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]

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.

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

[bostonman]

I'll get the stove model number and see if I can find a wiring diagram online, and I'll also look at the plug type to see if it's three or four prongs.

If its to calculate power used, amperage at 240V is not equivalent to amperage at 120V,

Why is current at 240 different? If I measure 10A on 240, then it's still 10A going through the breaker panel.

As for why I'm doing this, long story short, it's to roughly calculate current through the panel on some of the higher power appliances; the electric stove in this case.

[bostonman]

At the moment I can't access the plug to see if it's 3 or 4 pins because it's behind the stove, however, I found the model number. Unfortunately I can't locate the manual online for it (which I hope contains the wiring diagram).

It's model number: JBP220K2AD

[james_s]

Why is current at 240 different? If I measure 10A on 240, then it's still 10A going through the breaker panel.

As for why I'm doing this, long story short, it's to roughly calculate current through the panel on some of the higher power appliances; the electric stove in this case.

The calculation is exactly the same either way, 10A is 10A whether 120 or 240V. The difference is power, 10A at 120V is 1200W while 10A at 240V is 2400W. In the case of a pure resistive load like an oven the power factor is unity so VA and W are the same thing.

[bostonman]

Thanks for clarifying that. The previous statement about amperage at 120 is different than 240 confused me, however, I knew power would double.

I'll dig deeper into finding the manual; hopefully it contains a wiring diagram.

[IDEngineer]

Initially I thought 240V was "two phase"

Strictly and accurately speaking, it IS two phase.

With "three phase", you have three sine waves with equally spaced phase offsets of (360 / 3 =) 120 degrees. You also have the option of taking any one of those phases to neutral for a reduced, single phase voltage.

With "two phase", you have two sine waves with equally spaced phase offsets of (360 / 2 =) 180 degrees. You also have the option of taking any one of those phases to neutral for a reduced, single phase voltage.

Nice and simple, right?

Except that a long time ago there was a two phase system where the phases were offset by 90 degrees instead of 180. THAT system became known as "two phase", so to differentiate from it a 180 degree offset system acquired the moniker of "split phase". Thus in many electrical power conversations when you're discussing "two hots and a neutral" where the two hots are a balanced 180 degrees out of phase, you'll hear such a system referred to as "split phase" instead of the more obvious "two phase".

I try to correct this error when discussing electrical power with people. Just because someone made a mistake in the past doesn't mean we are compelled to perpetuate it.

[james_s]

That's a matter of semantics really. The split phase 240V we use in North America is still called single phase, it comes from a single one of the 3 phases that come from the power grid. 2 phases implies a shift that can be used to create a rotating magnetic field, which a 180 degree shift cannot. It is not 2 phases, it is a single split phase from a center tapped transformer.

[fordem]

Also fairly common in the US is the use of two phases of the standard three phase feed, with the 120 degree phase angle, this gives 120V phase to neutral and 208 phase to phase - this is the primary reason I prefer to use the term three wire single phase, split phase to describe the 120/240 system.

[IDEngineer]

That's a matter of semantics really. The split phase 240V we use in North America is still called single phase, it comes from a single one of the 3 phases that come from the power grid.

The semantics is about how the power is generated/derived. The resulting "signal" speaks for itself. On the bench, with a scope, you'd describe the two "hot" wires as each being a separate phase with 180 degrees of separation between the two. The fact that it's high(er) voltage, or that its symmetry doesn't automagically yield a rotating magnetic field, doesn't obviate that there are two phases, easily distinguished. For confirmation, "single phase" has one phase (a single sine wave) and "three phase" has three sine waves, which (like two phase) are equidistantly separated at (360 / # of phases) degrees. If we had 5, 6, 7, or 11 phase power this relationship would still hold true. There's no reason to discount two phase power just because it (might) originate from a center-tapped transformer secondary.

Which raises an interesting question: On a portable gasoline generator, do the two phases come from two generator windings 180 degrees apart? Or is there a transformer in there with a center tap? I strongly suspect the former, in which case the argument for "two phase" is strengthened.

[james_s]

Fairly common at point of use yes, but I've never seen 2 phase power brought into a structure, there is no real savings over just bringing in all 3 phases if you're going to bring in two of them. Service is either single phase or 3 phase, individual appliances on a 3 phase service may use anywhere from 1 to 3 of the available phases.

