240V Oven Question

[bostonman]

As I mentioned (or believe I mentioned) some messages ago, I am trying to get an idea of how much current is going through the breaker box.

It's not necessarily the need to know what the oven draws, or to know the current to within a few hundred milli-amps, but just a general idea of what the draw is.

Once I saw the substantial difference in current when the oven was on bake, then that got my attention.

[richard.cs]

As I mentioned (or believe I mentioned) some messages ago, I am trying to get an idea of how much current is going through the breaker box.

In a split-phase system "the current through the breaker box" doesn't have a useful or well defined meaning. It better to say the current is X on L1, Y on L2 and Z on Neutral, or perhaps to say the highest current is on L1 and is X Amps.

If you are interested in how hot the box gets then the highest current is probably useful (as cable and breaker losses go as current-squared). If you're interested in your electricity bill then work in Watts or kilowatts.

[bostonman]

I had been waiting for GE to send a wiring diagram, however, they never got back to me.

I've verified the plug is a three prong plug. I also confirmed the assumption: when the oven is on 'bake', the bottom heating element gets red and the top gets hot, but doesn't seem to get as hot (i.e. it doesn't glow red).

When the oven is on 'broil', the top heating element turns red, however, one thing I failed to do (since I almost never use 'broil') is turning the temperature dial to 'broil'. Instead, I just turned the temperature up to around 400 degrees F. After I shut down everything, I noticed the temperature knob had a 'broil' temperature. Not sure if that just maximizes the temperature whereas setting it to 400 degrees F does the same thing; just cooler.

Also, on a side note, when I turn on the largest top "burner", it seems 'high' has its own section because when I continue turning the knob from 'high' to a lower temperature, it clicks out of 'high', but turns smoothly as it goes through the other lower temperatures.

Knowing it's a three prong outlet, back to my question: do I add the 15A and the 8.3A when on 'bake' to make a total current draw of 23.3A or is there something else going on?

[Monkeh]

No, you do not add them. You have 15A, and you have 8.3A. That's it. You have two values, for two lines.

[james_s]

I think there is no more mystery here, we can figure out exactly how this thing is wired by the way it behaves. The broil element is run from 120V in bake mode and 240V to broil. The "broil" position on the temperature dial is just going to keep the element on constantly rather than turning it off when the rest of the oven cavity reaches the setpoint. The "high" setting on the burner controls stops the burner controller from cycling and keeps it fully on steady. The click is a cam that either pushes the contact so the bimetal strip that cycles it can't open the contact, or disconnects power to the heater wrapped around the strip, same result either way, the burner stays fully on.

No, you don't add anything. You have a 15A load on one side and an 8.3A load on the other, with a 6.7A neutral current due to the imbalance. The highest current anywhere is 15A.

[bostonman]

I never realized how odd this oven works. I thought everything saw 240V and that was it making it simple.

No, you don't add anything. You have a 15A load on one side and an 8.3A load on the other, with a 6.7A neutral current due to the imbalance. The highest current anywhere is 15A.

The two questions I have (since it seems 240 wiring is a bit confusing to me than I thought - especially since some of the oven uses 120V). The simplest question: if I have a 100A breaker box, am I using 15A leaving me with 85A for the rest of the house or 23.3A leaving me with 76.7A for the rest of the house?

As for a 6.7A neutral current due to the imbalance, where is the rest of the current going from the 15A and the 8.3A?

[richard.cs]

The two questions I have (since it seems 240 wiring is a bit confusing to me than I thought - especially since some of the oven uses 120V). The simplest question: if I have a 100A breaker box, am I using 15A leaving me with 85A for the rest of the house or 23.3A leaving me with 76.7A for the rest of the house?

A 100 A breaker box allows you to have up to 100 A on each phase. So you have 85 A left on L1 and you separately have 91.7 A left on L2. You could therefore add another 85 A worth of 240 V load (leaving you 8.3 A spare on L2), or you could add 85 A of 120 V load to L1 and 91.7 A of 120 V load to L2.

As for a 6.7A neutral current due to the imbalance, where is the rest of the current going from the 15A and the 8.3A?

The 6.7 A of neutral current is the rest of the current. 15 A flows from the breaker box on L1. At the appliance this passes through some loads and splits, 6.7 A flows back to neutral and 8.3 A flows through other loads and into L2.

[james_s]

The two questions I have (since it seems 240 wiring is a bit confusing to me than I thought - especially since some of the oven uses 120V). The simplest question: if I have a 100A breaker box, am I using 15A leaving me with 85A for the rest of the house or 23.3A leaving me with 76.7A for the rest of the house?

There are two ways you can think of a 100A (North American, split phase) breaker box. Either as 100A of 240V or as 2x 100A so 200A total of 120V. It's the same amount of power either way, 100A worth of 240V loads or 200A worth of 120V loads are both 24kVA, and in the real world it will almost always be a mix. This oven is itself a mix and can be thought of as a combination of 240V and 120V loads in the same box, wired just like the combination of 240V and 120V loads in your house. Even if it didn't wire the elements this way the small aux loads like the oven lamp will be 120V.

