By Mike Holt, NEC Consultant

Load calculation requirements for commercial applications are scattered throughout the NEC.

The NEC contains detailed requirements for calculating service and feeder loads in many types of occupancies. Although most of these requirements are in Article 220, other rules affecting service and feeder loads are scattered throughout the NEC. And that's the case with commercial occupancies.

Load calculation for commercial occupancies is an important and challenging subject, because there are many different types of commercial buildings and installations, and some have unique rules for calculating service and feeder loads.

Commercial occupancies include many different types of businesses and uses. These include banks, stores, restaurants, and office buildings. Some locations, such as marinas and mobile home parks, have their own special requirements. Table 220.3 has a list of additional calculation references to help you located some of these special requirements.

Lighting demand factors for occupancies

NEC Table 220.12 requires a minimum load per square foot for general lighting depending on the type of occupancy. For the guest rooms of hotels and motels, hospitals, and storage warehouses, you can apply the demand factors of Table 220.42 to the general lighting load.

There’s usually a diversity of usage in the occupancies that receive demand factors from Table 220.42. By diversity of usage, we mean not all of the lighting is in use continuously. The variations in usage are reflected in sizing the electrical service, but to what extent depends upon the type of occupancy.

Hospitals

You can reduce the general lighting unit load of 2 VA per sq ft [Table 220.12] for hospitals, using the in Table 220.42 demand factors.

Demand factors:

• First 50,000 VA at 40 percent demand factor.
• Remainder VA at 20 percent demand factor.

Example question: What’s the general lighting load for a 100,000 sq ft hospital after demand factors are applied?

(a) 20 kVA      (b) 30 kVA      (c) 50 kVA      (d) 75 kVA

Answer: (c) 50 kVA

[Tables 220.12 and 220.42]

100,000 sq ft x 2 VA = 200,000 VA

First 50,000 VA at 40% x 0.40 20,000 VA

Remainder at 20% 150,000 VA x 0.20 = 30,000 VA

Total demand load

50,000 VA 50,000 VA/1,000 = 50 kVA demand load

Hotel or motel guest rooms

You can reduce the general lighting unit load of 2 VA per sq ft [Table 220.12] for the guest rooms of hotels and motels, using the Table 220.42 demand factors. These are units without permanent provisions for cooking. Units with permanent cooking provisions are multifamily dwellings, rather than hotels or motels.

Demand factors:

• First 20,000 VA at 50 percent demand factor.
• Next 80,000 VA at 40 percent demand factor.
• Remainder VA at 30 percent demand factor.

Warehouses

You can reduce the general lighting unit load of 1/4 VA per sq ft [Table 220.12] for warehouses, using the Table 220.42 demand factors.

Demand factors:

• First 12,500 VA at 100 percent demand factor
• Remainder VA at 50 percent demand factor

Keep in mind that the demand factors of Table 220.42 will not apply to service or feeder loads that supply areas of hospitals, hotels, and motels where the entire lighting is likely to be used at one time such as operating rooms, ballrooms, dining rooms and similar areas. Calculate these types of areas separately at 100 percent demand factor.

Other occupancies

For commercial occupancies other than guest rooms (of motels and hotels), hospitals, and warehouses, use a 100 percent demand factor in your load calculations.

The lighting loads in Table 220.12 are minimum requirements. If the actual lighting load is known and it’s larger than the Table 220.12 value, use the actual load.

Specific applications

The NEC has load calculation requirements for specific types of applications. For example:

• Each commercial occupancy that’s accessible to pedestrians must have at least one 20A branch circuit for a sign [600.5(A)]. The load for the required exterior sign or outline lighting must be at least 1,200 VA. If the specific sign or outline load is known to be larger, the actual load must be used. [220.14(F)].
• The feeder/service demand load for each linear foot of show-window lighting must be calculated at 200 VA per ft [220.14(G)]. Treat show-window lighting as a continuous load. See Example D3 in Annex D for an example calculation.

