208V vs 240V

[kf4zht]

Just won a process oven in an auction, plan on using it for powder coating and painting. Here it is:

http://www.atlbid.net/cgi-bin/mnlist.cgi?robinhuff1/379


The electrical plate states that it requires 208V Single Phase 12.5a. Is there a real difference between 208V and 240V that I need to worry about for this unit? The blower motor on top is stamped for 220V so guessing that it is ok.

If not do I need a step down transformer?

 

[theoldwizard1]

Yes, there is a difference of 32V ! (Sorry, someone had to say it.) 208V is a commonly derive voltage from 3 phase.

Can you run it on 240V (typical residential split phase voltage). Probably, but I would check with the manufacturer.

 

[Norcal]

Only proper way to run a 208V item w/ a 240V supply is to use a buck/boost transformer to buck the voltage down, this will be a lot cheaper then changing elements as some of the ones used in food service equipment can be costly.
One should only operate equipment at the voltage it was intended to be run at.

 

[tylernt]

A resistance heating device (like an oven) will run hotter at 240V than it will at 208V. Whether this makes no difference, causes the device to catch fire, or something in between, depends on a lot of factors...

I would try to contact the manufacturer, and if you can't get them to confirm or deny, I wouldn't risk it on 240V and would start shopping for a transformer.

 

[Milton Shaw]

You might check and see if the mfg makes a 240 volt element for that furnace. That would be a lot cheaper than an external transformer. The 220 motor should be good to go just the heater needing to be changed. Going the other way using a 240 volt unit on 208 results in the unit heating a lot slower. For instance a 240 volt oven on 208 might take 30 - 45 minutes to heat to 375 to cook something. A lot of appliance stores have 208 instead of 240 and they have a lot of trouble having cooking demo's because they take so long to heat up. Some of the higher end stoves used to have an extra oven heating element to wire into the circuit if they were going to be used on 208 all the time. (these were high end $7,000 and up dual fuel, gas cooktop and electric double oven).

 

[jerryd68]

240 volt to the heating elements will not make the oven run hotter! It will increase the speed at which the element get hot. If the unit has decent temperature controller once the oven reaches set point the power will be reduced or turned off to the heating elements.

 

[LXCam]

240 volt to the heating elements will not make the oven run hotter! It will increase the speed at which the element get hot. If the unit has decent temperature controller once the oven reaches set point the power will be reduced or turned off to the heating elements.

Ah yup.

OP, are you sure it's a single phase unit. Being rated for 208 kind of makes me think the blower motor might be three phase. As far as the elements go, they are always single phase. And like Jerry point out, that is what will occur and it'll also draw a little less current.

So using what info you provided and assuming it is single phase. If it draws 12.5 amps at 208 (2600 watts), it'll draw 10.8 amps at 240V.

 

[tylernt]

So using what info you provided and assuming it is single phase. If it draws 12.5 amps at 208 (2600 watts), it'll draw 10.8 amps at 240V.

While this logic (more V = less A) may apply to inductive loads such as a motor, I beleive it is incorrect for a resistive load. Ohm's law:

http://www.electriciantalk.com/f2/208v-vs-240v-heater-elements-35009/#post655951

 

[Stuart in MN]

Generally, unless a device is marked 208/240 on the nameplate it's not designed to run at either voltage, but why not call Blue M and ask them? Contact information is on their website: http://www.thermalproductsolutions.com/brand/bluem.aspx?tid=1

 

[hmbemis]

While this logic (more V = less A) may apply to inductive loads such as a motor, I beleive it is incorrect for a resistive load. Ohm's law:

http://www.electriciantalk.com/f2/208v-vs-240v-heater-elements-35009/#post655951

Resistance of the heating elements is fixed, they are just really big power resistors... In this case the voltage is changing (from 208V to 240V).

Current = Voltage / Resistance

 

[tylernt]

So the current will change--it will decrease as the voltage increases.

Again, that's inductive (motor) logic. By that logic, applying 240V to a 120V light bulb will make it half as bright. In reality, the light bulb will get REALLY bright for a heartbeat or two, right before it blows.

12.5A @ 208V = 2,600W

From this we find that the heater element has a resistance of 16.64Ω (208V / 12.5A).

If we apply 240V to a 16.64Ω resistor: 240V / 16.64Ω = 14.42A

14.42A @ 240V = 3,460W

That's 860 more watts at 240V than at 208V.

I admit math is not my strong suit, so I'm happy to be corrected.

 

[ishiboo]

A lot of good points.

As jerry said, current may actually decrease. This device uses a temperature controller. Depending on how it works, it may either reduce the current to produce the same heat output, or it will consume more current at 240v (due to Ohm's law and the fixed resistance then) but have a lower on time if it is cycling on/off instead of modulating current/etc.

 

[hmbemis]

Again, that's inductive (motor) logic. By that logic, applying 240V to a 120V light bulb will make it half as bright. In reality, the light bulb will get REALLY bright for a heartbeat or two, right before it blows.

12.5A @ 208V = 2,600W

From this we find that the heater element has a resistance of 16.64Ω (208V / 12.5A).

If we apply 240V to a 16.64Ω resistor: 240V / 16.64Ω = 14.42A

14.42A @ 240V = 3,460W

That's 860 more watts at 240V than at 208V.

I admit math is not my strong suit, so I'm happy to be corrected.

Link to Book @ Google Books

A resistive heating element will only produce the rated wattage at a rated voltage. It will operate a lower voltage with a reduction in wattage. If connected to voltages above their rating they will have a very short life.

...

In resistive circuits, current is directly proportional to the voltage and can be simply calculated using a ratio and proportion formula.

When all is said and done, running the 208V element at 240V might work for a short period, but it might also run it too hot causing it to burn out faster--like the light bulb.

 

[rlitman]

240 volt to the heating elements will not make the oven run hotter!

So long as it is not on fire, yes, the thermostat should still control the temperature.

Running a 240V element at 208V is fine (but the ELEMENT will run cooler). Running a 208V element at 240V may not be, because while the thermostat will still control the oven temperature, while it is heating, the element may be heated beyond its design temperature.

 

[jerryd68]

The element won't run cooler, it is a resistive load, it will take longer to get to temperture and won't catch up as fast as if it is designed for a 208 circuit. Depending on the controller though I would bet that it will work at either voltage, if it is designed for 208 and wired to 240 you may eventually burn out the resistive elements, if it is designed for 240 and wired to 208 it will take longer to reach its setpoint.

 

[Socophreak]

I commonly see 208V elements running on 240V. It isn't recommended, but for a convection oven, it should do just fine. I work for a commercial kitchen company, This oven won't be far off from a regular convection oven.

Unless these elements are special, they typically are a nichrome wire with plaster casing in a steel tube. They are designed to get VERY hot.

 

[trainer]

It'll work fine. Single phase, 208 and 240volts are generally interchangable.

 

[Norcal]

It'll work fine. Single phase, 208 and 240volts are generally interchangable.

No they are not.

 

[kf4zht]

As a follow up on this - I checked the motor and it was 220/240v rated. Figured that the elements wouldn't be hurt too much. The manual costs more than the unit cost me, so I put a plug on the cord and tried it. Heated right up to the correct temp. I am going to run it the first few times away from everything and under a close eye, but I think it will work in this case.