How to apply demand factors
The electrical load requirements for commercial installations result in a great deal of diversity in usage. In other words, while some types of equipment and electrical loads are in use for extended periods, others are only used occasionally or for short periods of time. In addition, there are often two different types of electrical loads on the same service or feeder that will not be brought into service simultaneously by their very nature, such as heating and air conditioning. For this reason, we apply demand factors when calculating service and feeder loads. Different sets of demand factors apply for different types of electrical loads — and even for different types of commercial buildings.
Although most of the requirements for service and feeder commercial load calculations are found in Art. 220, other rules affecting these loads are scattered throughout the Code. For instance, Chapter 3 of the NEC provides information on the wiring methods used. Other Articles may provide a more in-depth snapshot of the requirements for particular equipment or applications, such as the specific requirements for motor circuits found in Art. 430.
Common commercial occupancies include banks, stores, restaurants, and office buildings. Some other locations with their own special requirements include marinas and mobile home parks. The NEC also provides specific requirements for calculating the loads for restaurant equipment, show-window lighting, sign lighting, multi-outlet assemblies, and electric welders.
When doing commercial load calculations, you have to know when the Code allows the application of a demand factor and when. On the other hand, it's necessary to consider a load as "continuous duty."
The ampacity of a conductor is the rating (in amperes) that a conductor can carry continuously without exceeding its insulation temperature rating [Art. 100]. The allowable ampacities listed in Table 310.16 are affected by conductor insulation, ambient temperature, and conductor bundling [310.10 and 310.15(B)] (Fig. 1).
Section 110.14(C)(1)(a) states that terminals are rated 60ºC for equipment rated 100A or less, unless marked 75ºC. Although most terminals are now rated 75ºC, be careful of assumptions —some equipment is still rated 60ºC. Always read the specifications and manufacturer's labeling information carefully to know what you're working with. If in doubt, be sure to use the rules of 110.14(C).
15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 500, 600, 800, 1,000, 1,200, and 1,600.
Article 100 defines a continuous load as a load where the maximum current is expected to continue for 3 hours or more. Some NEC sections tell you when certain loads are continuous. For example, 422.13 requires that water heaters with a capacity of 120 gal or less be considered continuous loads for purposes of sizing branch circuits. Most commercial lighting and electric signs are considered continuous loads. Unfortunately, the Code does not always spell out clearly when to consider a load as a continuous load for calculation purposes.
Ungrounded conductors for branch circuits, feeders, and services are sized at a minimum of 125% of the continuous load before applying any adjustment factor [ 210.19(A)(1), 215.2(A)(1), and 230.42(A)]. Likewise, the overcurrent protection devices (OCPD) for branch circuits and feeders are sized at a minimum of 125% of the continuous load [210.20(A), and 215.3].
Neutral conductors that aren't connected to an overcurrent protection device can be sized at 100% of the continuous and noncontinuous load [210.19(A)(1) Ex 2 and 215.2(A)(1) Ex 2].
Example: If a 60A continuous load with 75ºC-rated terminals is supplied by a feeder with four current-carrying conductors, it's necessary to adjust the conductor ampacity for four current-carrying conductors.
Step 1. Take the continuous load times 125% to find the minimum size for conductors and overcurrent protection.
60A x 1.25 = 75A
Step 2. Select the conductors using the column of Table 310.16 that corresponds with the temperature rating of the terminals, which was given in this example as 75ºC [110.14(C)(1)]. 4 AWG is rated 85A at 75ºC [Table 310.16].
Step 3. Verify that the conductor is large enough for any necessary deratings, using the column that corresponds to the conductor's temperature rating. If you're using THHN conductors, use the 90ºC column of Table 310.16. According to the 90ºC column of Table 310.16, 4 AWG is rated at 95A [110.14(C)]. Adjust the ampacity by 80% for four current-carrying conductors [Table 310.15(B)(2)(a)].
95A x 0.80 = 76A
This verifies that a 4 THHN conductor is large enough. An 80A breaker is allowed, because 240.4(B) allows rounding up to the next standard size listed in 240.6(A), and 80A is the next standard size above the 76A adjusted conductor ampacity.
