Pushing 240V through a 208V heating element

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A/A Fuel GTX

A/A Fuel GTX

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I have a convection oven made in China:(. It is 10KW, single phase, 208 Volt. I only have 240V available. Assuming the current draw will coincide with 10KW @ 208V even though there is 240V applied. Are there any negative ramifications with applying 240V through 208V loads?
 
A/A Fuel GTX said:
... Assuming the current draw will coincide with 10KW @ 208V even though there is 240V applied. ...
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For all practical purposes, the resistance of the heating element does not change, so when you apply a higher voltage the power and the current both go up. Most US based equipment would be fine with this. For something made in China, it may have been very tightly designed to work on 208 without enough "beef" in the element to work at 240 volts. It will be operating at ~ 13.3 kW on 240 volts.
 
Would anyone consider this to be a violation is the unit is specifically marked for only 208 volt operation? IMO whether it works or not is not relevant as to whether or not it's permissible to connect it to 240 volts.
 
infinity said:
Would anyone consider this to be a violation is the unit is specifically marked for only 208 volt operation? IMO whether it works or not is not relevant as to whether or not it's permissible to connect it to 240 volts.
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I would consider it a violation. Heat transfer and dissipation needs to be considered. Also, internal wiring may never have been designed with the generous tolerances, electricians are used to, based on NEC loading tables.
 
don_resqcapt19 said:
For all practical purposes, the resistance of the heating element does not change, so when you apply a higher voltage the power and the current both go up. Most US based equipment would be fine with this. For something made in China, it may have been very tightly designed to work on 208 without enough "beef" in the element to work at 240 volts. It will be operating at ~ 13.3 kW on 240 volts.
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I know of some radiant heaters that are made in both 208 and 240 volt versions. They clearly state in instructions they will not warranty 208 units that are connected to 240 volts.

Seen control transformers for HVAC equipment over the years that often have 208 and 240 input taps, usually factory connected to 240. They work for a few years but when it does fail you find it is applied to 208 and nobody changed the tap. May also find on those that other control components have failed because of improper voltage applied. Here you often do get away with this one easier because supply voltage is often near the high end of tolerable range which is not much below the low end of tolerance for 240.
 
OP also said convection oven which suggests an internal fan; while unlikely, the fan operation at 240 should be considered. I agree with infinity and Jim Dungar.
 
I'm just thinking if I'm inspecting this equipment I'm failing the installation if the supply voltage is 240 volts.
 
110.4 Voltages. Throughout this Code, the voltage considered shall be that at which the circuit operates. The voltage rating of electrical equipment shall not be less than the nominal voltage of a circuit to which it is connected.
 
augie47 said:
110.4 Voltages. Throughout this Code, the voltage considered shall be that at which the circuit operates. The voltage rating of electrical equipment shall not be less than the nominal voltage of a circuit to which it is connected.
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Thanks Augie I was too lazy to look up the code section. (y)
 
I'm pretty skeptical about a single phase electric oven truly being rated for 208V but not 240V. As far as I can recall every electric oven label I've ever seen had markings for both. It'd be pretty ridiculous to design and market something that only does 208, as there are no major 208V-only markets. Also from an engineering standpoint as far as I can see, with proper safety factors any heating element should be able to handle the difference. The difference is just a matter of how fast the oven heats up and how often the thermostat is turning on and off. I'd be trying to confirm first whether the oven is really not also rated 240V, and can I get any other documentation on it.
 
jaggedben said:
I'm pretty skeptical about a single phase electric oven truly being rated for 208V but not 240V. As far as I can recall every electric oven label I've ever seen had markings for both. It'd be pretty ridiculous to design and market something that only does 208, as there are no major 208V-only markets. Also from an engineering standpoint as far as I can see, with proper safety factors any heating element should be able to handle the difference. The difference is just a matter of how fast the oven heats up and how often the thermostat is turning on and off. I'd be trying to confirm first whether the oven is really not also rated 240V, and can I get any other documentation on it.
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I agree, 240 doesn't mean it won't work, at least for a while.
 
jaggedben said:
I'm pretty skeptical about a single phase electric oven truly being rated for 208V but not 240V. As far as I can recall every electric oven label I've ever seen had markings for both. It'd be pretty ridiculous to design and market something that only does 208, as there are no major 208V-only markets. Also from an engineering standpoint as far as I can see, with proper safety factors any heating element should be able to handle the difference. The difference is just a matter of how fast the oven heats up and how often the thermostat is turning on and off. I'd be trying to confirm first whether the oven is really not also rated 240V, and can I get any other documentation on it.
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Yeah I spent some time looking once and could not find residential ranges or dryers that were 208 specific. Commercial stuff is different though. Last year had one of those commercial ranges, and it was 208 only. I called them and asked if 240 was ok, they said no. May have been fine for a while, maybe forever who knows, but I ended up getting a buck transformer.
 
Probably shorten the life of the elements, I had a customer that I added buck/boost transformer to their kiln in order to lengthen the life of the elements. ( bucked it to 208, kiln was 240) If it was a radiant heater with an external thermostat, it would probably trip the high limit, but since this has direct temp control, that shouldn't be an issue.
 
One possible problem with applying a voltage higher than rated is what's called the "watt density" of the heating elements themselves, basically how hot the SURFACE temperature of the element will get. By increasing the voltage and thereby the watt rating, but not the amount of surface area of the elements, the surface temperature of the heating elements will increase. That may cause issues with the systems that hold the element in place, insulate it, etc. It may not as well, but unless the mfr. specifically SAYS it is OK to apply 240V, we really have now way of being sure.
 
augie47 said:
110.4 Voltages. Throughout this Code, the voltage considered shall be that at which the circuit operates. The voltage rating of electrical equipment shall not be less than the nominal voltage of a circuit to which it is connected.
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So how can we use NEMA standard motors? The nameplates are required to be marked 230 volts for use on a nominal 240 volt system, and 460 for use on a nominal 480 volt system. :)
 
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