I'm upgrading my stove and cooktop to electric units. Currently the house has a 50amp 220V circuit, run using 6AWG Al wire. New oven is rated at 20amp and cooktop is at 40amp. Can I replace the breaker to 60Amp or should I run a new circuit?
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:
- 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.
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.
There is really no need to change the conductor or the breaker size if the cook-top and the range meet the requirements of NEC 210.19(3) including the Exception 1. This will allow you to use the current range circuit which you can then tap and connect to your two appliances. In order for us verify that you can do this we would need to know the KW or KVA loads of the appliances being served not the amperage sizes for over current protection of 20 and 40 amps. The KW or KVA can be found on the nameplates of both appliances. Usually the nameplate on the range is on the inside of the door. The cook-top would be somewhere on an unexposed part of the appliance.