Article 220 Branch-Circuit, Feeder, and Service Calculations

I. General

220.1

Scope

Figure 220.1 indicates the location in this section of the various calculation requirements.

Figure 220.1. Branch-circuit, feeder, and service calculation methods.

220.3

Application of other Articles

Table 220.3 lists additional load calculation references for specilized areas.

Table 220.3. Additional Load Calculation References

Calculation

Article

Section
(or Part)

Air-Conditioning and Refrigerating Equipment, Branch-Circuit Conductor Sizing

440

Part IV

Cranes and Hoists, Rating and Size of Conductors

610

610.14

Electric Welders, ampacity calculations

630

630.11, 630.31

Electrically Driven or Controlled Irrigation Machines

675

675.7(A),
675.22(A)

Electrolytic Cell Lines

668

668.3(C)

Electroplating, Branch-Circuit Conductor Sizing

669

669.5

Elevator Feeder Demand Factors

620

620.14

Fire Pumps, Voltage Drop (mandatory calculation)

695

695.7

Fixed Electric Heating Equipment for Piplines and Vessels, Branch-Circuit Sizing

427

427.4

Fixed Electric Space Heating Equipment, Branch-Circuit Sizing

424

424.3

Fixed Outdoor Electric De-icing and Snow-Melting Equipment, Branch-Circuit Sizing

426

426.4

Industrial Machinery, Supply Conductor Sizing

670

670.4(A)

Marinas and Boatyards, Feeder and Service Load Calculations

555

555.12

Mobile Homes, Manufactured Homes, and Mobile Home Parks, Total Load for Determining Power Supply

550

550.18(B)

Mobiles Homes, Manufactured Homes, and Mobile Home Parks, Allowable Demand Factors for Park Electrical Wiring Systems

550

550.31

Motion Picture and Television Studios and Similar LocationsSizing of Feeder Conductors for Television Studio Sets

530

530.19

Motors, Feeder Demand Factor

430

430.26

Motors, Multimotor and Combination-Load Equipment

430

430.25

Motors, Several Motors or a Motor(s) and Other Load(s)

430

430.24

Over 600 Volt Branch Circuit Calculations

210

210.19(B)

Over 600 Volt Feeder Calculations

215

215.2(B)

Phase Converters, Conductors

455

455.6

Recreational Vehicle Parks, Basis of Calculations

551

551.73(A)

Sensitive Electrical Equipment, Voltage Drop (mandatory calculation)

647

647.4(D)

Solar Photovoltaic Systems, Circuit Sizing and Current

690

690.8

Storage-Type Water Heaters

422

422.11(E)

Theaters, Stage Switchboard Feeders

520

520.27

220.5

Calculations

The nominal voltages to be used for branch-circuit and feeder load calculations are 120, 120/240, 208Y/120, 240, 347, 480Y.277, and 600 unless there are others specified. If there is a fraction of an ampere left when making the calculations, then if the fraction is less than 0.5 it can be dropped.

II. Branch-Circuit Load Calculations

220.10

General

Sections 220.12, 220.14, and 220.16 are to be used for branch circuit calculations.

220.12

Lighting Load for Specified Occupancies

Table 220.12 lists the minimum load required based on floor area, which is calculated using outside dimensions. Where an area cannot be used in dwelling units, garages, and open porches, it does not have to be included. It should be noted that these values are minimum load conditions and 100% power factor and may not provide sufficient capacity for the proposed installation.

Table 220.12. General Lighting Loads by Occupancies

Type of Occupancy

Unit Load

Volt-Amperes per Sq. Meter

Volt-Amperes per Sq. Ft.

Armories and auditoriums

11

1

Banks

39[b]

3½[b]

Barber shops and beauty parlors

33

3

Churches

11

1

Clubs

22

2

Court rooms

22

2

Dwelling units[a]

33

3

Garagescommercial (storage)

6

½

Hospitals

22

2

Hotels and motels, including apartment houses without provisions for cooking by tenants[a]

22

2

Industrial commercial (loft) buildings

22

2

Lodge rooms

17

Office buildings

39[b]

3½[b]

Restaurants

22

2

Schools

33

3

Stores

33

3

Warehouses (storage)

3

¼

In any of the above occupancies except one-family dwellings and individual dwelling units of two family and multi-family dwelling units:

 

Assembly halls and auditoriums

11

1

Halls, corridors, closets, stairways

6

½

Storage spaces

3

¼

[b] See Section 220.14(K).

