I have placed two induction surface units on top of our 30-year-old GE coil element stove (model JSP31GOP1WH). I have removed all four coil burners. Each of the induction units is rated at 1800 W at 120 V so each draws 15 A. The instructions for each unit states to plug into a 20 A circuit, but we have only one 20 A circuit available. Right now we have both connected to that circuit through a 20 A GFCI duplex receptacle. We only turn on one unit at a time.
Obviously I could install another 120 V 20 circuit, but I would like to get power through the heavy 4-wire aluminum cable (original 53 year-old 50 A GE breaker) that the stove appliance cord connects to through a NEMA 14-50 receptacle behind the stove.
My first thought is to connect a 12 or 10 AWG cable (3 + gnd) to the connection block on the back of the stove where the cord connects. I would route this behind the stove to a box surface mounted on the top back of the stove (magnetic hold fast?) or just resting there. This would be a multiwire branch circuit sharing a neutral or two separate neutrals wired to a 20 A duplex receptacle.
Another thought is to snake the 12 AWG cable through the back of the stove into the 3 inch high space under the stove top but I have pretty much rejected that.
The oven of the this stove still works and we are reluctant to buy a new induction range if this cheaper solution would work.
The suggestion in one answer or comments is to install a subpanel to obtain both 240 V and 120 V power from a 6-6-6-6 aluminum cable fed by a 50 A breaker in the main panel currently going to a NEMA 14-50R.
The purpose of this would be to get two 120 V 20 A circuits to operate the two countertop induction cooktops sitting on top of our 30-year-old GE electric range. I have removed all four coil elements to prevent damage if a coil were accidently turned on. Currently both induction cooktops (each rated at 1800 W so 15 A at 120 V are plugged into one dedicated 20 A circuit. I cannot use both at high power. One unit is a 2-burner and one is a 1-burner but each is rated at 1800 W total. The 2-burner will allow all 1800 W on one burner, but will divide power if both burners are on to keep the total power at 1800 W or less.
I want to keep the 240 V power to the NEMA 14-50R because the GE range is plugged into that and the oven of the GE range is working perfectly. It is not a forced air oven, but an old fashioned "still" oven, but we find it OK. I have a sentimental attachment to this oven because a now deceased friend was instrumental in getting the faulty oven timer/control unit repaired since a new one was not available.
I would install the subpanel in the wall of the room the kitchen stove backs on and would put a picture over the subpanel. I want to bring existing 6-6-6-6 aluminum cable into the subpanel and take 6-6-6-6 copper from the subpanel into the box for the NEMA 14-50R so I can use the highly rated Bryant NEMA 14-50R which requires copper conductor.
The subpanel would have two 20 A 1-pole breakers for two separate 120 V circuits for the two separate induction cooktops. Should the subpanel also have a 2-pole breaker (40 A minimum per intructions for GE range) for the 6-6-6-6 copper to the 14-50R or can I just connect through lugs?