Existe uma disposição no Código Elétrico Nacional que permitirá uma instalação de 3 fios, em situações onde exista fiação existente (exceção 250.140). Você deve estar bem para instalar um receptáculo de três pinos e usar um cabo de três pinos.

Deve haver instruções especiais nas instruções de instalação do fabricante incluídas na faixa, que especificarão exatamente como conectar o cabo de 3 fios.

Se você está realmente preocupado com isso, ou deseja atualizar para seguir o código moderno. Você pode instalar um novo cabo de 3 fios com fio terra e usar um conector e cabo de 4 pinos.

National Electrical Code 2014

Chapter 2 Wiring and Protection

Article 250 Grounding and Bonding

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 120/240-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 serviceentrance cable and the branch circuit originates at the service equipment.

(4) Grounding contacts of receptacles furnished as part of the equipment are bonded to the equipment.