[a] See Section 220.14(J).

220.14

Other LoadsAll Occupancies

There is a list of requirements for all other loads. An outlet for an appliance or other specific load not covered below is to be calculated on the ampere rating of the appliance or load. The load for electric dryers is based on Section 220.54 and electric ranges and other cooking devices on Section 220.55. Motor loads are to be calculated in accordance with Sections 430.22, 430.24, and 440.6. Luminaires (lighting fixtures) are to be calculated based on the maximum volt-ampere rating of the equipment and lamps. Heavy-duty lampholders are calculated at a minimum of 600 VA. Sign and outdoor lighting is to be calculated at a minimum of 1200 VA for each branch circuit noted in Section 600.5(A). Show windows are calculated at 200 VA per linear foot of window or the unit load as noted elsewhere in this section. Multioutlet assemblies need only be counted as one outlet (180 VA) for each 1.5 m (5-ft.) length unless many appliances are used simultaneously. If appliances are used simultaneously, then each 1.5 m (1-ft.) length is calculated at 180 VA. The portion containing the receptacle outlets can be used for calculations. Receptacle outlets on one strap other than those covered by the calculations in Section 220.14(J) are calculated at 180 VA each. If a single piece of equipment has multiple receptacles, then each receptacle is to be calculated at not less than 90 VA. Receptacles noted in Sections 210.11(C)(1) and (2) are not bound by the previous two sentences. In dwelling occupancies all receptacle outlets as noted in Sections 210.11(C)(3), 210.52(E), 210.52(G), and 210.70(A) and (B) are included in the general lighting load calculations in Section 220.12. In a bank or office building use the larger of either the calculated load of 220.14(I) or 1 volt-ampere/ft2. All other outlets are calculated at 180 VA.

220.16

Loads for Additions to Existing Installations

Loads for additions or unwired portions greater than 46.5 square meters (500 square feet) to dwelling units are to be calculated in accordance with Sections 220.12 and 220.14. Additions to circuits should be calculated as noted in Sections 220.12 and 220.14. Loads for other than dwelling units are to be calculated as noted in Section 220.12 and 220.14.

220.18

Maximum Loads

The load should never exceed the rating of the branch circuit. Article 430 applies when only motor-operated loads are used. Article 440 is used for air-conditioning and refrigeration equipment. When more than one motor fastened in place is used, then you use 125% of the largest motor and the sum of the rest of the motor loads. Inductive lighting is calculated on the total ampere rating of the unit including ballasts, transformers, lamps, etc. Range load demand factors can be used in accordance with Table 220.55 including note 4.

Table 220.55. Demand Factors and Loads for Household Electric Ranges, Wall-Mounted Ovens, Counter-Mounted Cooking Units, and Other Household Cooking Appliancesover 1¾ kW Rating
(Column C to be used in all cases except as otherwise permitted in Note 3.)

Number of Appliances

Demand Factors (Percent) (See Notes)

Column C Maximum Demand (kW) (See Notes)

Column A (Less Than 3½ kW Rating)

Column B (3½ kW to 8¾ kW Rating)

(Not over 12 kW Rating)

1

80

80

8

2

75

65

11

3

70

55

14

4

66

50

17

5

62

45

20

6

59

43

21

7

56

40

22

8

53

36

23

9

51

35

24

10

49

34

25

11

47

32

26

12

45

32

27

13

43

32

28

14

41

32

29

15

40

32

30

16

39

28

31

17

38

28

32

18

37

28

33

19

36

28

34

20

35

28

35

21

34

26

36

22

33

26

37

23

32

26

38

24

31

26

39

25

30

26

40

26-30

30

24

31-40

30

22

41-50

30

20

51-60

30

18

61 and over

30

16

Note 1. Over 12 kW through 27 kW ranges all of same rating. For ranges individually rated more than 12 kW but not more than 27 kW, the maximum demand in Column C shall be increased 5 percent for each additional kW of rating or major fraction thereof by which the rating of individual ranges exceeds 12 kW.

