Male Plug

NEMA 5-20P

Female Receptacle

NEMA 5-20R

This receptacle will also accept the 15A/125V 2-wire or 3-wire,  male plug above.

20 amp. 125 volt
2-Pole
3-Wire Grounding

Uses include old style window air conditioners, large electric heaters, certain machines and office equipment in commercial applications.
1 hp rating

Notice that you can plug a standard 15A/125V male plug into this female receptacle but not the 
other way around.
Most codes require that this receptacle be used when wiring a single receptacle on a 20 amp. circuit. 

Male Plug

NEMA 6-20P

Female Receptacle

NEMA 6-20R

This receptacle will also accept the 15A/250V male plug above.

20 amp. 250 volt
2-Pole
3-Wire Grounding

Most common household 250 volt plug. Used in many larger window air conditioners and some large home use air compressors, certain machines and office equipment in commercial applications.  2 hp rating

Notice that this configuration is the opposite of 20A/125V but can be confusing when looking at the plug.  They are mirror images of each other.

Male Plug

NEMA 6-30P

Female Receptacle

NEMA 6-30R

30 amp. 250 volt
2-Pole
3-Wire Grounding

Uses include large style window air conditioners (30,000 btu), china and pottery kilns, certain machines and office equipment in commercial applications.  
2 hp rating

Actual face diameter is 2 1/8" 

Male Plug

NEMA 10-30P

Female Receptacle

NEMA 10-30R

30 amp. 125/250 volt
3-Pole
3-Wire 

Mainly used on old electric dryers.  This is a dual voltage device when the appliance uses both 125 volt for the controls and drum motor and 250 volt for the electric heating element.
2 hp rating

Actual face diameter is 2 1/8" 

Male Plug

NEMA 14-30P

Female Receptacle

NEMA 14-30R

Same as above but with a ground wire and used on new electric dryers.  This is a dual voltage device when the appliance uses both 125 volt for the controls and drum motor and 250 volt for the electric heating element.
2 hp rating.

Actual face diameter is 2 1/8" 

Male Plug

NEMA 6-50P

NEMA 6-50R

50 amp. 250 volt
2-Pole
3-Wire Grounding

Uses include electric welders.
  3 hp rating

Actual face diameter is 2 1/8" 

Male Plug

NEMA 10-50P

Female Receptacle

NEMA 10-50R

50 amp. 125/250 volt
3-Pole
3-Wire 

Mainly used on old ranges.  This is a dual voltage device when the appliance uses both 125 volt for the controls, clock and lights and 250 volt for the electric heating elements.
2 hp rating

Actual face diameter is 2 1/8" 

Male Plug

NEMA 14-50P

Female Receptacle

NEMA 14-50R

Same as above but with a ground wire and used on new electric ranges.  This is a dual voltage device when the appliance uses both 125 volt for the controls, clock and lights and 250 volt for the electric heating elements.
2 hp rating

Actual face diameter is 2 1/8"