When you have an uneven balance you look at it on a per-leg basis, you have a 15A load on one leg so you're left with 85A on that leg and you have a smaller load on the other leg, leaving you with a bit more remaining on that one. If your entire house was 240V loads then whichever leg hit 100A first would be your limit and the remaining capacity on the other leg would be unusable. Pure 240V loads draw perfectly evenly from both legs, imbalanced loads like your oven do not. If you put 110 amps on one leg and 5 amps on the other leg it would trip the main breaker even though you're under the total capacity of the service because you've overloaded one leg and the breaker is tied together. In practice it is impossible to ever have a perfectly balanced panel, but when wiring a house one attempts to spread the estimated loads as evenly as possible between the two legs and in practice there is sufficient capacity that it doesn't matter, I don't recall ever seeing a main breaker trip in a house. I'm not sure how to better explain this, it seems really simple in my head, hopefully it will eventually click and make sense for you.

[bostonman]

Your explanation makes sense and this has been quite the educational exchange of messages. I've looked inside a breaker box plenty of times, and, when my friend (an electrician) added a 240V breaker, told me how both "legs" should be balanced. I don't exactly remember, nor am I asking, but I seem to remember each leg wasn't separated left and right, but odd and even. Maybe I'm wrong, but it's irrelevant to this conversation; I'm just adding this to show I've taken time to listen when people tell things to me.

It's just that most older homes in my area have 100A service (with a 100A main breaker). Until now, I thought (assuming a perfect layout) 50A for one leg and 50A for another leg.

Prior to this conversation, I thought if I had perfect 10A loads to plug into my outlets, assuming I plug five in rooms that are wired on one leg, and the other five in rooms wired on the other leg, then I would be at the maximum my service can provide (50A on one leg and 50A on the other leg totaling the 100A).

If I understand correctly, I'd only be at 50% capacity because I can put another 50A on both legs before tripping the 100A main breaker.

So the main breaker, even though it's marked 100A, is actually 100A for each leg? Obviously it's not a "200A" breaker, but it's handling 100A on both legs?

I get why it's called a 100A service, but say I built a house that required I run strictly 120V, single phase, devices, then why would it not make more sense to say I have a 200A service panel?

My house has been upgraded to 200A, this means it's technically 400A (200A per leg)?

[james_s]

Yes that's exactly it, the 100A breaker is two poles, each rated at 100A that are mechanically linked together so that if either one is overloaded they both trip. The service is 100A at 240V, which is the same amount of power as 200A at 120V. You can power 200A worth of 120V loads but you CANNOT power any one 120V load that draws more than 100A, not that you're likely to encounter one.

The two bus bars in the panel are interleaved so that two side by side single pole breakers are each fed from a different leg. This is done so that you can install a double pole breaker and it straddles both bus bars because that's what it has to do to deliver 240V. The North American service is a 240V transformer with a center tap that is tied to ground/neutral. This is how we get the dual voltages, 240V loads are fed from both hot wires that have 240V between them. 120V loads are fed from one or the other hot wire which has 120V to neutral.

[bostonman]

Wow, this has been a good educational experience.

Now I understand why my question of adding 15A and 8.3A was incorrect and I wasn't getting answers that made sense to me. Seeing the 100A breaker, and not really thinking about the entire picture, I was visualizing 100A coming in from the street, a series 100A breaker, and every load after is subtracted from 100A, thus leaving me with how many more loads I can add.

Not that I'm going to do this, nor can I buy such an item, but I can basically run a 90A 120V air conditioner in one bedroom (that's connected to one leg) and a 90A 120V air conditioner in another room (that's connected to the other leg), with a 100A service, and not trip the main 100A breaker.

Obviously I'd be restricted to just a few light bulbs, but I'd have a cold house, and a breaker box that wouldn't trip.

[james_s]

Yes, technically you could do that, and your neutral current would be zero because you'd be drawing the same amount from both sides with no imbalance. Electrically it would be the same as a single 240V appliance drawing 90A. A pure 240V load with no neutral has to draw exactly the same current from both sides according to Kirchoff's law.

[bostonman]

That's a good point about the neutral.

I was thinking (referring to my 90A on each leg with a 120V line) that the neutral would see 180A of current since it's a single leg.

But I guess they'd be out of phase making the current zero?

[richard.cs]

Correct, it cancels.

The neutral sees 100 A under the worst case, which is when one live leg is fully loaded and the other has zero load. In all other cases the neutral sees less current because any current from the opposing leg causes cancellation of the neutral current.

The fully-loaded-neutral is sufficiently rare that historically some jurisdictions have allowed the neutral to be smaller (1/2 to 2/3 the line conductors). No idea if it was ever allowed in the USA.

[NiHaoMike]

Here's an explanation of how split phase 120/240V works:

[Zero999]

Yes, the currents are anti-phase so cancel. If you think split-phase is confusing, try reading up on three phase theory.