Note 2. Over 8¾ kW through 27 kW ranges of unequal ratings. For ranges individually rated more than 8¾ kW and of different ratings, but none exceeding 27 kW, an average value of rating shall be calculated by adding together the ratings of all ranges to obtain the total connected load (using 12 kW for any range rated less than 12 kW) and dividing by the total number of ranges. Then the maximum demand in Column C shall be increased 5 percent for each kilowatt or major fraction thereof by which this average value exceeds 12 kW.

Note 3. Over 1¾ kW through 8¾ kW. In lieu of the method provided in Column C, it shall be permissible to add the nameplate ratings of all household cooking appliances rated more than 1¾ kW but not more than 8¾ kW and multiply the sum by the demand factors specified in Column A or B for the given number of appliances. Where the rating of cooking appliances falls under both Column A and Column B, the demand factors for each column shall be applied to the appliances for that column, and the results added together.

Note 4. Branch-Circuit Load. It shall be permissible to calculate the branch-circuit load for one range in accordance with Table 220.55. The branch-circuit load for one wall-mounted oven or one counter-mounted cooking unit shall be the nameplate rating of the appliance. The branch-circuit load for a counter-mounted cooking unit and not more than two wall-mounted ovens, all supplied from a single branch circuit and located in the same room, shall be calculated by adding the nameplate rating of the individual appliances and treating this total as equivalent to one range.

Note 5. This table also applies to household cooking appliances rated over 1¾ kW and used in instructional programs.

FPN No. 1: See Example D5(a) in Annex D.

FPN No. 2: See Table 220.56 for commercial cooking equipment.

FPN No. 3: See the examples in Annex D.

 

III. Feeders and Services

220.40

General

Feeders must be able to supply the required load. They must be sized to supply the connected load with applied allowable demand factors.

220.42

General Lighting

To calculate the general lighting load, use the demand factors in Table 220.42.

Table 220.42. Lighting Load Demand Factors

Type of Occupancy

Portion of Lighting Load to Which Demand Factor Applies (Volt-Amperes)

Demand Factor Percent

Dwelling units

First 3000 or less at

100

 

From 3001 to 120,000 at

35

 

Remainder over 120,000 at

25

Hospitals[1]

First 50,000 or less at

40

 

Remainder over 50,000 at

20

Hotels and motelsincluding apartment houses without provision for cooking by tenants1

First 20,000 or less at

50

 

From 20,001 to 100,000 at

40

 

Remainder over 100,000 at

30

Warehouses (storage)

First 12,500 or less at

100

 

Remainder over 12,500 at

50

All others

Total volt-amperes

100

[1] The demand factors of this table shall not apply to the calculated load of feeders or services supplying areas in hospitals, hotels, and motels where the entire lighting is likely to be used at one time, as in operating rooms, ballrooms, or dining rooms.

220.43

Show-Window and Track Lighting

Use at least 660 VA per linear meter or 200 VA per linear foot for show windows and 150 VA for every 600 mm (2 ft.) for track lighting in other than dwelling units or hotels and motels. When multi-circuit track is used the load is to be considered divided equally between circuits.

220.44

Receptacle LoadsNondwelling Units

If a receptacle load is calculated in accordance with Section 220.14(H) Fixed Multioutlet Assemblies and 220.14(I) Receptacle Outlets, then the demand factors in Table 220.42 or Table 220.44 can be used.

Table 220.44. Demand Factors for Nondwelling Receptacle Loads

Portion of Receptacle Load to Which Demand Factor Applies (Volt-Amperes)

Demand Factor Percent

First 10 kVA or less

100

Remainder over 10 kVA at

50

220.50

Motors

Calculate motor loads as directed by Sections 430.24, 430.25, and 430.26 and 440.6 for hermetic refrigerant motor compressors.

220.51

Fixed Electric Space Heating

Generally, the load is to be calculated at 100% of the connected load. In no case can the service or feeder current rating be less than that of the largest branch circuit supplied. If, however, the total load is less as a result of intermittent operation or diversity, the authorities can reduce the calculated load provided the conductors have a rating for the determined load.