[james_s]

Which raises an interesting question: On a portable gasoline generator, do the two phases come from two generator windings 180 degrees apart? Or is there a transformer in there with a center tap? I strongly suspect the former, in which case the argument for "two phase" is strengthened.

It is indeed the former, it would be a pointless waste to include a separate multi-kVA transformer in there to split the phase. In my mind it's not really any different than a transformer with multiple secondaries that can be wired in series or parallel. Even if you have 3 secondaries you still can't make 3 phase power from it.

[IDEngineer]

Fairly common at point of use yes, but I've never seen 2 phase power brought into a structure

Every house I've lived in had two phase power. 120V per phase, 180 degrees out of phase, use one or both phases depending upon the load.

Yeah, I know. {grin} 

[NiHaoMike]

Fairly common at point of use yes, but I've never seen 2 phase power brought into a structure, there is no real savings over just bringing in all 3 phases if you're going to bring in two of them. Service is either single phase or 3 phase, individual appliances on a 3 phase service may use anywhere from 1 to 3 of the available phases.

Quite common in US apartments since it would be overkill to have 3 phase in every room but very reasonable for the whole building. Each room gets 2 of the 3 phases, spread out to even the load.

[bdunham7]

Strictly and accurately speaking, it IS two phase.

No, it really isn't.  There are fundamental differences between all single-phase systems (split or not) and all polyphase systems.  You can call it two-phase if you wish, but the term 'phase' in that context means something quite different than same term used in reference to a polyphase system. 

[IDEngineer]

I can't quite agree. In every case, a voltmeter across different phases will show a voltage potential which can cause current to flow.

Indeed, the 90 degree offset case is the oddball. It is the exception to the (360 - # of phases =) phase offset relationship that describes the others. I understand the convenience of having the built-in rotating magnetic field behavior but other than Philadelphia(?) I know of nowhere in the United States that has offered such utility power for decades. Literally everywhere else uses one phase (no offset), two phase (in every case 180 degree offset), or three phase (in every case 120 degree offset). I suspect that's true worldwide.

Why refer to 180 degrees of phase offset "differently" than 0 or 120 degrees of offset, other than in magnitude? The sine waves do not know what you plan to do with them... resistive loads, inductive loads....

[bdunham7]

Why refer to 180 degrees of phase offset "differently" than 0 or 120 degrees of offset, other than in magnitude?

Because your two 'phases' are simply the result of a single phase with an entirely arbitrary selection of the zero or ground point that happens to usually be right in the middle due to a center-tapped transformer.  You may as well hook up 2 9 volt batteries in series, ground the center point and call it 'two-phase DC'.  In theory, you can efficiently derive any polyphase system from any other polyphase system (I think--see the Scott-T transformer for an example) with just a set of transformers and if one system is balanced, the other will be as well.  You can't do anything like that with your 'two phase', the phases are degenerate.  So as I said, you can call it 'two phase' if you like--it's not particularly confusing--but 'phase' in that context has a different meaning.  Polyphase is generated, distributed and delivered as polyphase, it has to be 3 (or however many) phases the whole way through.  Split phase is just the way a single phase is wired at the point of use.

Look up 'high leg delta' and tell me if you would call that 3, 4 or 5 phase!

[bostonman]

I found the manual online, and I also found the installation instructions that were stuffed in a drawer (see attached pictures).

Unfortunately it doesn't seem to specify much about the electrical connections - or at least I didn't see anything pointing to whether it feeds off the 120V too.

To answer some previous questions: when I turn the top dial s for the top burners (are they actually burners since they are electric?) they click past 'off', but seem to be smooth turning.

Correct me if I'm wrong, but I assume the heating elements are controlled by triacs controlled by the hand dials. When I measured the currents, it seemed the current didn't change (within reason - obviously the current changed a bit from initial turn on to steady state - although I didn't keep the oven on long) regardless of the temp setting, however, the heating elements I think get more red the higher I set the temperature.

[james_s]

Fairly common at point of use yes, but I've never seen 2 phase power brought into a structure, there is no real savings over just bringing in all 3 phases if you're going to bring in two of them. Service is either single phase or 3 phase, individual appliances on a 3 phase service may use anywhere from 1 to 3 of the available phases.

Quite common in US apartments since it would be overkill to have 3 phase in every room but very reasonable for the whole building. Each room gets 2 of the 3 phases, spread out to even the load.