One application with many variations is the receptacle:

• Each 5 ft, or fraction thereof, of a multioutlet receptacle assembly is considered to be 180 VA for feeder/service calculations if it’s unlikely that appliances will be used simultaneously [220.14(H)]. When a multioutlet receptacle assembly is expected to have appliances used simultaneously, each foot or fraction of a foot must be considered as 180 VA for feeder/service calculations.
• The load for a general-use receptacle outlet in a nondwelling occupancy is 180 VA per yoke [220.14(I)]. The maximum number of receptacle outlets permitted on a commercial or industrial circuit depends on the circuit rating. To calculate the number of receptacles per circuit, divide the VA rating of the circuit by 180 VA for each receptacle yoke. Note that a duplex receptacle is two receptacles on the same yoke [100]. For this calculation, a single receptacle or a duplex receptacle each count as 180 VA.
• In other than dwelling units, receptacle loads computed at not less than 180 VA per outlet yoke per with 220.14(I), and fixed multioutlet assemblies computed per 220.14(H), can be added to the lighting loads and made subject to the Table 220.42 demand factors for those occupancies in Table 220.42.
• For banks or office buildings, 1 VA for each square foot of the building is used for the receptacle load if the actual number of receptacles is unknown [220.14(K)(2)]. If the receptacle count is known, apply the Table 220.44 demand factors to the total of 180 VA per receptacle yoke, and compare the results to the 1 VA per square foot figure, then use the larger of the two.

Manufactured home parks

When sizing the service demand load for a mobile/manufactured home park, apply the demand factors of Table 550.31 to the larger of:

• 16,000 VA for each mobile/manufactured home lot, or
• The demand load for each mobile/manufactured home site per 550.18.

The NEC allows you to size service and feeder conductors to an individual mobile home using 310.15(B)(7) [550.33(B)].

Example question: What’s the demand load for a mobile/manufactured home park that has facilities for 35 sites? The system is 120/240V, single-phase.

(a) 400A      (b) 560A      (c) 800A      (d) 1,000A

Answer: (b) 560A

A manufactured home park’s electrical wiring system must be calculated at 120/240V based on the larger of the following [550.31]:

(1) 16,000 VA for each manufactured home lot.

(2) The load calculated with 550.18 for the largest typical mobile home that each lot will accept.

The feeder or service load can be calculated using the demand factor percentages in Table 550.31. Since no calculated load per 550.18 is given, just use 16,000 VA per site [550.31(1)].

Demand Load = VA x Sites x Demand Factor

Table 550.31, 35 sites = 0.24 demand factor.

Demand Load = 16,000 VA x 35 sites x 0.24

Demand Load = 134,400VA

I = VA/E

I = 134,000 VA/240V

I = 560A demand load

Parallel two conductors per phase: 560A/2 raceways = 280A

300 kcmil rated 285A at 75degrC, Table 310.15(B)(16)

This is large enough for the demand load. You can use 600A overcurrent protection; 240.4(B) allows you to round up to the next standard size overcurrent protection not exceeding 800A.

The neutral must be large enough to serve the neutral load [220.61] but the minimum neutral size when paralleling conductors is 1/0 AWG [250.24(C)(2) and 310.10(H)].

Recreational vehicle parks [551.73]

Recreational vehicle (RV) parks are typically equipped with several different sizes of receptacles. Each size of receptacle must be present based on a minimum percentage requirement provided in 551.71. You can exclude dedicated tent sites when calculating these percentages.

Calculate RV parks using the demand factors of Table 551.73(A). The total demand load is based on the VA per site values listed in 551.73(A):

• 9,600 VA for each site with 50A, 120/240V supply facilities.
• 3,600 VA for each site with 20A and 30A, 120V supply facilities.
• 2,400 VA for each site with 20A, 120V supply facilities.

Example question: What’s the demand load for a recreational vehicle park that has ten 20A supply facilities, fifteen 20A and 30A supply facilities, and twenty 30A and 50A supply facilities? The system is 120/240V, single-phase.

(a) 420A      (b) 461A      (c) 520A      (d) 600A

Answer: (b) 461A

The service for the recreational vehicle park is sized using the Table 551.73(A) demand factors.

Ten 20A sites (2,400 VA x 10 sites)                 24,000 VA

Fifteen 20A and 30A sites (3,600 VA x 15 sites)                54,000 VA

Twenty 50A sites (9,600 VA x 20 sites)                192,000 VA

Connected Load for 45 sites                270,000 VA

Demand Load = 270,000 VA x 0.41 [Table 551.73(A) for over 36 sites]

Demand Load = 110,700 VA

I = VA/E

I = 110,700 VA/240V

I = 461A demand load

Parallel two conductors per phase: 461A/2 raceways = 231A

250 kcmil rated 255A at 75degrC, Table 310.15(B)(16)

The neutral must be large enough to serve the neutral load [220.61] but the minimum neutral size when paralleling conductors is 1/0 AWG [250.24(C)(2) and 310.10(H)].

Demand factor caution

Be careful when applying demand factors to commercial installations. For example, the demand factors for dryers in Table 220.54 are for dwelling units only. You can’t use them for laundromats or similar locations. There are different rules, and in some cases different demand factors, for commercial occupancies. In Part 2, we will examine more applications that have their own demand factors and rules.