**Residential Services (General Method)**

Calculating service requirements is one of the most rewarding accomplishments that an electrician can achieve. It utilizes all the skills you’ve learned; basic theory, motors, ranges, branch circuits, demand factors, and ties it all together to size the main service for a single- family dwelling.

The General Method

Article 220 covers the general method of calculating single family dwelling service requirements. It’s based on determining demand (reduced) loads in the following individual areas..

General Lighting and Receptacles

Small Appliances

Laundry

Fixed Appliances

Cooking Appliances

Dryers

Air-Conditioning and Heating

Motors and Other Loads

Small Appliances

Laundry

Fixed Appliances

Cooking Appliances

Dryers

Air-Conditioning and Heating

Motors and Other Loads

After calculating the demand loads in volt-amps (or watts) they’re all added together to determine the main breaker (or fuse) size and feeder conductor size. To simplify, we’ll try one service calculation a step at a time.

General Lighting And Receptacles

Article 220.12 covers single family dwelling service requirements for both general lighting and receptacles. It refers to Table 220.12 for the unit load per square foot in volt-amps (vA).

220.12 Lighting Load for Specified Occupancies. A unit load of not less than that specified in Table 220.12 for occupancies specified therein shall constitute the minimum lighting load. The floor area for each floor shall be calculated from the outside dimensions of the building, dwelling unit, or other area involved. For dwelling units, the calculated floor area shall not include open porches, garages, or unused or unfinished spaces not adaptable for future use.

Table 220.12 calls for 3 volt-amps (vA) per square foot for general lighting and receptacles in Dwelling Units...

220.12 Lighting Load for Specified Occupancies. A unit load of not less than that specified in Table 220.12 for occupancies specified therein shall constitute the minimum lighting load. The floor area for each floor shall be calculated from the outside dimensions of the building, dwelling unit, or other area involved. For dwelling units, the calculated floor area shall not include open porches, garages, or unused or unfinished spaces not adaptable for future use.

Table 220.12 calls for 3 volt-amps (vA) per square foot for general lighting and receptacles in Dwelling Units...

For example: take a house with an 1,800 square foot of floor area (measured by outside dimensions), excluding open porches, garages, or unused or unfinished spaces not adaptable for future use. The unit load per square foot would be...

1,800 square feet x 3 vA = 5,400 vA

Does this mean that all of our general lighting and general receptacles (inside and outside) will amount to a total of 5,400 vA? Yes sir ! Let’s break that down to 15 amp branch circuits using the formula I = vA/E.

5,400 vA ÷ 120 volts = 45 amp

That would equal three 15 amp circuits (45 ÷ 15 = 3). But, how does this relate to lights and receptacles. Well, if 600 square feet x 3 vA = 1,800 vA, and one 15 amp branch circuit equals 1,800 vA (15 amps x 120 volts), then wouldn’t 600 square feet equal one 15 amp branch circuit? Exactly !

So, for every 600 square feet of floor area in a house we would need one 15 amp circuit for all general lighting and general use receptacles. The Code bases this on floor area, not on volt- amp or watt ratings of individual lights or outlets. This is an estimate because we don’t really know what light fixtures will be used or what will be plugged in to the receptacles. If we did, we would be able to come up with a more precise estimate.

So, for every 600 square feet of floor area in a house we would need one 15 amp circuit for all general lighting and general use receptacles. The Code bases this on floor area, not on volt- amp or watt ratings of individual lights or outlets. This is an estimate because we don’t really know what light fixtures will be used or what will be plugged in to the receptacles. If we did, we would be able to come up with a more precise estimate.

Small Appliances

Article 210.11(C)(1) and 220.52(A) specifies service requirements for small appliance circuits. These are 20 amp circuits for small cord-and-plug connected appliances in the kitchen, pantry, breakfast and dining room.

The Code requires a minimum of two 20 amp small appliance circuits in all dwelling units and the load that will be added to the service shall be computed at 1,500 vA each. Refrigeration equipment can be supplied by these circuits or by a separate 15 or 20 amp circuit. If we put at least two 20 amp small appliance branch circuits in our house this is what we would add to our service calculation...

210.11(C)(1) Dwelling Units: Small-Appliance Branch Circuits. In addition to the number of branch circuits required by other parts of this section, two or more 20-ampere small-appliance branch circuits shall be provided for all receptacle outlets specified by 210.52(A).

