The 1999 Code required that new 240-volt receptacles be grounded also, which necessitates a fourth slot in their faces. U.S. 240 centertapped single phase has two of these slots being 'hot', with the neutral being the center tap. There is only one standard for these circuits, but 240 V receptacles come in two incompatible varieties. In one the 'neutral' slot accepts a flat blade-prong. In the other the neutral slot accepts a blade with a right angle bend. These are officially NEMA types 14-50R (commonly used with number 8 wire for electric ranges) and 14-30R (commonly used with number 10 wire for electric clothes dryers), respectively, and differ only in current rating (50 A versus 30 A); previous installations would have used the 10-30 or 10-50 configuration.
These changes in standards often cause problems for people living in older buildings.
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