According to the National Electrical Code (NEC), it is not necessary to continue the increased size conductor to the device being wired. The conductor size can be decreased to the size that would normally be required for the last 25 ft. before the device. However, the splice must be made in a junction box at or around 25 ft. before the device.The reason for this is that the voltage drop over the last 25 ft. of the circuit is negligible. Therefore, there is no need to use a larger conductor size for this portion of the circuit.In your case, you are using a 30 amp 240 v single phase receptacle. The NEC requires that a no. 10 AWG copper conductor be used for this type of circuit. Therefore, you can decrease the conductor size to no. 10 AWG for the last 25 ft. of the circuit.Please note that this is only a general guideline. The actual conductor size that you need to use will depend on the specific circumstances of your installation. You should always consult with a qualified electrician to determine the correct conductor size for your application. Flag for review