220.52(A) Small-Appliance and Laundry Loads - Dwelling Unit: Small-Appliance Circuit Load. In each dwelling unit, the load shall be calculated at 1500 volt-amperes for each 2-wire small-appliance branch circuit as covered by 210.11(C)(1). These loads shall be permitted to be included with the general lighting load and subjected to the demand factors provided in Table 220.42.210.11(C)(1) Dwelling Units: Small-Appliance Branch Circuits. In addition to the number of branch circuits required by other parts of this section, two or more 20-ampere small-appliance branch circuits shall be provided for all receptacle outlets specified by 210.52(A).

220.52(A) Small-Appliance and Laundry Loads - Dwelling Unit: Small-Appliance Circuit Load. In each dwelling unit, the load shall be calculated at 1500 volt-amperes for each 2-wire small-appliance branch circuit as covered by 210.11(C)(1). These loads shall be permitted to be included with the general lighting load and subjected to the demand factors provided in Table 220.42.

The Code requires a minimum of two 20 amp small appliance circuits in all dwelling units and the load that will be added to the service shall be computed at 1,500 vA each. Refrigeration equipment can be supplied by these circuits or by a separate 15 or 20 amp circuit. If we put at least two 20 amp small appliance branch circuits in our house this is what we would add to our service calculation...

1,500 vA x 2 = 3,000 vA

Remember, the Code requires a minimum of two 20 amp circuits, more could be required.

Laundry

Article 210.11(C)(2) and 220.52(B) requires at least one 20 amp circuit for laundry equipment. The 20 amp laundry circuit can be used by both the washing machine and other laundry related equipment. Dryers fall under Article and Table 220.54 and we’ll deal with them a little later.

*210.11(C)(2) Laundry Branch Circuits. In addition to the number of branch circuits required by other parts of this section, at least one additional 20-ampere branch circuit shall be provided to supply the laundry receptacle outlet(s) required by 210.52(F). This circuit shall have no other outlets.*

220.52(B) Laundry Circuit Load. A load of not less than 1500 volt-amperes shall be included for each 2-wire laundry branch circuit installed as covered by 210.11(C)(2). This load shall be permitted to be included with the general lighting load and subjected to the demand factors provided in Table 220.42.220.52(B) Laundry Circuit Load. A load of not less than 1500 volt-amperes shall be included for each 2-wire laundry branch circuit installed as covered by 210.11(C)(2). This load shall be permitted to be included with the general lighting load and subjected to the demand factors provided in Table 220.42.

General Lighting Demand Factors

As in cooking equipment there are service and feeder demand factors for general lighting, general use receptacles, small appliance and laundry circuits.

*220.42 General Lighting. The demand factors specified in Table 220.42 shall apply to that portion of the total branch circuit load calculated for general illumination. They shall not be applied in determining the number of branch circuits for general illumination.*Table 220.42 gives the lighting load feeder demand factors. This requires that the first 3,000 vA be calculated at 100%, beyond that, we can apply a 35% demand factor. Articles 220.52(A) and (B) allows us to include small appliance and laundry circuits in these demand factors.

Let’s take our 1,800 sq. ft. house and calculate the minimum loads we would have to add to the service and feeder.

(1) General Lighting & Receptacles (1,800 sq. ft. x 3 vA) = 5,400 vA

(2) Small Appliances (2 x 1,500 vA) = 3,000 vA

Total Connected Load = 9,900 vA

Applying the demand factors of Table 220-42.

(4) First 3,000 vA at 100% = 3,000 vA

Total Demand Load = 5,415 vA

Guess the guys who write the Code figure that the first 3,000 vA of general lighting is very important. But, they feel that the next 6,900 vA is less likely to be used as much, so they allow us reduce it in our service calculation. When the dust clears, though, we’re going to have to add 5,415 vA to our service.

Let’s take our 1,800 sq. ft. house and calculate the minimum loads we would have to add to the service and feeder.

(1) General Lighting & Receptacles (1,800 sq. ft. x 3 vA) = 5,400 vA

(2) Small Appliances (2 x 1,500 vA) = 3,000 vA

__(3) Laundry (1 x 1,500 vA) = 1,500 vA__Total Connected Load = 9,900 vA

Applying the demand factors of Table 220-42.

(4) First 3,000 vA at 100% = 3,000 vA

__(5) Remaining 6,900 vA (9,900 - 3,000) at 35% = 2,415 vA__Total Demand Load = 5,415 vA

Guess the guys who write the Code figure that the first 3,000 vA of general lighting is very important. But, they feel that the next 6,900 vA is less likely to be used as much, so they allow us reduce it in our service calculation. When the dust clears, though, we’re going to have to add 5,415 vA to our service.