If you need to add an additional electrical outlet to a room, there's no need to run a line all the way to the breaker box. You can tap into an existing junction box.
You should first look inside the junction box to be certain that there is room for another connection. Junction boxes are sold open (nothing inside) or supplied with connection terminals.
An open junction box should have room enough for three additional wires, while a terminal supplied box must have empty terminals. If the junction box in question can handle another line, you'll need a few tools and supplies.
Checklist for adding a line to a junction box
Outlet and cover plate
You'll need a regular 15 amp outlet, even if you intend to add the outlet to a 20 amp circuit from the junction box. A 20 amp outlet has one round slot, one rectangular slot, and one "T" shaped slot, and should be the sole outlet on a circuit that is wired directly to the breaker box. This type of outlet is for higher powered appliances that need a dedicated line.
A 15 amp outlet has the familiar two parallel rectangular slots and single round hole. this type of outlet is for general use and can be used on either 15 or 20 amp circuits.
The cover plate is installed over the outlet for protection and aesthetics.
Single gang box
A gang box is an open box of metal or plastic that is placed inside a wall to hold an electrical outlet. You will need an "old work" gang box that is designed to be placed inside existing walls, rather than a "new construction" box, which is attached to exposed wall studs.
You will need to find the proper gauge (thickness) of wire by looking at the circuit breaker that controls power to the junction box. If the breaker is stamped "15," then it is a fifteen amp line, and 14 gauge wire can be used.
If the breaker is stamped "20," you have a 20 amp line and you must use at least 12 gauge wire. Don't try to save money by using thinner, higher gauge wire, or overheating or fire can occur.
You must measure the distance from the junction box to the proposed outlet location to determine the length of wire needed, and purchase a 3 wire sheath of either 14 or 12 gauge wire according to the amp rating of the line.
The rolls of wire sheath will be marked either "14-2" or "12-2" according to the gauge needed. Be sure to buy at least several feet of wire more than your measurement from junction box to outlet.
You'll also need a few tools such as:
Drill with 1/2" bit for making holes in wall and ceilings
Utility knife or hole saw for creating hole for outlet box in wall
For professional help, contact a company like Central Heating and Cooling.