With the National Electric Code sections and tables, the minimum service load for a dwelling 40 feet by 25 feet, or 1,000 square feet may be calculated as follows.
(A) Step 1, calculate the lighting load requirements.
1000 sq. ft. by three watts per ft. = 3000 watts (two circuits for lighting load)
(B) Step 2, calculate the small appliance circuits code states.
We use two circuits = 3000 watts.
So far in our calculations, we have four circuits and 6000 watts.
So far in our calculations, we have four circuits and 6000 watts.
(C) Step 3. It is reasonable to believe that not all lights and outlets for the appliance circuits will be in use at the same time, so a demand factor is used to calculate the feeder demand factor. NEC Table 220-11 shows the first 3,000 watts of lighting and appliance circuits shall be computed at 100%. The remainder of the 3,000 watts are to be computed at 35%. Example:
Steps 1 and 2 showed a total of 6000 watts
First 3000 watts computed at 100% = 3000 watts
Remainder, 3000 watts computed at 35% = 750 watts
The 6000 watts are now reduced to 3750 watts
(D) Step 4, feeder load for the oven and countertop. NEC Table 220-19 Column C shows taking a demand factor of 65% of the sum of the nameplate rating of the two units. Example:
Oven is rated at 5000 watts
Countertop rated at 6500 watts
11500 watts
11500 watts
Using the demand factor - 65%
65% of 11,500 watts = 7475 watts
The 11,500 watts are now reduced to 7475 watts
(E) Step 5, compute the feeder load.
Lighting and appliance circuits | 3750 watts |
Countertop and oven | 7475 watts |
Dishwasher (cannot be reduced) | 1500 watts |
Garbage disposer unit | 750 watts |
Exhaust fan | 300 watts |
13,775 watts |
(F) Step 6, compute the feeder size. The utility company has supplied a 115/230-volt overhead service to the house. The total computed feeder load is 13,755 watts. Example:
13,755 watts - 230 volts = 59.8 amperes
Table 310-16 of the NEC shows that 59.8 amperes require a #6 AWG, Type THW, copper conductor good for 65 amperes. Table 3A, Chapter 9 of the NEC shows that a 1-1/4 inch conduit is required for 3 #6 AWG, Type THW, copper conductors.
(Prior Code, § 152.35)