April 25, 2007
this is a simplified method for doing a very rough calculation. remember, electricians and designers use rules that are much more complicated, so don't get ahead of yourself. let's look at a simple formula. house load = basic load + large appliances + special loads basic load
- include lighting, outlets, and smaller appliances including oil or gas furnaces, central vacuums, pool pumps, waste disposals, etc., but not baseboard heaters
- based on house size (square feet)
- include electric stove, electric dryer and electric water heater
- include saunas, fixed electric heaters (most commonly baseboard heaters), electric furnaces, central air conditioners, kilns, etc.
follow these five steps:
- basic load estimate or measure house square footage. include 75% of any basement floor area. don't use measurements based on the outside wall surfaces. calculate interior space. for the first 1,000 square feet, allow 5,000w. for each of the next 1,000 square feet, or part thereof, add 1,000w. this gives a basic load, in watts.
- large appliances a typical house has an electric stove and dryer, and an electric water heater. note: include these in the calculation even if the house presently has a gas stove, for example, if you plan on changing appliances. allow 6,000w for the stove. allow 1,000w for the dryer. allow 750w for the electric water heater. allow 1,500w for any additional stoves. the total of these gives you the large appliances load, in watts.
- add the basic and large appliances load, and divide by 240v to get a subtotal, in amps. subtotal = (basic + large appliances) ? 240 = amps.
- special loads find out any special loads the house may have. the load in amps is read off the data plate on each appliance. common special loads are a. sauna b. electric furnace c. electric baseboard or wall mounted heaters add up the wattage on each data plate; divide the total by 240v to get amps d. central air conditioner use the fla (full load amps) e. kiln if you can't find or can't read the data plate, use the amp rating of the supply cable to the appliance, or the rating of the fuse or breaker.
- house load the house load is the sum of the subtotal and the special loads. let's run through an example with a 2-story, 1,500 square foot home with 100-amp service.
- basic load each story is 750 square feet (1,500 2). seventy-five percent of the basement area is: 75% x 750 = 562 sq. ft. total area for calculations = 1500 + 562 = 2062 sq. ft. use 5,000w for first 1,000 sq. ft.: 5,000w use 1,000w for next 1,000 sq. ft.: 1,000w use 1,000w for the last 62 ft.: 1,000w basic load: 7,000w
- large appliances electric stove: 6,000w dryer: 1,000w large appliance load: 7,000w
- subtotal: (7,000w + 7,000w) 240v = 58.3 amps
- special loads 2-ton air conditioner with full load amperage rating (fla) = 12 amps 2 x 1,000w baseboard heaters = 2,000w 240v = 8.3 amps sauna data plate says 25-amps
- house load total (subtotal plus special loads) 58.3a + 12a + 8.3a + 25a = 103.6 amps
this house doesn't look like it has quite enough electricity. it's close, though, so we recommend that the homeowner try it before committing to a service upgrade. remember that the baseboard heaters won't be on when the a/c is, and the sauna will only be used occasionally. but this service cannot support any additional loads.