Electric oven and cooktop

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green2012

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highland park, il
I have a cooktop 120/240 3 wire 10.22kva and a double wall oven 120/240 3 wire 11.4 kva. The prior owner wired them wrong and they cant be run together. They have them both on the same 30amp breaker using 10 awg! lol The manufacturer says I need 2 50 amp breakers? That seams a little excessive. Does this sound right? I would have thought 2 40 amp breakers would be enough. I also want to utilize the same pipe run if I can. I need number 8awg for the hots but can I downsize the neutral?

1. Can I downsize the neutral? (70 % rule)
2. What size breakers?
 
You may be in an area of some controversy. If you apply Art 220, the loads you specify would be calculated at 8 kw and a 40 amp circuit to each would suffice.According to 210.19(A)(3) you can use the 70% rule for your neutral (no smaller than a #10)
 
does the table in 220 apply to the branch circuit or just when calculating services? I have only ever used it when I was trying to do a load calculation for an entire home.
 
mjf said:
My understanding is that the manufacturers specs trump the NEC.
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If those are actual manufactures specs I would agree.

green2012 said:
does the table in 220 apply to the branch circuit or just when calculating services? I have only ever used it when I was trying to do a load calculation for an entire home.
Click to expand...

Look at Note 4 to Table 220.55

My opinion is that (2) 40 amp circuits would meet Code MINIMUM. My opinion is also that I would use 50 amp circuits.
 
Note 4 of table 220.55 says "4. Branch-Circuit Load. It shall be permissible to calculate the branch-circuit load for one range in accordance with Table 220.55. The branch circuit load for one wall-mounted oven or one counter-mounted cooking unit shall be the nameplate rating of the appliance. The branch-circuit load for a counter-mounted cooking unit and not more than two wall-mounted ovens, all supplied from a single branch circuit and located in the same room, shall be calculated by adding the nameplate rating of the individual appliances and treating this total as equivalent to one range."

Don't ask me how to reconcile the first two sentences, as they seem contradictory. However, it seems just about every new house comes with a 40A range circuit and typical ranges are about 10KW to 12KW in size. If the branch circuit had to be sized to nameplate, you'd need a 50A circuit.

Based on the highlited note, I would think you could use one 50A circuit to feed the combination of two ovens and the counter mounted unit. Sounds crazy, but this 22KW "range" load would be 8KW + (21.62 - 12 = 10 = 50% increase over 8KW) = 12KW maximum demand. 12000 / 240 = 50A circuit.
 
I think I understand the branch circuit part I didn't get the first time. It is" It shall be permissible to calculate the branch-circuit load for one range in accordance with Table 220.55. The branch circuit load for one wall-mounted oven or one counter-mounted cooking unit shall be the nameplate rating of the appliance."

The first sentence covers a range (not what you have). The second covers a circuit for a cooktop OR an oven (what you have if wired separately). So I think you have two choices:

Run all this from one branch circuit, and 50A just barely covers it (a 60A may be better).
Wire the oven and cooktop separately, and wire each circuit per the nameplate of the oven/cooktop.

Seems kind of strange with this approach that one circuit per appliance needs to be 50A and that a combined circuit serving both could also be 50A. Seems like they should allow a demand factor for the large ovens and cooktops too, but those are limited to the smaller units.
 
Mark, You very well may be correct and I'm embarrassed to say I don't recall, but I think that Note is telling us you confirm that you can use Column C values by the nameplate bring within that allowable range. As long as your nameplate is not over 12kw, the 8 kw figure will suffice.
In some way that males sense as in a great majority of situations, on new home construction, you are installing for an unknown value but knowing most ranges are 12kw or less.
 
Note 1 says a range over 12KW but not more than 27KW can still use column C. You just have to increase by 5% for each KW above 12. That's what the (21.62 - 12 = 10 = 50% increase over 8KW" was about in my earlier post.

That rule allows a single range of up to 16KW to be connected to a 40A circuit (16 - 12 = 4 -> 4*5% = 20% increase over 8KW which is 9600KW which is 40A at 240V).
 
Forget what code says for a minute or two, and look at reality.

How often do you use both ovens and all the surface elements at same time?

If you do use all at same time, how long do you have them at "high" setting?

Once the ovens are up to setpoint temperature, they only cycle as needed to maintain temperature. Same with cooktop, with exception of when an element is in the "high" position, but leave it there for very long and you are likely burning whatever you are cooking.

Very good chance a 40 amp breaker, even though more than 40 amps may be drawn at times doesn't trip nearly as often, and probably even better chance you will not trip a 50 amp breaker if both appliances were connected to a single breaker.

This is exactly why the demand table is there - the demand generally is not at nameplate level, and if it is, it is not for very long.
 
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