Connecting modern 4 wire appliances in older homes with 3 wire systems.
Older 3-wire wiring may not be NEC compliant. NEC 250.140 states that “Frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet junction boxes that are part of the circuit for these appliances shall be connected to the equipment grounding conductor (ground wire). Articles 250.134 and 250.138 also state that the grounding conductor may be fixed and run with the power conductors, or may be a separate flexible wire or strap.
These requirements pose a problem for houses that have been wired with the older 3-wire multiconductor cables as opposed to the newer standard of 4-wires. The older wiring consisted of 2 separate phase conductors rated at 240 volts from phase to phase and 120 volt to ground, and also a neutral, or “grounded conductor”, not to be confused with a “grounding conductor” or ground wire. The larger loads from the range or appliance, such as the heating elements, would be fed by the 240-volt circuit, while the smaller loads would be supplied by one of the phase conductors and there would be a return path through the neutral conductor for the incidental 120 volt loads such as the stove light, clock, and timers. This wiring was permitted prior to the 1996 edition of the NEC.
Problems posed by older home 3-wire appliance circuits
The problem with this method of wiring was that the neutral conductor at the appliance was then attached to the metal frame of the appliance. This meant that the metal of the appliance was connected to the return path for the 120-volt loads. If the neutral conductor became loose or disconnected then there would be no return path for the load and the metal of the appliance could be energized waiting to make contact with an alternate return path. This path could be supplied by a person touching the appliance while at the same time touching a separate appliance that was properly grounded.
Today’s standard is the 4 wire system which separates the grounded (neutral) from the grounding (ground) conductors. Therefore the return path would remain on the neutral conductor, which is NOT connected to the metal frame of the appliance, while the metal frame would be connected to the grounding conductor which has not current normally flowing through it. The grounding conductor is only there to provide a low resistance, alternate grounding path in the event of a ground fault. That is, in the event the metal frame of the appliance ever became energized, there would be a path for the current to flow without becoming a hazard to people in contact with the appliance.
What can you do and still be in compliance with the NEC?
So what do you do if you live in a home with permanently installed 3-wire wiring, but have a new appliance that is wired for the new 4-wire requirements? Well, there are a few exceptions to the rule in Article 250.140. These exceptions are given because there are so many homes that do not have the extra 4th wire, or grounding conductor permanently installed.
The exceptions will allow the metal frame of the equipment to be connected to the grounded conductor, or neutral wire, as long as ALL four of the exceptions are met.
These exceptions are:
- The supply circuit is 120/240-volt, single-phase, 3-wire; or fed from a 208/120-volt 3 phase, 4-wire, wye connected system.
- The grounded conductor, neutral, is NOT smaller than #10AWG copper, or #8 AWG aluminum.
- The grounded conductor, neutral, is insulated, or is uninsulated as part of a Type SE service entrance cable and the branch circuit originates at the service equipment, not a sub-panel.
- Grounding (ground wire) contacts of receptacles furnished as part of the equipment are bonded to the equipment.
Older appliance in a new house
Therefore, an older appliance that is connected to a newer 4 wire system must have its 3-wire cord replaced with a 4 wire cord and one of those 4 wires must be an equipment grounding conductor. The appliance bonding jumper between the neutral wire and the metal frame of the appliance must be removed.
A newer appliance in an old house
A new appliance that is connected to an older 3-wire branch circuit without an equipment grounding conductor must have an equipment bonding jumper connected between the metal frame of the appliance and the grounded, neutral, conductor. The neutral is permitted in this case to be used to ground the frame of the appliance as long as ALL four of the exceptions listed above and found in Article 250.140 are met.