220.52

Small Appliance and Laundry LoadsDwelling Units

(A) Small-Appliance Circuit Load. Use 1500VA for each two-wire small-appliance branch circuit that is required in 210.11(C)(1). When the load is divided through two or more feeders, use 1500VA for each two-wire small-appliance branch circuit. This load can be included in the general lighting load and demand factors of Table 220.42 can be applied.

(B) Laundry Circuit Load. Use not less than 1500VA for each two-wire laundry branch circuit that is required in 210.11(C)(2). This load can be included in the general lighting load and demand factors of Table 220.42.

220.53

Appliance LoadDwelling Units

A demand factor of 75% can be applied to nameplate ratings of four or more appliances in a dwelling supplied by the same feeder or service. This does not apply to electric ranges, clothes dryers, and space heating or air-conditioning equipment.

220.54

Electric Clothes DryersDwelling Units

Use a load of 5000 watts (volt-amperes) or the nameplate rating, whichever is larger. Demand factors listed in Table 220.54 can be used. If two or more single-phase dryers are served by a three-phase four-wire circuit, the total load is calculated at twice the maximum number connected between two phases.

Table 220.54. Demand Factors for Household Electric Clothes Dryers

Number of Dryers

Demand Factor Percent

14

100

5

85

6

75

7

65

8

60

9

55

10

50

11

47

1222

[47(number of
dryers over 11)]%

23

35

2442

[350.5(number of
dryers over 23)]%

43 and over

25

220.55

Electric Ranges and Other Cooking AppliancesDwelling Unit(s)

The load for household electric ranges, wall-mounted ovens, counter-mounted cooking units, and other household cooking units larger than 1¾ kW are shown in Table 220.55. kVA and kW are considered equivalent for this section.

For single-phase ranges on a three-phase four-wire feeder or service, use twice the number of units between two phases.

220.56

Kitchen EquipmentOther Than Dwelling Units

This section applies to commercial cooking equipment, dishwasher booster heaters, water heaters, and other kitchen equipment. Demand factors shown in Table 220.56 can be used. They apply to thermostatic controlled or intermittently used kitchen equipment, not air conditioning, space heating or ventilating equipment. The feeder or service must always be larger than the sum of the two largest kitchen equipment loads.

Table 220.56. Demand Factors for Kitchen EquipmentOther Than Dwelling Unit(s)

Number of Units of Equipment

Demand Factors (Percent)

1

100

2

100

3

90

4

80

5

70

6 and over

65

220.60

Noncoincident Loads

If two loads on the same feeder or service will not be operating simultaneously, the smaller of the two can be omitted for calculation purposes.

220.61

Feeder or Service Neutral Load

The feeder or service neutral load is by definition the maximum unbalanced load between the neutral and one ungrounded conductor. In three-wire two-phase or five-wire two-phase systems the load is then multiplied by 140%. A factor of 70% is used in the case of household electric ranges, wall-mounted ovens, electric dryers, and counter-mounted cooking units, when the maximum unbalanced load for ranges is in accordance with Table 220.55, and for dryers, Table 220.54. A factor of 70% may also be used for load more than 200 A in systems with three-wire dc or single-phase ac; three-phase, four-wire; two-phase, three-wire; and two-phase, five-wire. Reduction of the neutral or the grounded conductor capacity of a 3 wire circuit with two phase wires and neutral of a four-wire wye-connected three-phase system is not permitted. Reduction of the neutral or grounded conductor capacity is also not permitted in a portion consisting of non-linear loads on a 3-phase, 4-wire, wye-connected system.

IV. Optional Feeder and Service Load Calculations

220.80

General

An optional calculation for feeder and service loads is permitted in accordance with this part (Part IV).

