When was it first required that 240-volt receptacles for a clothes dryer and range be four slot (equipment ground connection)?

Sunday, January 13, 2019

The requirement for 4-slot receptacle outlets and 4-prong cord plugs began with the 1999 edition of the National Electrical Code (NEC), primarily affecting residential electric ranges and dryers. An older 3-slot receptacle can continue to be used when an old appliance is replaced with a new one, although the cord may have to be changed out if it comes with a 4-prong plug. But often the cord is sold separately from the appliance. 

     The older 3-slot receptacles provided connections for two hot wires and a neutral, but the 4th slot adds a separate connection for any current to ground.

    When installating a new or replacement cord, it’s important to also use a strain relief clamp at the hole where the cord enters the appliance. See our blog post Why is a strain relief clamp necessary for the cord connection to some electric appliances?


    The latest edition of the NEC has to be “adopted” by each local building department before it becomes effective in their jurisdiction, and that sometimes happens more than a few years later than the cover date. For more on this, see our blog post Is the latest edition of the National Electrical Code (NEC) the standard used for the electrical system of new homes?

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  To learn more about electrical wiring, devices, and receptacles, see these other blog posts:

 • What is the difference between what trips a GFCI (ground fault) receptacle and a circuit breaker?

Why is an opening in the wall around the side of an electrical receptacle outlet a safety defect?  

When was GFCI-protection for kitchen dishwasher receptacle outlet first required? 

When should I replace electric receptacle outlets? 

What are "self-contained" electrical receptacle outlets and switches?

What is the difference between an electrical receptacle, an outlet, and a plug?

Does a washing machine receptacle outlet require GFCI protection?

What is the building code requirement for receptacle outlets at stairs and stair landings?

What is a "backstab" receptacle outlet? 

Why are some electric receptacle outlets upside down (ground slot up) in a house?   

What is the height requirement for an electric receptacle outlet? 

Where are GFCI receptacle outlets required?

 Does a home inspector remove receptacle outlet cover plates?

When was the current receptacle/outlet spacing of 12-feet first required? 

When was the three-slot (grounding) outlet/receptacle first required? 

Why does painting an electric receptacle (outlet) make it unsafe?

Why are electrical outlets and plugs polarized? 

How many electrical receptacles (outlets) are required in a hallway?  

Is a house required to have outdoor electric receptacle outlets?

How I can tell if a receptacle outlet is tamper resistant?

What is a false ground, bootleg ground, or cheated ground receptacle? 

How can adding wood paneling or a wainscot create an electrical safety hazard?

How far apart should kitchen counter receptacles be spaced?  

What is reversed polarity at an outlet/receptacle? Why is it dangerous? 

Is an ungrounded electric receptacle outlet dangerous?

My bathroom electric receptacle/outlet is dead and there are no tripped breakers in the electric panel. What's wrong?  

Is there an adapter that can be placed on a two-slot receptacle to make it safe? 

How do the new tamper-resistant electric outlets work?

Why is there no bathroom electric receptacle in this old house?

How can I tell if the electric receptacle outlets are grounded? 

How far apart should the electrical receptacles be placed?

• What are the most common problems/defects found with electric receptacle outlets during a home inspection?

    Visit our ELECTRICAL page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles. 

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