The purpose of overcurrent protection is to protect conductors and equipment from excessive temperatures [240.1 FPN]. There are many different rules for protecting conductors and equipment. The general rule is that conductors must be protected at the point where they receive their supply in accordance with their ampacities, which are listed in Table 310.16. Other methods of protection are permitted or required, as listed in 240.4. These include:
Overcurrent protection devices rated 800A or less. You can use the next higher standard rating see Standard Size OCPDs) of overcurrent protection device above the ampacity of the ungrounded conductors, if all of the following conditions are met:
A 400A overcurrent device can protect 500kcmil conductors, where each conductor has an ampacity of 380A at 75°C, per Table 310.16 (Fig. 2).
This "next size up" rule doesn't apply to feeder tap conductors [240.21(B)] or transformer secondary conductors [240.21(C)].
Overcurrent protection devices rated over 800A. If the overcurrent protection device exceeds 800A, the conductor ampacity must have a rating of not less than the rating of the overcurrent protection device. A 1,200A overcurrent protection device can protect three sets of 600kcmil conductors per phase, where each conductor has an ampacity of 420A at 75°C per Table 310.16 (Fig. 3).
Unless other voltages are specified, branch circuit, feeder, and service loads are to be calculated at a nominal system voltage of 120V, 120/240V, 120/208V, 240V, 277/480V, or 480V. [220.5(A)]
Where calculations result in a fraction of less than 0.50A, you can drop the fraction.
Table 220.12 requires a minimum load per square foot for general lighting, depending on the type of occupancy. For certain types of occupancies, Table 220.42 allows a demand factor that can be applied to the calculated lighting load. For instance, the guest rooms of hotels and motels are allowed the following demand factors for the general lighting load:
Let's do an example problem. What is the general lighting calculated load for a 40-room hotel? Each unit contains 600 sq ft of living area (Fig. 4).
As per Tables 220.12 and 220.42, 40 units x 600 sq ft x 2VA = 48,000VA. The first 20,000VA is calculated at 50% (20,000VA x 0.50 = 10,000VA). The next 80,000VA is calculated at 40% (28,000VA x 0.40 = 11,200VA). The sum of these two totals is 21,200VA.
The feeder/service general lighting load for commercial occupancies other than guest rooms of motels and hotels, hospitals, and storage warehouses is assumed to be continuous. Calculate it at 125% of the general lighting load, as listed in Table 220.12.
The lighting loads listed in Table 220.12 are minimum requirements. If the actual lighting load is known — and it is larger than the Table 220.12 value — use the actual load.
The feeder/service calculated load for each linear foot of show-window lighting must be calculated at 200VA per ft. Consider show-window lighting to be a continuous load. Example D3 in Annex D of the NEC provides a good example calculation that includes show-window branch circuits.
A 3,000 sq ft store has 30 ft of show window with a total of 80 duplex receptacles. The service is 120/240V, single-phase. The actual connected lighting load is 8,500VA [Annex D, Example D3].
General lighting = 3,000 sq ft at 3VA per sq ft
General lighting = 9,000 VA
Window lighting load [220.14(G)] = 30 ft at 200VA per ft
Window lighting load = 6,000VA
Outside sign circuit [220.14(F)] = 1,200VA
Lighting total = 9,000VA + 6,000VA + 1,200VA
Lighting total =16,200VA
(This is a continuous load, and will be taken times 125% for the feeder/service sizing.)
In the example, 125% of the actual connected lighting load (8,500VA x 1.25 = 10,650VA) is less than 125% of the load from Table 220.12 (9,000VA x 1.25 = 11,250VA), so the minimum lighting load of 9,000VA from Table 220.12 is used in the calculations. Had the actual lighting load been greater than the value calculated from Table 220.12, 125% of the actual connected lighting load would have been used. See NEC Annex D, Example D3 for the full calculation on this store building.
As you can see, it's important to determine what kinds of loads you have before starting your commercial load calculations. You can avoid confusion and prevent errors by mapping it all out. For example, use a simple spreadsheet on a computer, or graph it out on paper. If you list each load in the first column, you can name the applicable tables in the other columns from the more than half dozen tables that you may need to select from.
In part two of this installment, we'll look at calculating receptacle loads and introduce the optional calculation method for commercial occupancies. The commercial calculations we've discussed thus far have used the standard method, but the Code does provide an optional method for some calculations.
The following is a list of some of the standard ampere ratings for fuses and inverse time circuit breakers: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 500, 600, 800, 1,000, 1,200, and 1,600.