this is from the 2002 .......latest, greatest...)250.140 Frames of Ranges and Clothes Dryers.
This section shall apply to existing branch-circuit installations only. New branch-circuit installations shall comply with 250.134 and 250.138. 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 grounded in the manner specified by 250.134 or 250.138; or, except for mobile homes and recreational vehicles, shall be permitted to be grounded 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 service-entrance 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. this is the 'commentary' after it..
Section 250.140(4) applies only to existing branch circuits supplying the appliances specified in this section. The grounded conductor (neutral) is no longer allowed to be used for grounding the metal non–current-carrying parts of the appliances listed in 250.140. Branch circuits installed for new appliance installations are required to provide an equipment grounding conductor for grounding the non-current-carrying parts.
Caution should be exercised to ensure that new appliances connected to an existing branch circuit are properly grounded. An older appliance connected to a new branch circuit must have its 3-wire cord and plug replaced with a 4-conductor cord, with one of those conductors being an equipment grounding conductor. The bonding jumper between the neutral and the frame of the appliance must be removed. If a new appliance is connected to an existing branch circuit without an equipment grounding conductor, where the neutral conductor was previously used for grounding the appliance, a bonding jumper must be installed at the appliance terminal to connect the frame to the neutral.
The grounded circuit conductor of an existing branch circuit is still permitted to be used to ground the frame of an electric range, wall-mounted oven, or counter-mounted cooking unit, provided all four conditions of 250.140 are met. The grounded circuit conductor is also permitted to be used to ground any junction boxes in the circuit supplying the appliance, and a 3-wire pigtail and range receptacle is permitted to be used, even though the circuit to the receptacle contains a separate equipment grounding conductor.
Where service-entrance cable was previously installed, an uninsulated covered neutral conductor was allowed. However, the circuit was required to originate at the service equipment to avoid neutral current from downstream panelboards flowing on metal objects, such as pipes or ducts. Exhibit 250.51 shows an existing installation in which Type SE service-entrance cable was used for ranges, dryers, wall-mounted ovens, and counter-mounted cooking units. Junction boxes in the supply circuit were also permitted to be grounded from the grounded neutral conductor. i can post more if need be...
[This message has been edited by sparky (edited 04-29-2002).]