I am replacing an old dryer and am told I should also replace the wall receptacle (flush mount, 30 Amp, 3 Prong, Non-grounding). The dryer information says the receptacle should be 120/240 volt and the hardware store only sells 125/250 volt receptacles. Does this matter?

 

the device show nominal ratings. same thing. make sure it is 30 amp 120/240 volt(nominal) with 4 prongs. 2 hot, one neutral, one ground. the ground was probably missing on the old device. it is very important that you connect the dryer to a grounding conductor, not the neutral. typically if this was installed with conduit it would require bonding the metal box/conduit to a grounding conductor(10AWG). if it was installed with NM cable there should be a ground conductor in the back of the box if it wasnt cut off. that ground should have been bonded to the back of the box already. if it was AC cable (BX) you would again bond the ground terminal on the device to the box. I suggest you test this to make sure you have a good ground.

 

Do not replace the receptacle unless you are also replacing the entire cable run between the panel and that receptacle. It would be nice to replace that two+ground cable with a three+ground cable, but code does not require you to do so. If you leave the cable as is, then also leave the receptacle as is. Your new dryer will probably come with a four-prong cord and plug, but you cannot use that. Go to any home center or appliance center and buy a three-prong dryer cord and plug that will fit your existing receptacle. The dryer installation instructions will give two sets of instructions, one for a four-prong cord and plug, and one for a three-prong cord and plug. The difference will involve a bonding strap on your dryer.

 

as John said the exception is for existing installation only with a few qualifications.

250.140 Frames of Ranges and Clothes Dryers.
This section shall apply to existing branch-circuit installations only. New branch-circuit installations shall comply with 250.134 and 250.138. Frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be grounded in the manner specified by 250.134 or 250.138; or, except for mobile homes and recreational vehicles, shall be permitted to be grounded to the grounded circuit conductor if all the following conditions are met.
(1) The supply circuit is 120/240-volt, single-phase, 3-wire; or 208Y/120-volt derived from a 3-phase, 4-wire, wye-connected system.
(2) The grounded conductor is not smaller than 10 AWG copper or 8 AWG aluminum.
(3) The grounded conductor is insulated, or the grounded conductor is uninsulated and part of a Type SE service-entrance cable and the branch circuit originates at the service equipment.
(4) Grounding contacts of receptacles furnished as part of the equipment are bonded to the equipment.
Caution should be exercised to ensure that new appliances connected to an existing branch circuit are properly grounded. An older appliance connected to a new branch circuit must have its 3-wire cord and plug replaced with a 4-conductor cord, with one of those conductors being an equipment grounding conductor. The bonding jumper between the neutral and the frame of the appliance must be removed. If a new appliance is connected to an existing branch circuit without an equipment grounding conductor, where the neutral conductor was previously used for grounding the appliance, a bonding jumper must be installed at the appliance terminal to connect the frame to the neutral.

 

If it's still in good shape, remove the 3-wire cord from your old dryer and install it on your new one. Just make sure the bonding strap (piece of metal or green wire connecting frame of dryer to middle screw on terminal bar) is in place. If the cord is suspect, a new one will only cost you 6 or 7 bucks.

As was stated before, the only way you can change from a 3-prong to 4-prong outlet is if the circuit wiring contains 4 conductors. You can look to see, but chances are pretty good that your cable with be 3 insulated conductors (black, red, white) without a 4th ground wire.