According to NEC 240.4 (D), 14 AWG must be protected at 15A. (3). A 14 AWG cable cannot be used on a circuit with a 20A breaker. If you wish to connect a 15-amp receptacle to a 20-amp circuit using 12 gauge wire, screw terminals are the best option. The conductor size of the cable needs to be larger than 9 inches to be connected to a breaker panel capable of handling 20 amperes.
As long as you stay below the maximum load rating of any one outlet, you should have no problems with 14 AWG cable up to 20 amps. If you need to connect more than two outlets or if you plan to feed power into receptacles from these circuits, then you will need to use 16 AWG or 18 AWG cable instead.
The smallest size of cable suitable for a given application can be found by using the tables in NEC Table 310.15(B) and (C). For example, for a branch circuit with a maximum load of 10 amps, a 14 AWG cable is large enough.
The voltage drop across electrical wiring will reduce the amount of power that can be delivered from an outlet. As your load increases, so does the voltage drop across each section of cable. Unless you use a larger conductor, such as 18 or 24 awg, your voltage drop will increase too. Large cables are more expensive than smaller ones and they cannot be reused if they become damaged. It is best to choose a size conductor that is large enough to handle your load while keeping voltage drops low enough to avoid reducing the efficiency of your circuit.
If you are not sure whether your circuit will need 20 amp service, then you should select a 22-24 awg cable as this will give you room to grow.
On a circuit with a 20A breaker, you cannot use 14 AWG anyplace. When connecting 15-amp receptacles to a 20-amp circuit with 12 gauge wire, you MUST utilize screw terminals rather than back-stab terminals. Simply connect to the side terminals. If you are not sure what size wire you have, call your electric company and ask for their recommendation on minimum wire size for a given number of amps.
The reason for this limitation is that if you were to put more than 15 amps through 14 AWG wire it would overheat and possibly burn up. The same thing would happen if you tried to run 18 or 22 AWG wire on a circuit capable of handling 20 AMP.
If you are confident that you can handle the current you will be carrying and your electrician has not raised any concerns about stress tests on your wiring, you should be fine using 14 AWG wire on a circuit capable of handling 20 AMP.
A 14-gauge wire can apparently safely transport 20 amps, but the "powers-that-be" declare it can only carry 15 amps as a built-in safety mechanism. At 15A, 14 AWG must be protected. The code prohibits it.
The current in a 14-AWG copper wire is about 7 amperes. This means that if there are no other factors involved, then it will heat up enough to cause damage in less than 21 years. If it were me, I would either use 16 or 18 AWG instead or even choose another type of metal for parts of the wiring system where more current doesn't flow at once.
The current in a 16-or 18-AWG copper wire is around 10 or 11 amperes respectively. This means that these wires should last much longer than 21 years.
There are many types of metals used in electrical wiring. Each one has different properties such as resistance to corrosion when exposed to moisture and ambient conditions. For this reason, different types of metal are used in different areas of the wiring system. For example, 14 AWG is commonly used for trunk lines (high currents) because it's able to carry a lot of power without getting too hot. It also works well for small wiring jobs because it's easy to work with and costs less than some other types of metal.
The voltage needs to be at least 11.4 volts for every wire in order to make and break the current properly. A breaker only sees the total amperage of all the wires it is connected to, so if one piece of wiring has 14 AWG wire on it and another piece has 16 AWG wire, then the breaker can't know that they are both carrying 20 amps and will open itself.
The easiest way to fix this problem is to replace the 14-2 with 16-2 cable. This will keep the voltage up to 21 volts instead of dropping down to less than 10 volts which would require replacing some of the wiring inside your house. If you can't replace the cable now but want to use smaller wire for part of the project, then you'll need to put splitters or transformers between each set of wires on different circuits to increase the voltage enough for the smaller gauge wire.