What size wire to use for 30 Amp? 

For a 30-Amp circuit, the standard wire size is 10 AWG copper or 8 AWG aluminum. The ampacity of 10 AWG copper is 30 amps at the 60°C rating used for NM-B cable. The ampacity of 8 AWG aluminum is also 30 amps at the 60°C rating, which makes it the correct aluminum equivalent for a 30-amp breaker.

Many sources also mention 8 AWG copper wire for 30-amp breakers. While 8 AWG copper is safe, it is a conservative choice and generally only needed for longer runs to keep voltage drop low. It is not required for standard 30-amp circuits, but can be used.

All things considered, 10 AWG copper is the correct and most practical wire size for a 30-Amp circuit, while 8 AWG copper or 6 AWG aluminum can be used when extra distance or voltage drop is a concern.

What is a 30 amp breaker good for?

A 30 amp 240-volt circuit can carry up to 7,200W of electrical power. The National Electric Code allows one 30-amp 240v receptacle on a dedicated 30-amp circuit. These circuits are mostly used in residential applications. They are used for home appliances or RV vehicles, such as:

• Central air conditioning units
• Electrical system of an RV vehicle. 30 Amp RV plugs and 30 Amp RV extension cords are the most common.
  
• Electric clothes dryers
• Kitchen appliances like smaller ovens and electric ranges
• Small electric furnaces

Generally speaking, a 30-amp breaker is meant for a dedicated, single appliance or device. The receptacles for 30-amp circuits are different from the typical household receptacle and are intended for a single appliance or device. This is why they have a different plug configuration. 30-amp circuits differ from 15-amp or 20-amp outlets that may be used for multiple devices or one device.

This is because 30 amp devices, such as electric clothes dryers, some air conditioners, and electric water heaters, generally need a dedicated circuit to operate safely.

The outlets for these circuits usually have a different configuration than standard 15-amp or 20-amp outlets to prevent the plugging in of inappropriate appliances or devices. For example, a typical 30-amp receptacle for a dryer has a unique configuration that won't accept plugs from devices that aren't required or rated for 30-amp service.

Which cables for a 30 amp circuit breaker?

Most common cable for a 30-Amp circuit breaker is NM-B Romex non-metallic sheathed cable. If the circuit is impacted by water or weather, it is best to use UF-B ( f.e., in case air conditioners are partly located outside, water heaters, or RV electrical systems). Here is a breakdown of how each cable can be used for a 30-Amp service:

• NM-B Non-Metallic Sheathed Romex® Cable for interior applications is the most popular cable for 30 Amp circuit breakers as long as all units are located indoors and cables do not need extra mechanical protection.
• UF-B cables are suitable for devices that are fully or partly located outdoors or the ones that are in contact with water and moisture, such as water heaters, RV hookups, or window-mounted air conditioning units.
• MC metal-clad cables can be used for 30-Amp circuit breakers if the cable will have an exposed run or can experience mechanical damage. However, you should not use MC cable if there is direct exposure to moisture, such as in the case of the water heater, where it is better to use UF-B cable. MC metal-clad cables can be used in damp conditions, but they are more difficult to install than UF-B. 
• Alternatively, you can use copper and aluminum THWN in conduit for exposed applications. However, it is more challenging to install, as it requires separately placed hot wires, a neutral wire, and a ground wire.

Before choosing a 30 Amp circuit cable, consult local codes, as rules may vary from state to state, and some cables may be locally prohibited for certain applications.