Given the following:
Wall mounted oven with nameplate rating of 240V, 8KW, 40Amps.
Cooktop with nameplate rating of 240V, 8KW, 40Amps.
Can I run an #8/3 with 40A OCPD to a JBox. Two whips tapped from the JBox, one to the oven and one to the cooktop. Each whip is #10THHN in greenfield?
Table 220.19 note 4 allows the oven and cooktop to be to have their nameplate ratings added together and treated as one range. Thus I effectively have a range with a 16KW rating. For a 16KW range, table 220.19 note 1 applies. This gives me 8KW+[4(.05*8)] =9.6KW. 9.6KW/240=40Amps. Thus, I should be able to use a 40Amp branch circuit to feed both units. Anyone see any problems with this?

I'm not familiar with the NEC, but aren't standard breakers only to be loaded to 80%?

So 40A * 125% = 50A

That would require #6 wire running to the JB and 50A OCP.

I would size the conductors running to each unit from the JB as though they were each their own range. (That's probably how you arrived at #10 wire?)

I believe the 80% rule you refer to is for continuous loads only. I don't believe a range is considered a continuous load.

I would run two dedicated circuits 40A each.

That way i know when they have both of the units on during a party or Thanksgiving cookoff they will not trip the breakers.

Edward

I think the approach is wrong, here!

Electric Ian your looking at the rules for "Calculating Demand Factors" which basically used for calculating Total Loads.

Edward is correct, the rules that would apply would be rules that relate to Protection of Conductors and Equipment. (fusing)

Due to this, each unit gets a seperate feed, unless the unit comes with protection for the remote unit buit-in.