[james_s]

The fully-loaded-neutral is sufficiently rare that historically some jurisdictions have allowed the neutral to be smaller (1/2 to 2/3 the line conductors). No idea if it was ever allowed in the USA.

The neutral feeding the panel still can be smaller AFAIK. It's been about 12 years since I've installed a panel but I'm pretty certain the neutral was a size or two smaller than the live conductors coming in from the service entrance. I haven't looked at the current NEC requirements but I suspect that hasn't changed. Typically most of the large loads in a panel are 240V so they're always going to draw from both sides, the remaining 120V branch circuits are normally a lot less. I have an old split bus 200A panel that has 6 separate main breaker slots, one of which is a 50A double pole that powers the lower bus with all of the 120V branch circuits.

[bostonman]

This has been one heck of an educational discussion.

I'll never look at my circuit breaker panel the same now.

[bostonman]

After several weeks, GE sent a wiring diagram, however, I'm uncertain if it's correct because this shows a glass touch panel and fan. This oven is very basic and doesn't have any of that, plus it has an analog clock.

In any case, thought I'd share it since everyone contributed their input on helping dissect the connections.

[james_s]

That schematic is for a JT930 which is a fancy built in wall oven that combines microwave and convection.

https://assets.ajmadison.com/image/upload/c_limit,f_auto,fl_lossy.progressive,h_1000,q_auto,w_1000/v1/ajmadison/images/large_no_watermark/JT930SKSS.jpg

I don't think it is even remotely similar to the oven you have, isn't yours an old one? The classic electric ovens didn't even have a PCB in them.

[bostonman]

Yes, the one in question is a basic model and most likely no PCBs since it seems all mechanical.

I was trying to dig into it last night thinking maybe the general wiring is the same, but you're correct, it looks completely different.

[james_s]

Well if you REALLY want to find out, I did stumble across a website that has a lot of manuals for vintage appliances but they cost money so I didn't get one. I didn't bookmark the site but I'm sure you can find it with google.

[XOIIO]

Found this thread after posting in the general chat section looking for a two phase power meter, this is one of those things that twisted my brain a bit too lol, since 240v didn't work quite as I expected.

I have a kiln that I'm retrofitting to a pid controller for use as a burnout oven, and I want to add a power meter to it so I can track the energy usage, I was just going to use a cheap panel mount inductive monitor clipped around neutral, but after talking with a local appliance repair guy he said that it wouldn't work since neutral is just an unbalanced return line.

The two kiln sections run at 120v it turns out, each half of it connect to one of the 120v lines, and the neutral, they don't run the elements at 240v like the manual claimed, so I was wondering then, for this specific appliance, would measuring current on the neutral line only, and powering the meter across the two live connections be correct, or would I have to measure each phase independently?

I don't currently (heh) have a clamp meter to test with which is why I'm having a bit of trouble with this. I could just order two of the cheap power meters and test with those but hey, if I can save $20 and only use one, it would be good to know.

Then again, since I could in theory make a different cable and run one half of the kiln on medium on a standard outlet, maybe monitoring the power use of each 120v line alone is the way to go, if its as simple as adding them both up, since then I could also monitor it when in that half config.

[james_s]

Just get one of the power meters that's meant to monitor your entire house. I have one made by Efergy, it has a transmitter that mounts next to my panel and a pair of current transformers that clamp over the incoming live wires. It does the math automagically and displays the real time and cumulative consumption on a wireless console. If you want DIY there are some open source energy monitor projects that will work, and you can probably get hardwired panel meters that have two CTs.

If it really only ever runs one element at a time then yes, measuring the neutral current will work. I'm surprised it would work like that though, there is no reason to use 240V in the first place, the whole thing could run off a 120V circuit if it really only ever powers one side at a time. That seems unlikely. Are you sure it doesn't have a more complex cycle that runs one, the other or both depending on the point in the cycle?

[Ian.M]

Your repair guy is right - a meter on Neutral would only show you the imbalance, as the current in Line1 and Line2 are in opposite directions. If you were working with a wired meter shunt that would be it, game over, no way to get the total power, but as the meter uses a clip-on current transformer, you can get tricky and get it to add up the currents.   Pass the (insulated) Live1 and Live2 wires feeing the kiln controller through the sensor in opposite directions so the currents add up.  Do *NOT* pass Neutral through the sensor.  Then if the display is fed with 120V between one of the Line wires (the 'hots') and Neutral it should display total current and total power.   If it reads negative current or power swap its feed to the other 120V line.

If there is an imbalance drawing heavily from the Line the display isn't connected to, there will be some loss of accuracy, depending on how 'stiff' the supply is due to the voltage being multiplied by the current not coming from the same wire.   If your kiln is in an outbuilding, at the  end of a long minimum permitted CSA feed, and *ALL* the elements on one of the 'hots' are off it could over-read by as much as the 5% voltage drop permitted by the NEC.  However in most cases with less voltage drop and a better balanced load, the accuracy will be significantly better.