O National Electrical Code permite que um cabo de 3 fios existente alimente um novo forno.
Aqui está o artigo pertinente da NEC de 2014:
250.140 Frames of Ranges and Clothes Dryers. Frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be connected to the equipment grounding conductor in the manner specified by 250.134 or 250.138.
Exception: For existing branch-circuit installations only where an equipment grounding conductor is not present in the outlet or junction box, the frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be permitted to be connected to the grounded circuit conductor if all the following conditions are met.
(1) The supply circuit is 1201240-volt, single-phase, 3-wire; or 208Y/120-volt derived from a 3-phase, 4-wire, wye-connected system.
(2) The grounded conductor is not smaller than 10 AWG copper or 8 AWG aluminum.
(3) The grounded conductor is insulated, or the grounded conductor is uninsulated and part of a Type SE service-entrance cable and the branch circuit originates at the service equipment.
(4) Grounding contacts receptacles furnished as part of the equipment are bonded to the equipment.
Se todos os 1-4 forem verdade, você pode fazê-lo.
O forno deve ter uma correia de ligação do fabricante para permitir a ligação do neutro e do aterramento no forno. E então você pode conectar tanto o terra quanto o neutro do forno ao fio neutro do cabo de 3 fios.
Boa sorte e fique seguro!