Column C
First off you'll notice the text "Column C to be used in all cases except as otherwise permitted in Note 3.)", in the title of table 220.55. This makes it simple. You have 2 appliances, so follow that over in the table, and you'll see 11 in Column C. So there you go, you can just use 11 kW. Done.
11,000 W / 240 V = 45.8333 A
So you'll need a 50 ampere breaker, and wire sized appropriately for the load.
Note 3
Note 3 says:
- Over 1 3⁄4 kW through 8 3⁄4 kW. In lieu of the method provided in Column C, it shall be permissible to add the nameplate ratings of all household
cooking appliances rated more than 1 3⁄4 kW but not more than 8 3⁄4 kW and multiply the sum by the demand factors specified in Column A or
Column B for the given number of appliances. Where the rating of cooking appliances falls under both Column A and Column B, the demand
factors for each column shall be applied to the appliances for that column, and the results added together.
Perfect, so instead of just using the value from column C you can do math. Let's step through it.
...it shall be permissible to add the nameplate ratings of all household
cooking appliances rated more than 1 3⁄4 kW but not more than 8 3⁄4 kW...
8.4 kW + 5.0 kW = 13.4 kW
...and multiply the sum by the demand factors specified in Column A or
Column B for the given number of appliances...
Let's check the table again... You have 2 appliances, both between 3 1/2 and 8 3/4 kW. So You'll look at column B, and find 65%.
13.4 kW * 65% = 8.71 kW
8710 W / 240 V = 36.2916 A
So using this method you can use a 40 ampere breaker, and appropriately sized wire. However, keep in mind that if you change the equipment, you'll have to do the calculation again. So while you can use this value, you may have to upgrade the circuit later if you change equipment.
Note 4
I'm not exactly sure how note 4 comes into play, but I think it can be used if this is the only equipment on the branch circuit. Just for fun, let's run through that one too.
- 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.
You're only concerned with the second half of this note, since you have one counter-mounted cooking unit, and one wall-mounted oven, all supplied by a single branch-circuit, and located in the same room. So you can add the nameplate values, and treat it as a single range.
8.4 kW + 5.0 kW = 13.4 kW
So you can treat the units as a single 13.4 kW range. Check the column C again, this time for a single range. You'll find a value of 8 kW. But wait... The column header says "(Not over 12 kW Rating)". Your range is 13.4 kW. That's bigger than 12 kW. Now you'll have to check note 1
- Over 12 kW through 27 kW ranges all of same rating. For ranges individually rated more than 12 kW but not more than 27 kW, the maximum
demand in Column C shall be increased 5 percent for each additional kilowatt of rating or major fraction thereof by which the rating of individual
ranges exceeds 12 kW.
That's easy enough.
13.4 kW - 8 kW = 5.4 kW
Since .4 is not a "major fraction", you can just use 5 kW. So you'll have to add 5% 5 times.
5% * 5 = 0.25
8000 W * 0.25 = 2000 W
8000 W + 2000 W = 10,000 W
That means you'll have to use 10 kW as your demand.
10,000 W / 240 V = 41.666 A
Which means you can use a 50 ampere breaker, and appropriately size wire.
Best Answer
There are a few different issues here:
A typical oven for installation in a system like the US (split 120/240) will use 240V for the heating elements (i.e., needs 2 hot wires) and 120V for the controls (i.e., needs 1 hot, 1 neutral). So you can't run it on only 120 or only 240. That is different from, for example, a computer power supply that might list 120 - 240 (or more typically something like 100 - 250) which means that any voltage in that range is OK. That is because a computer uses a power supply that can take "anything" and convert to a single lower voltage. An oven will actually use the power "as is" (at least for heating elements and lights) and has to get what it needs or it won't work properly.
An oven is designed with specific power requirements. It will not, unless damaged, exceed those requirements. In addition, as a device with a continuous load (i.e., it can be on for a while), it will actually be designed to use less (I think the normal number is <= 80% of the rated current) power. So there is plenty of safety margin even at 40A, by design. As far as 50A, there are some ovens that require 50A instead of 40A. By designing a 40A oven so that it can operate safely on a 50A circuit - i.e., that it is engineered such that the breaker not tripping until 50A instead of 40A is "OK", that gives a little bit of flexibility on the installation. Otherwise, if you had a 50A circuit (both appropriate wire and a 50A breaker), you would need to replace the 50A breaker with a 40A breaker in order to install this oven - but that is not the case here and you are fine with 40A or 50A.
You didn't ask about this part, but I will tell you anyway. For historical reasons, many ovens used to have no ground and then later ground was piggybacked on the neutral wire. That is not safe (though marginally better than no ground at all) and is not recommended, even if local code permits it. If you have a hardwired oven then you need to connect the oven ground to either a ground wire or (if you have a metal junction box attached to metal conduit going all the way back to the breaker panel) to a metal box. If you have a plug-in installation then you need to use a properly wired 4-wire socket (NEMA 14-50) connected to 2 hots, 1 neutral and 1 ground (ground again can be metal box/conduit). If you have an older 3-wire socket, now is the time to replace it - and yes that may require running a new cable.