few more options but don't get tempted to put things in series that the manufacturer doesn't approve or run heaters at too high a voltage.

I have an idea, correct me if I am way off your needs!
Can you get three heaters and wire them in star formation? you could then use three standard stats as they should all be at 240v as long as the loads were matched evenly, better phase balancing too....
would that do the trick? obviously the star point would have to be sensibly marked!
miniman