Electricity: Understanding How It Works: Circuit Mapping and Planning


The first step in assessing your electrical system is to determine whether you have two-wire (120-volt) or three-wire (240-volt) service. To do this, check the number of wires entering the service head out side. Also look for the system's service rating in amps, usually stamped on the service disconnect.

Mapping an Electrical System

The first step in assessing your electrical system is to determine whether you have two-wire (120-volt) or three-wire (240-volt) service. To do this, check the number of wires entering the service head out side. Also look for the system's service rating in amps, usually stamped on the service disconnect.

Before doing any electrical work, make a map of all existing circuits. Create a panel index card and assign a number to each circuit breaker or fuse in the service panel, noting its ampere rating. Draw a floor plan of your home that includes the attic, basement, garage and exterior wiring. Use a full sheet of graph paper for each floor and label all rooms and spaces. Using the common electrical symbols shown below, record the location of all receptacles, switches, lights and other equipment. Simply turn off a circuit breaker or remove a fuse at the service panel, then walk through the house with a tester or portable light to determine which outlets are dead. Turn that circuit back on and repeat the process for each additional circuit, recording the assigned circuit number next to the symbols on your floor plan.

Are you looking for home electrical items -- such as switches and fuse boxes -- or parts and accessories for ones you already have? Try our dedicated electrical supply pages here:






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Last modified: Friday, 2016-03-18 6:29 PST