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
You don't need a subpanel, or a breaker upgrade, for this
As with most range situations, we start with NEC Table 220.55, Note 4:
This lets us add the 4.2kW of the oven and the 7.4kW of the stove together to create a single range load of 11.6kW, which we then apply Table 220.55 column C to to yield an 8kW factored load for your circuit. This is well within the 12kW capability of a 50A/240VAC circuit, so you don't need to upgrade the breaker at all; furthermore, unless your cooktop or oven requires (not just recommends) a dedicated branch circuit (most don't require it) in the installation instructions, you don't need to put a subpanel in either. Considering that putting a subpanel in a kitchen is difficult at best due to the 110.26(A) clear working space requirements, avoiding a subpanel in this situation is a good thing as far as I am concerned.
As to making that splice (or tap)...
Since you are dealing with aluminum wiring here, your instinct to use a mechanical screw-lug tap connector (Polaris™ or equivalent) is correct; a 14-4AWG, Cu9Al, 3-hole tap (or 2-hole splice if you're just splicing here) will do the trick for the hots and neutral, as well as the ground if you're using a plastic box. If you're using a metal box, though, you'll want to use a UL467 listed, dual-rated, 14-4AWG equipment grounding bar with sufficient room for the grounds, such as the Penn-Union NA-402 (or NA-401 for the splice case); this mounts to the box using a 1/4"-28 machine screw through a tapped hole in the back of the box.
Speaking of the box, you'll need a larger box than your typical junction box in order to accommodate all those mechanical setscrew tap connectors. A 6x6x3 NEMA 1 enclosure should provide ample space for all the splicing work needed here for not too much more cost, though. Given that we're using an enclosure this size, this also opens us up to the option of using UL 1953 listed, Cu9Al rated, 14-4AWG power distribution blocks, such as a Mersen MPDB63163 for tapping or MPDB63103 for splicing, instead of the mechanical tap connectors; this makes the inside of the box tidier than having connectors floating loose, and may also be cheaper as well.