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Old 11-15-2008, 10:38 AM   #1
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240 vac to 208 vac


I am no electrican and need some help. I just had a 208 vac 32 amp electric kiln given to me. It states in the literature not to hook a 208 vac into a 240 vac. What procedures are required to make this application work.

Thanks for any information you can give me.
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Old 11-15-2008, 10:40 AM   #2
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240 vac to 208 vac


Get your self an electrician to hook up a buck/boost transformer.

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Old 11-15-2008, 11:01 AM   #3
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240 vac to 208 vac


In which case it would probably be cheaper to sell that kiln and get the right one.

Actually, you may be able to just replace the elements. That may be cheaper still.
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Old 11-15-2008, 11:10 AM   #4
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240 vac to 208 vac


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In which case it would probably be cheaper to sell that kiln and get the right one.

Actually, you may be able to just replace the elements. That may be cheaper still.

Good call, I was just taking a look at buck/boost prices and thought wow, not a great free gift at all.
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Old 11-15-2008, 11:30 AM   #5
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240 vac to 208 vac


if the unit tag only shows 240VAC on the voltage tag and not 208/240VAC then that the manufacturers specs.that lower voltage would cause a higher amperage on elements in the long run.what voltages in your house do you read coming into the main panel.
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Old 11-15-2008, 11:59 AM   #6
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240 vac to 208 vac


I want to thank everyone for thfeir help with the voltage problem with this kiln. The specs on the kiln tag says 208vac and 32 amps. Didn't realize it is such a major problem.
I appreciate everyone assistance.

Thanks
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Old 11-15-2008, 12:01 PM   #7
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240 vac to 208 vac


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Originally Posted by snapcutter View Post
a 208 vac 32 amp electric kiln
not to hook a 208 vac into a 240 vac.
208v/32A = a 6.5 ohm heating element, at 6.7kW.
If you can find a 1 ohm 32A 1kW heating element for this kiln you can put it in series with the existing element and convert the oven to 240v, 7.7kW.
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Old 11-15-2008, 08:04 PM   #8
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240 vac to 208 vac


You will need to purchase a 1 kVA 120/240 to 16/32 Volt transformer, and wire it in series as an autotransformer. Sometimes called a "buck/boost" transformer, you will be using it as "buck" in this situation.

By wiring the 32 Volt secondary in series with the 240 primary windings, you can effectively reduce the output by 28 volts, with a net nominal output of 212 Volts.

The wiring diagram is found on page 10 of this file, look for schematic #4.

HV = 240 Volts input
LV = 212 output.
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Old 11-16-2008, 10:41 AM   #9
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240 vac to 208 vac


I would run it on 240. I think it will work fine. Measure the voltage you have. It's probably not at 240 anyway. You do not have a warranty to worry about. Just run it on 240 and if down the road the elements fail you can replace them. Have element access ahead of time.
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Old 11-16-2008, 06:28 PM   #10
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240 vac to 208 vac


kbsparky, a 1KVA transformer with 6000 watts+ running through it? Is that a good idea?

snapcutter, another option might be to run it on 120v at a much lower wattage. It would still get hot, just not as fast and not as hot.
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Old 11-17-2008, 10:48 AM   #11
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240 vac to 208 vac


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kbsparky, a 1KVA transformer with 6000 watts+ running through it? Is that a good idea?
The xformer only sees 32v x 32A and in this case it is returning power to the grid.

Last edited by Yoyizit; 11-17-2008 at 02:49 PM.
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Old 11-17-2008, 03:13 PM   #12
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240 vac to 208 vac


That transformer only sees about 1/7 of the total load. That's the beauty of using autotransformers. You only need capacity for the difference of the supplies.
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Old 11-17-2008, 10:00 PM   #13
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240 vac to 208 vac


Despite all the fancy engineering mathematical formulas, it comes down to this.

When sizing a buck-boost transformer, the secondary (lower voltage) winding needs to be big enough to handle the current of the load. It really is that simple.

The primary voltage of a buck-boost transformer needs to match either the line or the load voltage. Usually 120, 240, 480. They usually come in two different secondary voltages, 12/24, or 16/32. There are two secondary windings in the same case. A lot of them are dual primary as well. The voltage it operates at depends on how it's connected. Using the 12/24 as an example, a parallel connection will give 12 volts, a series connection will give 24 volts.

When sizing these transformers, the primary voltage doesn't matter, only the secondary. Lets say we have a 20 amp load. If we want to raise or lower the voltage by 12 volts, 12 X 20 = 240. The VA (volt-amp) rating of the transformer needs to be 240VA or more. 250VA is a common size, so that's what we'd use. Using the same example, but raising or lowering 24 volts, 24 X 20 = 480. The 250VA model won't work anymore, because we're making it do more work (24 volt difference rather than 12). In this case, we'd use a 500VA unit.

A 250VA transformer isn't very big at all. Including the built-in splice box, it's about 4" wide, 4" deep, and 6" high. It weighs about 10 lbs. But it can raise or lower a 20 amp load by 12 volts.

Suppose we need to go 32 volts. 240 - 208 = 32. Very common. 32 volts X 20 amps = 640 volt-amps. In this case, we'd need a 750 VA transformer. This one is about 6" X 6" X 8", and weighs about 25 lbs.

As you can see, the more we want to raise or lower the voltage at the same amperage, the larger the transformer. In any case, they're quite small for what they do.

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Old 11-17-2008, 10:11 PM   #14
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240 vac to 208 vac


I personally don't think it will be a huge problem to run the thing on 240. It might slightly shorten the life of the elements, but probably not so much as to be noticeable. Just my opinion.
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Old 11-18-2008, 08:45 AM   #15
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240 vac to 208 vac


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Originally Posted by InPhase277 View Post
I personally don't think it will be a huge problem to run the thing on 240. It might slightly shorten the life of the elements, but probably not so much as to be noticeable. Just my opinion.
This reduction in service life can be guesstimated if we know the kiln temp at 208v.

If 208v at 32A gives a 200C temp rise above ambient, 240v will give a 230C temp rise above ambient. This will reduce the kiln lifetime to 1/8th of what the factory design lifetime is.

I don't think this effect can be cancelled out by using a thermostat in the kiln.


Last edited by Yoyizit; 11-18-2008 at 02:48 PM.
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