Lately, inspectors from a third-party inspection company have been asking me about determining the number of circuits required for outlets installed in dwelling units. The supervisor wanted to know what method his inspectors could use to get a general idea of the number of circuits necessary to supply these outlets. The lack of a unified method was creating a problem because each inspector had his or her own way of doing things, and there was no consistent procedure. Arguments among his inspectors, contractors and electricians (wiremen) were holding up construction jobs.
When I was chief electrical inspector, we divided the circuits into three types:
1 General lighting and receptacle outlets (general purpose circuits)
2 Small-appliance circuits
3 Special appliance circuits (individual circuits)
General lighting and receptacle outlets
The general lighting and receptacle outlets can be determined and inspected by multiplying the square footage of the dwelling unit by 3 volt-amperes (VA) per square foot as listed in Table 220.12 of the National Electrical Code (NEC). All the lighting outlets are installed per Section 210.70(A) and the receptacle outlets per Section 210.52(A) through (H).
Such outlets are located in bedrooms, bathrooms, dens, living rooms, halls, garages, kitchen areas, outside areas, etc.
Determining the number
The general lighting load and number of circuits for a 3,000-sq.-ft. dwelling unit can be determined and inspected like this:
Step 1: Based on voltage and amperes (A)
Table 220.12, 220.14(J)
120-volt (V) circuit 20A OCPD = 2,400 VA
Step 2: Outlets per square foot
2,400 VA ÷ 3 VA = 800 sq. ft.
Answer: There is no limit to the number of outlets that are permitted in the 800-square-foot area per Table 220.12 and NEC sections 220.14(J) and 210.11.
Based on the square-foot concept
Table 220.12, 210.11(B)
3,000 3 VA = 9,000 VA
9,000 VA ÷ 120V = 75A
75A ÷ 20A = 3.75
Answer: Rounding up, four 20A, general-purpose branch circuits can supply a dwelling unit having a 3,000-sq.-ft. area.
Note that local codes may limit the number of outlets on a circuit so that nuisance tripping of the overcurrent protection device (OCPD) is prevented under normal conditions of uses. For example, many jurisdictions will allow only 10 outlets on a 20A circuit and eight on a 15A circuit, while others allow 13 on a 20A circuit (20A ÷ 1.5A = 13) and 10 on a 15A circuit (15A ÷ 1.5A = 10) as calculated per 220.14(I) and (L). Note that the 1.5A is found by dividing 180 VA by 120V, which equals 1.5A. Note that the 180 VA requirement is not applicable to dwelling-unit calculations and is used in the above examples for guidance only.
The outlets installed in the kitchen, pantry and breakfast and dining rooms require a minimum of two 20A small-appliance circuits. These circuits must be routed to the kitchen countertop(s), and the outlets must be proportioned between the two circuits as evenly as possible to prevent unbalanced loading of such circuits. If too many portable appliances are plugged into the same small-appliance circuit and overloading of the circuit occurs, then nuisance tripping of the OCPD can occur.
The two 20A small-appliance circuits wired with 12-2 AWG with ground nonmetallic-sheathed cable are only permitted to supply outlets located in the kitchen, pantry and breakfast and dining rooms. Note that the general-purpose circuits must supply all other outlets or individual circuits as outlined in Table 220.12, 210.23(A) or 210.19(A)(1).
Special appliance circuits
The special appliance circuits are known in the industry as individual circuits that serve directly installed appliances. For example, these circuits feed such loads as circulating pump motors, air conditioners and similar appliances in effort to prevent opening or tripping the upstream overcurrent protection device.
There are three types of countertops that must be inspected in dwellings:
1 Countertops with a wall per 210.52(C)(1)
2 Island countertops per 210.52(C)(2)
3 Countertops without a wall (peninsular) per 210.52(C)(3)
At least one 20A branch circuit [per 210.52(D)] must be provided to supply the bathroom, and no other outlets are permitted to be connected to this circuit. This circuit with its connected outlets must be inspected in this manner to be considered Code-compliant.
Number of circuits and outlets needed
Four general-purpose circuits and two small-appliance circuits are needed. One 20A circuit is needed for the bathroom, and whatever number in size is needed for the special appliance circuits. A 20A branch circuit is also required for the laundry in a dwelling unit.
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the National Electrical Code and other standards, including those from OSHA. Contact him at 817.581.2206.