Why did the code change from a 2 w/ground to a 3 w/ ground on stove and dryers? Is it the increased electronic 110 loads involved in the new units? I have people ask me why the change was made in the code........honestly I never knew.
The change your referring to I believe was part of the 96 NEC but don't have a copy in front of me right not to verify if this is the proper addition.
A neutral (grounded conductor) has always been installed as part of range and dryer circuits but it was also permitted to be used for grounding the frame of the range or dryer. New installations are now required to have an equipment ground wire run with the circuit. 250-140 (2002 NEC)
#24211 - 04/08/0306:54 AMRe: Stove and Dryer Wiring Change
I have heard that the practice was permitted in order to save on materials during the WW2 era. I guess that could be checked historically. The practice was allowed to remain because of it's "good track record" regarding safety. Overall though, it is not an ideal method (problems can arise during renovations when service panels become subpanels) and the time finally came to prohibit it.
#24213 - 04/08/0309:59 AMRe: Stove and Dryer Wiring Change
A three-wire electric stove or dryer plug and cord would be like this:
Hot #1, Neutral, Hot #2
240 volts across hot #1 and hot #2. 120 volts across each hot and the neutral. You would also have to bond the chassis (the metal case) to the neutral.
A four wire plug and cord would be like this:
Hot #1, NEUTRAL , Hot #2
The added ground (always a green wire in the cord) is to bond the metal housing of the device to EARTH, so that way if any loose current-carrying conductor were to short-circuit with the metal cabinet the charge would get safely carried to the ground instead of through YOU.
The voltages across the two hots and across each hot and neutral (the white wire in the cord) are the same as the three wire configuration. There must be no voltage across neutral and ground (EARTH) conductors!!
I tried to use the simplest layman language I could to explain this. Please correct me if I have made ANY error.
[This message has been edited by SvenNYC (edited 04-09-2003).]
#24216 - 04/09/0311:56 AMRe: Stove and Dryer Wiring Change
So why the neutral in the first place? AIUI, electric ranges used 120V for lower heat, and 240V for higher. 120V clocks and a convenience outlet on electric ranges were once common. For dryers, the motor has been classically 120V and the heater 240V. I don't know of that's the case for new appliances.