Commercial occupancy calculations

Harlow Commons has six units. The Copper Kettle takes the end unit, and the unit next door is becoming Lakeview Outfitters, a 3,000 square foot outdoor-gear store with a 20-foot show window, a lighted sign over the door and one rooftop heating and cooling unit. The landlord's panel serves each tenant at 208Y/120 volts, three phase. Priya needs a load calculation for the permit and for the landlord, who wants to know whether the existing 100-ampere tenant feeder is enough. Commercial calculations look different from dwelling calculations, but they are built from the same pieces: unit loads, outlet loads, continuous load factors and a few demand factors that apply only in specific situations.

This lesson works through a nondwelling feeder calculation: general lighting from Table 220.12, the 125% factor for continuous lighting, receptacle loads and the demand factor in 220.44, show windows, signs and multioutlet assemblies, the special receptacle rule for offices and banks, and motor loads.

General lighting for nondwelling occupancies

Table 220.12 lists a unit load in volt-amperes per square foot for many occupancy types: offices, retail stores, restaurants, schools, warehouses, hospitals, hotels and so on. The general lighting load is the floor area times that unit load. The 2023 edition also allows, under conditions in 220.12(B), the use of an energy code's lighting power density in place of the table value where the building is designed and monitored to that code; exam questions that intend this will say so.

General lighting in a store or office is a continuous load, because it runs for three hours or more. For a feeder or service, 215.2(A) and 230.42(A) require the conductors and overcurrent devices to carry the noncontinuous load plus 125% of the continuous load, the same arithmetic you met for branch circuits in Lesson 3. In this course's examples, the general lighting of a commercial occupancy is therefore multiplied by 1.25.

Unlike dwelling lighting, nondwelling general lighting usually gets no demand factor. Table 220.42 lists demand factors for a few occupancies with large lighting loads that never all run at once, such as hospitals, hotels and warehouses, but stores, offices and restaurants take their lighting at 100% before the continuous factor.

Receptacle loads and the 220.44 demand factor

Receptacles in a nondwelling occupancy are counted at 180 VA per yoke under 220.14(I), as in Lesson 3. When you total them for a feeder, 220.44 lets you apply a demand factor: the first 10 kVA of receptacle load at 100% and the remainder at 50%. The factor applies only to general-use receptacles counted under 220.14(H) and (I), not to receptacles for specific appliances.

Receptacle loads are not continuous by default, so they do not get the 125% factor unless the question says they serve continuous equipment.

Show windows, signs and multioutlet assemblies

Several kinds of outlet have their own load values. For a feeder, a show window is calculated at 200 VA per linear foot of window, measured horizontally along its base, under 220.14(G) and 220.43(A). Show window lighting typically runs into the evening and is treated as continuous in this course's examples. Each commercial building and occupancy accessible to pedestrians must have at least one sign outlet under 600.5, and 220.14(F) counts it at no less than 1,200 VA; sign lighting is also a continuous load. Multioutlet assemblies, the plug strips mounted along benches and counters, are counted under 220.14(H) at 180 VA for each 5 feet, or for each foot where appliances are likely to be used at the same time, as on a workbench with soldering irons.

The difference between the two answers is a factor of five, and it rests entirely on how the space will be used. Exam questions make the use explicit; in the field, Dana asks the tenant before she designs the circuits.

Offices and banks

Section 220.14(K) has a rule for banks and office buildings. Their receptacle load is the larger of two values: the receptacles counted at 180 VA each (or the actual load), or 1 VA per square foot of floor area. The rule exists because offices are often fitted out after the electrical design is done, and a minimum based on area guarantees capacity for the computers and printers that arrive later.

Motors and HVAC equipment

Motor loads on a feeder follow 430.24: the sum of the full-load currents of all the motors plus 25% of the largest motor's full-load current, plus the other loads. For packaged heating and cooling equipment, the nameplate minimum circuit ampacity already includes the 25% for its own largest motor. In this lesson's example the rooftop unit's nameplate load is given in volt-amperes with the compressor identified as its largest motor, and the 25% is added for the feeder. MELU-105 covers motor feeders in detail.

Heating and cooling are noncoincident in commercial buildings too, under 220.60. If the store had separate electric heat that could not run with the cooling, only the larger would be counted.

Putting the store together

At 95.8 amperes, the calculated load fits under the existing 100-ampere tenant feeder, but only just. Before telling the landlord the feeder is adequate, Dana checks that the feeder conductors' ampacity, the breaker and the terminal temperature ratings all support 100 amperes, which is the subject of Lesson 9. She also notes that the store has no spare capacity for a second rooftop unit or a heated sign upgrade, which the tenant mentioned might come later.

import math

def commercial_feeder(sqft, unit_va, receptacles, show_ft, signs, hvac_va, largest_motor_va):
    lighting = sqft * unit_va * 1.25
    rec = receptacles * 180
    rec_d = min(rec, 10000) + 0.5 * max(rec - 10000, 0)
    window = show_ft * 200 * 1.25
    sign = signs * 1200 * 1.25
    hvac = hvac_va + 0.25 * largest_motor_va
    return lighting + rec_d + window + sign + hvac

va = commercial_feeder(3000, 1.9, 60, 20, 1, 9000, 6000)
print("Store feeder:", va, "VA =", round(va / (208 * math.sqrt(3)), 1), "A at 208 V three phase")

Store feeder: 34525.0 VA = 95.8 A at 208 V three phase

Other nondwelling occupancies

The same pattern extends to almost any commercial building. A warehouse has a low lighting unit load and may qualify for a demand factor on lighting in Table 220.42. A hotel's guest rooms have their own lighting value and lighting demand factors, and their receptacles are included in the lighting load the way a dwelling's are. A hospital's lighting gets a demand factor too, but its feeders and essential electrical system have additional rules in Article 517. Schools and restaurants have optional methods of their own, covered in the next lesson. When an exam describes an occupancy you have not practiced, find its line in Table 220.12, look for it in Table 220.42, and check whether 220.14 has a special rule for its outlets.

Reading the occupancy correctly

The single most important choice in a commercial calculation is the occupancy type, because it sets the unit load. A space described as a clothing store uses the retail value; a space described as a doctor's office uses the office value, and also brings in the 220.14(K) receptacle rule; a self-storage building uses the storage or warehouse value. Exam writers sometimes describe a mixed space, such as a store with an office in the back, and expect you to calculate each area with its own unit load. When the question gives only a total floor area and one occupancy type, use that type for the whole area.

A second choice is whether a load is continuous. The definition turns on whether the maximum current is expected to continue for three hours or more, which depends on how the space is used, not just on the type of equipment. Office lighting during business hours is continuous; a storage room light switched on for a few minutes is not, but it is part of the general lighting load anyway, and general lighting in a commercial occupancy is treated as continuous as a whole. Receptacles become continuous only when they serve equipment that runs for hours, such as a bank of point-of-sale terminals that never turn off. Unless the question tells you that, leave receptacles at 100%.

Last, remember that the feeder calculation in this lesson gives a minimum load. The tenant feeder must also be checked against the conductors' terminal temperature ratings, voltage drop on a long run and any spare capacity the owner wants, and those checks often push the feeder to a larger size than the calculation alone would require.

The tenant feeder question also has a business side that the exam does not ask about. A landlord who leases space to restaurants, gyms or clinics needs far more capacity than one who leases to clothing stores, and a feeder sized exactly to one tenant's calculation may be too small for the next. Ray's habit with landlords is to show them both numbers: the calculated minimum for the tenant in hand, and the capacity the space would need for the most demanding tenant it is likely to attract.