Contact your local building department and ask for a copy of the electrical codes for building and remodeling. Talk over your plans with the building inspector and get any advice or tips they have for you before you begin. Building inspectors are generally people who are experts in the field who were previously contractors and have extensive knowledge of both the code requirements and how to get your job done the best way possible.
Determine if the service to your home is out of date and will need to be upgraded. For instance many older homes have only a 60 amp service. When these homes were built, electric appliances such as microwaves, air conditioning and electronics did not exist in homes. The building code handout will tell you what basic service is now required and when upgrades are mandated. If the basic service needs to be upgraded, it will need to be done by an electrical contractor.
Draw a diagram of your home and each electrical circuit. Include the additions you are making if you are adding on to the home and all new circuits you will be building, if you are simply remodeling. Mark each electrical circuit in the rooms and then determine how each room will be used. A bedroom will have standard uses, but something such as the kitchen or family room where televisions, computers and gaming systems may be in use will need more power and circuits.
Make a list for each room of all the electrical appliances. Most have a rating of power usage in watts. Be sure to add in the furnace, air conditioning and any other mechanical systems you have that require electrical service. For large home appliances look at the usage on the item itself. The plate has a usage rating on the appliance itself, and ranges vary. These items will be things like your dryer, dishwasher, stove, or water heater. List each item and the usage value on the same line across from the description. Add each room to get a value for that room. Multiply this number by 40 percent to get a total wattage calculation. The reason for using 40 percent is that appliances do not run 24 hours a day, or all at the same time.
Determine the load from your central heating system or air, whichever is largest by finding the rating on the appliance. Add this number to the total separately.
Calculate the square footage of the occupied areas of the home. Include the basement if you will use it for more than storage, such as a rec room. Multiply your square footage by 3 to determine a number that is calculated in watts required. Example, an 1800 square foot house the base watts for this formula are 3 x 1800 = 5400.
Add together the square footage to watts calculation, the total watts of appliances you calculated, and then the numbers from your large systems like the air conditioning. Divide this by 240 (volts) to determine your overall electrical load needs. For instance a 50,000 watt usage divided by 240 volts which is the basic voltage of a circuit, equals 208 amps of electrical service needed.
Consult both the wiring codes you received from the building department and any information you have from your electrical contractor, if you have hired one. Any large upgrades such as new service are required to be performed by a licensed contractor for the permit to be approved, but some smaller jobs can be done by the homeowner. The needs of your electrical system that you have just calculated and your building codes will tell you which applies to you.