I have a new range with a NEMA 14-50P plug. I find my 240V receptacle is for a dryer, with the top blade a 90° one. So I need to change the receptacle out. However, I think my range needs 50 Amps, and the 240V circuit breakers are ganged 30 amp. The reason I think this is the cord on the range is rated for 50 amps. The installed circuit breakers are type QP. Are these all plug in circuit breakers?. Is it sufficient to just upgrade the existing circuit breakers with 50 amp QP circuit breakers. Is that all I would need to do change out the circuit breakers, and change the receptacle? Do I need to change the circuit breakers or would the 30 Amp breakers suffice?
To increase the amp rating of your circuit breaker, you almost certainly need to increase the gauge of all the wiring on that circuit. Circuit breakers are there to protect the wiring for overloading, overheating, melting the insulation, and eventually starting a fire.
Typically, you have the following wire gauges (this may vary with long circuits and your local codes):
So going from 30 amps to 55 amps requires you to rewire that circuit from 10 gauge to 8 gauge wiring.
For your oven, check the specs on the oven itself. It may simply have a plug that's designed for multiple ovens and will work on a 30 amp circuit.
Before you do anything else, look at the appliance's rating plate. It is probably located somewhere inside the door or maybe on the back.
While this one is for an electric clothes dryer, all appliance ratings plates have the same information. You can see it clearly indicates the amperes required, in this case 24. So a 30 amp circuit would be fine. If your rating plate indicates less than 25 amps, all is probably well. Otherwise, a heavier than normal circuit is required.
Begin by turning off the circuit breaker, open up the circuit breaker box and the receptacle box and look at the appliance circuit's wires. If not ancient (1960s or later), the wire gauge is printed on them somewhere. If you can't find any printing, look at the bare wire itself (use a screwdriver to loosen the retaining screw and slide the wire out from the connector) and compare it to known wire gauges—another wire in the circuit breaker box, a gauging device, or wire samples from an electrical supply store. If the panel and receptacle ends do not exactly match in wire quality, color, number, and gauge, ignore the rest of what I say and call a handyman or electrician (it indicates you have the wrong wire, or there is a junction somewhere).
The breaker for the circuit cannot exceed the wire rating. That is, a 50 amp breaker cannot be used on a wire rated for 30 amperes: doing so would allow the wire to overheat leading to a house fire. @BMitch's table of values is only for copper wiring for lengths up to 80–100 feet (25–30 m). Smaller capacities apply to aluminum wire and for longer wire runs. It would be unusual to find a smaller installed breaker than the wire could handle, but it is acceptable. (I have done this in special situations in my house to protect the device as well as the house wiring when I knew a lot about the specific appliance installed—such as an electric vehicle.)
U.S. range circuits are most commonly intended for a 40 amp appliance (240 volts times 40 amps is 9,600 watts—with a 20% derating for continuous use, that means the most the circuit can use is 7,680 watts). Most run of the mill electric range/ovens are rated at 4,000 watts.
The type of cord ("pigtail") on the appliance is not necessarily indicative of its rating. A 50 amp pigtail and outlet is fine for a 30 amp circuit and appliance, though possibly confusing. An extra heavy component does not increase the circuit capacity—that is determined by the weakest link. If someone installed a 50 amp pigtail and outlet, but the appliance only needs 30 amps and the wire is 10 gauge, there is nothing to do—it is fine as is.
However, if the appliance needs more than the wires' rating, there are several ways to proceed:
The circuit is composed of the breaker, the wire, and the receptacle. Changing a receptacle is easy but the size and shape of the receptacle is dictated by the rest of the circuit and is an indication of the voltage and current capacity. Do not just swap it out.
You also should not change the breaker size because then the wire will likely be undersized. The purpose of the breaker is to cut off electricity to avoid a fire by overheating the wire. The circuit breaker is not really the limiting factor, it's the wire size.
If you need a 50 amp circuit you will need to upgrade all 3 components. Might be a good idea to call an electrician.