Computation of Branch Circuits,  NEC 220-3

 

Branch-circuit loads shall be computed as shown in (a) through (d) below:

(a) Continuous and Noncontinuous Loads. 
 
  • The branch-circuit rating shall not be less than the noncontinuous load plus 
    125 percent of the continuous load. 
 
  • The minimum branch-circuit conductor size, without the application of any 
    adjustment or correction factors, shall have an allowable ampacity equal to 
    or greater than the noncontinuous load plus 125 percent of the continuous load. 

    Exception: Where the assembly, including overcurrent devices, is listed for continuous operation of 100 percent of its rating. 

(b) Lighting Load for Listed Occupancies. 
 
  • A unit load of not less than that specified in Table 220-3(b) for occupancies 
    listed therein shall constitute the minimum lighting load for each square foot
    of floor area. The floor area for each floor shall be computed from the outside 
    dimensions of the building, dwelling unit, or other area involved. 
 
  • For dwelling units, the computed floor area shall not include open porches, 
    garages, or unused or unfinished spaces not adaptable for future use. 

    ***The unit values herein are based on minimum load conditions and 100 percent 
    power factor, and may not provide sufficient capacity for the installation contemplated.

(c) Other Loads -- All Occupancies. In all occupancies, the minimum load for each 
outlet for general-use receptacles and outlets not used for general illumination shall be 
not less than the following, the loads shown being based on nominal branch-circuit voltages.
 

(1) Outlet for a specific appliance or other load except for a motor load ......... ampere rating of appliance or load served.

 

(2) Outlet for motor load (see Sections 430-22 and 430-24 and Article 440).

 

(3) An outlet supplying recessed lighting fixture(s) shall be the maximum volt-ampere rating of the equipment and lamps for which the fixture(s) is rated. 

 

(4) Outlet for heavy-duty lampholder ......... 600 volt-amperes.

 

(5) Track lighting (see Section 410-102). 

 

(6) Sign and outline lighting ............... 1200 volt-amperes for each required branch circuit specified in Section 600-5(a). 

 

(7) Other outlets* ......... 180 volt-amperes per outlet. 

 

For receptacle outlets, each single or each multiple receptacle on one strap shall be considered at not less than 180 volt-amperes. 

*This provision shall not be applicable to receptacle outlets connected to the circuits specified in Sections 220-4(b) and (c). 

Exception No. 1: Where fixed multioutlet assemblies are employed, each 5 ft (1.52 m) or fraction thereof of each separate and continuous length shall be considered as one outlet of not less than 180 volt-amperes capacity unless appliances are likely to be used simultaneously. Where appliances are likely to be used simultaneously, each 1 ft (305 mm) or fraction thereof shall be considered as an outlet of not less than 180 volt-amperes. The requirements of this exception shall not apply to dwelling units or to the guest rooms of hotels or motels. 

Exception No. 2: Table 220-19 shall be permitted for computing the load of household electric ranges. 

Exception No. 3: A load of not less than 200 volt-amperes per linear ft (305 mm) of show window, measured horizontally along its base, shall be permitted instead of the specified unit load per outlet. 

Exception No. 4: The loads of outlets serving switchboards and switching frames in telephone exchanges shall be waived from the computations.

Exception No. 5: Section 220-18 shall be considered as a permitted method of computing the load for a household electric clothes dryer. 

(d) Loads for Additions to Existing Installations. 
 

(1) Dwelling Units. Loads for structural additions to an existing dwelling unit or to a previously unwired portion of an existing dwelling unit, either of which exceeds 500 sq ft (46.5 sq m), shall be computed in accordance with (b) above. Loads for new circuits or extended circuits in previously wired dwelling units shall be computed in accordance with either (b) or (c) above.

 

(2) Other than Dwelling Units. Loads for new circuits or extended circuits in other than dwelling units shall be computed in accordance with either (b) or (c) above. 


Table 220-3(b). General Lighting Loads by Occupancies


Type of Occupancy                                               Unit Load per Square Foot (Volt-Amperes) 
________________________________________________________________________
Armories and Auditoriums                                                     1 
________________________________________________________________________
Banks                                                                                 3 1/2** 
________________________________________________________________________
Barber Shops and Beauty Parlors                                          3 
________________________________________________________________________
Churches                                                                            1 
________________________________________________________________________
Clubs                                                                                 2 
________________________________________________________________________
Court Rooms                                                                      2 
________________________________________________________________________
Dwelling Units*                                                                 3 
________________________________________________________________________
Garages - Commercial (storage)                                           1/2 
________________________________________________________________________
Hospitals                                                                            2 
________________________________________________________________________
Hotels and Motels, including apartment houses 
without provision for cooking by tenants*                               2 
________________________________________________________________________
Industrial Commercial (Loft) Buildings                                   2 
________________________________________________________________________
Lodge Rooms                                                                    1 1/2 
________________________________________________________________________
Office Buildings                                                                 3 1/2** 
________________________________________________________________________
Restaurants                                                                      2 
________________________________________________________________________
Schools                                                                            3 
________________________________________________________________________
Stores                                                                             3 
________________________________________________________________________
Warehouses (storage)                                                      1/4 
________________________________________________________________________
In any of the above occupancies except one- 
family dwellings and individual dwelling units 
of two-family and multifamily dwellings: 
________________________________________________________________________
Assembly Halls and Auditoriums                                       1 
________________________________________________________________________
Halls, Corridors, Closets, Stairways                                   1/2 
________________________________________________________________________
Storage Spaces                                                               1/4 


*All general-use receptacle outlets of 20-ampere or less rating in one-family, two-family, and multifamily dwellings and in guest rooms of hotels and motels [except those connected to the receptacle circuits specified in Sections 220-4(b) and (c)] shall be considered as outlets for general illumination, and no additional load calculations shall be required for such outlets.

**In addition, a unit load of 1 volt-ampere per square foot shall be included for general- purpose receptacle outlets where the actual number of general-purpose receptacle outlets is unknown.