Electronics > Projects, Designs, and Technical Stuff
240V Oven Question
bostonman:
Long story short, I wanted to measure the current draw on my electric oven. Initially I thought 240V was "two phase", however, from research, it's not; or at least how I interpreted what I read.
It has an oven light and clock, so I imagine it also needs 120V too to power them, however, I'm not interested in that portion since it's low current.
Tonight I used a clamp on current meter and measured (what I thought) both phases (separately) with various parts of the oven on at different times. In almost all cases, usually when the inside of the oven was on (boil or broil), both wires had the same current going through them (I only have one meter, so I couldn't check them simultaneously). But when I would power the top external burner, one "phase" had 15A, and the other "phase" had 8.3A.
My two questions are: do I add the current in both "phases" (i.e. do I add the 15A and the 8.3A for a total of 23.3A)? If I understand this correctly, the current in both "phases" should be equal but opposite polarity, so why is the external burner two different current values, whereas the inside of the stove (boil or broil setting) equal?
Just to elaborate on one portion of all this, if I set the oven to "bake" and all four top burners on high (a somewhat worst case current draw), the current on one "phase" is 40A, and the other "phase" is 33A. If I need to add these together, that would trip the 50A breaker, so I assume the total current isn't 73A.
bob91343:
The currents don't add. Picture a 240 V line center tapped. You can get power from the entire 240 or from each 120 separately. If you draw from one side, the return current will go back on the center tap. If you draw from both sides, the currents subtract, reducing the current in the center wire. The loads can all be presumed to be pure resistance so there is no phase shift anywhere to deal with.
Ian.M:
CAUTION: Possible fault - further investigation required.
What's your stove powered from? If its a legacy 10-50R socket, (three pin ungrounded*, no longer to NEC code, but grandfathered in), your unbalanced readings for the top 'burner' indicate it may have an internal short to chassis part way along its element. If its on a 14-50R socket, (four pin), there are separate Ground and Neutral connections so *IF* the circuit is GFCI protected, *AND* the stove and its lead is internally properly wired (Ground and Neutral each on its own wire, *NOT* strapped together), any fault to ground would trip the GFCI.
There's also a small possibility that the top 'burner' consists of two or more 120V elements switched in in different combinations for different power levels. Due to the elements different wattage ratings, even if you distribute them over both phases as evenly as possible, the load will still be imbalanced at most or all power settings.
For what purpose do you want to know the current? If its to calculate power used, amperage at 240V is not equivalent to amperage at 120V, so if there is any imbalance its simplest to calculate the wattage as the sum of that of two 120V loads. If its for breaker or wire sizing, simply take the highest amperage of the two.
* Actually the third NEMA 10-50 pin is nominally Ground, used to ground the appliance chassis, but it carries the Neutral return current of any 120V loads such as timers and oven lamps, which AFAIK makes GFCI protection on that circuit impractical if the appliance isn't purely 240V.
wizard69:
IAN post above pretty much covers what you should expect. I would be concerned also if you have two dramatically different current readings on a single element burner. It would be most interesting to know if the other 3 "burners" display the same split. The possibility that the element has failed in some manner would be pretty high with such a wide split in current readings.
Also you can't assume that the lights and clock are 120 VAC. They might be or they could very well be 220 VAC devices. The age of the stove is a factor as is the price, that is if it is a cheaper unit the builder most likely too the route of minimal components.
Beyond all of that if you know the wattage ratings of the various burners you can quickly compute the expected current draws. Your current draws should be very close to the expected values.
I hope this is coherent as I just got off a 12 hour shift. One thing that might help to visualize or to keep in mind is that the elements are basically resistors in conventional ovens / ranges. At least on my simple low end stove the elements are all two wire. Effectively there is 220/240 VAC or so across those big resistors. If you have an extra fancy stove there could be other things going on and as noted new installations and stoves require the center tap to be brought to the stove. In general though the is about 240 VAC across those elements.
jmelson:
--- Quote from: bostonman on May 03, 2021, 03:24:28 am ---In almost all cases, usually when the inside of the oven was on (boil or broil), both wires had the same current going through them (I only have one meter, so I couldn't check them simultaneously). But when I would power the top external burner, one "phase" had 15A, and the other "phase" had 8.3A.
--- End quote ---
If the stove top burners have thermostatic controls that continuously dial from low to high, they should always run from 240. If it has controls that have warm-low-med-high click positions, then they may use 120 V for warm and low, and 240 for med and high, with two separate resistance elements in the burner. That could explain the different current on the two hots.
Jon
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