It occurred to me right after I posted my reply that someone would probably mention this. As far as I'm aware, the buildings are still fed by the full 3 phase feed from the transformer, and then the panels in the individual units get two of the three phases. I guess it's fair to call that 2 phase to the dwelling, but it isn't 2 phase power fed into a building.

[ejeffrey]

I can't quite agree. In every case, a voltmeter across different phases will show a voltage potential which can cause current to flow.

Indeed, the 90 degree offset case is the oddball. It is the exception to the (360 - # of phases =) phase offset relationship that describes the others. I understand the convenience of having the built-in rotating magnetic field behavior but other than Philadelphia(?) I know of nowhere in the United States that has offered such utility power for decades. Literally everywhere else uses one phase (no offset), two phase (in every case 180 degree offset), or three phase (in every case 120 degree offset). I suspect that's true worldwide.

Why refer to 180 degrees of phase offset "differently" than 0 or 120 degrees of offset, other than in magnitude? The sine waves do not know what you plan to do with them... resistive loads, inductive loads....

Because 180 degrees is equivalent to multiplication by -1.  Its just the negative of the other signal.  It is still a 1-D / scalar quantity.  And it doesn't have any of the other useful properties of what are normally called multi-phase systems such as that the instantaneous power never goes to zero.  You can generate an inverted signal from a single phase with a simple transformer (or just crossing the wires!) but you can't make a polyphase signal from single phase (split or not) without an energy storage device.

While utility power is never 2-phase any more, two phase BLDC and stepper motors are common, and they refer to 90 degree phase separation.

Split phase is called it because you take a single phase winding and add a center tap, splitting it in two.

It's sure a matter of definition and nomenclature, since you are certainly correct that the other live in a split phase system can be described as phase shifted by 180 degrees.  But it is pretty universally accepted nomenclature.  So coming out in threads and saying "well, actually US residential power distribution is a 2-phase system" is not really correct.

[james_s]

I found the manual online, and I also found the installation instructions that were stuffed in a drawer (see attached pictures).

Unfortunately it doesn't seem to specify much about the electrical connections - or at least I didn't see anything pointing to whether it feeds off the 120V too.

To answer some previous questions: when I turn the top dial s for the top burners (are they actually burners since they are electric?) they click past 'off', but seem to be smooth turning.

Correct me if I'm wrong, but I assume the heating elements are controlled by triacs controlled by the hand dials. When I measured the currents, it seemed the current didn't change (within reason - obviously the current changed a bit from initial turn on to steady state - although I didn't keep the oven on long) regardless of the temp setting, however, the heating elements I think get more red the higher I set the temperature.

They're still commonly called "burners" even though they're not actually burning. If you want to be fancy you could refer to them as calrod burners or calrod elements.

I'm not saying triac controlled burners don't exist but I've never seen one. The classic infinite controller is actually a cycling thermostat. Internally it has a heating element wrapped around a bimetal strip, this heater is fed from the same output that cycles power to the burner. The knob you turn is a mechanical adjustment that controls the spring tension against the bimetal strip thus adjusting the duty cycle. When you turn it all the way off the contacts are held open and when you turn it all the way up to high they're held closed but the rest of the time it will cycle. This design has been around a lot longer than triacs, it's more reliable and less likely to fail shorted.

[NiHaoMike]

It occurred to me right after I posted my reply that someone would probably mention this. As far as I'm aware, the buildings are still fed by the full 3 phase feed from the transformer, and then the panels in the individual units get two of the three phases. I guess it's fair to call that 2 phase to the dwelling, but it isn't 2 phase power fed into a building.

It's also possible for there to be a 3 phase transformer in the area with each building connected to 2 of the three phases, that might be the case for apartments with a few rooms each in many smaller buildings. It's also possible for individual houses to be connected that way in a neighborhood although I don't think it's very common.

I'm not saying triac controlled burners don't exist but I've never seen one. The classic infinite controller is actually a cycling thermostat. Internally it has a heating element wrapped around a bimetal strip, this heater is fed from the same output that cycles power to the burner. The knob you turn is a mechanical adjustment that controls the spring tension against the bimetal strip thus adjusting the duty cycle. When you turn it all the way off the contacts are held open and when you turn it all the way up to high they're held closed but the rest of the time it will cycle. This design has been around a lot longer than triacs, it's more reliable and less likely to fail shorted.

Some higher end electric stoves use triacs in order to modulate the heat at a faster rate to make it more like a gas stove, but I think those have largely been replaced by induction cookers.