220.82

Dwelling Unit

(A) Feeder and Service Load. This section can be used to calculate the load for a single three-wire, 120/240-V or 208Y/120-V feeder or service entrance with an ampacity of 100 A or larger. The neutral load can then be determined by Section 220.61. The load is the sum of (B) and (C) as follows:

(B) General Loads. These shall not be less than 100 percent of the first 10 kVA plus 40% of the remainder of the load listed as follows: each laundry and 20-A small appliance circuit at 1500 VA, for general lighting and receptacles use 33 VA per sq. m or 3 VA per sq. ft., the nameplate rating of all fastened in place appliances, all motors and low power factor loads use the nameplate ratings.

(C) Heating and Air Conditioning Load. Use the largest of the following.

1. 100% of the nameplate rating(s) of the air conditioning and cooling.

2. 100% of the nameplate ratings of the heating when a heat pump is used without supplemental heating.

3. 100% of the nameplate ratings of electric thermal storage and other heating systems where the usual load is expected to be continuous at the full nameplate value. Systems qualify under this selection shall not be figured under any other selection in (c).

4. 100% of the nameplate rating of the heat pump compressor and 65% of the supplemental electric for central electric space heating systems. If the heat pump compressor cannot operate at the same time as the supplemental heat, then it does not have to be added to supplemental heat for the total central space heating load.

5. 65% of the nameplate rating(s) of electric space heating if less than four separately controlled units.

6. 40% of the nameplate rating(s) of electric space heating of four or more separately controlled units.

220.83

Existing Dwelling Unit

This section applies only to existing 120/240-V and 120/208Y-V three-wire services. It gives an alternate means of calculating loads. Refer to the section in the Code.

220.84

Multifamily Dwelling

This section permits an alternate means of calculation of feeder and service loads when each dwelling is supplied by a maximum of one feeder, has electric cooking, and has electric space heating or air conditioning. The alternate calculation is based on the demand factors shown in Table 220.84. These demand factors apply to the small-appliance branch-circuit load, 33 VA per sq. m or 3 VA per square foot general load, the nameplate rating of all appliances and motors, and the larger of the air-conditioning or space heating loads. House loads are calculated in accordance with Part III of this article and are in addition to the dwelling unit loads calculated by Table 220.84. There is a listing of the connected loads which apply to the table.

Table 220.84. Optional CalculationsDemand Factors for Three or More Multifamily Dwelling Units

Number of Dwelling Units

Demand Factor (Percent)

3-5

45

6-7

44

8-10

43

11

42

12-13

41

14-15

40

16-17

39

18-20

38

21

37

22-23

36

24-25

35

26-27

34

28-30

33

31

32

32-33

31

34-36

30

37-38

29

39-42

28

43-45

27

46-50

26

51-55

25

56-61

24

62 and over

23

220.85

Two Dwelling Units

220.86

Schools

Refer to Table 220.86.

Table 220.86. Optional MethodDemand Factors for Feeders and Service-Entrance Conductors for Schools

Connected Load

Demand Factor (Percent)

First 33 VA/m2 Plus,

(3 VA/ft2) at

100

Over 33 to 220 VA/m2 Plus,

(3 to 20 VA/ft2) at

75

Remainder over 200 VA/m2

(20 VA/ft2) at

25

220.87

Determining Existing Loads

Actual demand data can be used if the maximum readings are known for at least 1 year, the existing demand multiplied by 125% and the new loads are not larger than the wire capacity, and proper feeder and service overcurrent protection are provided, in accordance with Sections 240.4 and 230.90, respectively. There is an exception if the maximum demand is not available for 1 year.

220.88

New Restaurants

Table 220.88 can be used to calculate a service load or a feeder for a new restaurant if the feeder serves the total load. The service-entrance overload protection must be in accordance with Sections 230.90 and 240.4. The feeder conductors do not have to be larger than the service-entrance conductors. If this optional calculation is used, the neutral load of service or feeder conductors is determined by Section 220.61.

V. Farm Load Calculations

220.100

General

220.102

Farm LoadsBuildings and Other Loads

220.103

Farm LoadsTotal

Note: Examples for the calculations referred to in Article 220 are shown in Annex D of the NEC®. They are shown in the same location in this book for correlation.





Pocket Guide to the National Electrical Code 2005
Pocket Guide to the National Electrical Code(R), 2005 Edition (8th Edition)
ISBN: 0131480014
EAN: 2147483647
Year: 2004
Pages: 120
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