Engineers often include demand factors in panel schedules and/or load calculations when preparing an electrical design. It is important to include these as permitted by the Code so that equipment is properly sized and correctly shown on drawings, resulting in an accurate determination of spare capacity. NFPA 70, National Electrical Code (NEC), allows engineers to take into account demand factors for various scenarios in electrical designs. By carefully following the NEC, engineers can avoid several negative scenarios and ensure the success of their projects.
The NEC defines a demand factor as a ratio of the maximum demand to the total connected load; this is in reference to a system or a part of a system that is under consideration.
Areas of oversight
Two common errors often emerge in electrical designs:
- Using 125% as a demand factor for continuous loads, and
- Approaching demand factors as a grey area and therefore devising demand calculations based on the engineer’s generalizations.
The NEC requires that branch-circuit conductors and overcurrent protection (as well as feeder overcurrent protection) shall be sized at 125% for continuous loads. See related requirements in Secs. 210.19(A)(1), 210.20(A), and 215.3. This requirement is sometimes misinterpreted. Consequently, the engineer applies a 125% demand factor for loads in a panel schedule and/or a load calculation. However, the 125% has already been applied at the branch and feeder level, so applying a second time to determine the load on the distribution equipment is neither necessary nor correct. Demand factors also are never more than 100%; the demand load should not exceed the connected load.
Refer to NEC Table 220.45 for lighting-load demand factors. This table outlines demand factors that may be used for lighting different occupancy types; demand factors range from 25% to 100% in this table.
Cases in point
Consider the hypothetical panel schedules in Table 1 (incorrect) and Table 2 (correct). These are both representative of the same fictional small commercial office project. The project scope for the examples includes circuiting for a few receptacles and some luminaires. Since lighting is a continuous load, some engineers mistakenly apply a 125% demand factor for lighting; this is shown in Table 1. One can see that the demand load exceeds the connected load in this panel schedule, which should never be the case. Per NEC Table 220.45, office lighting falls into the “other” category, and the associated demand factor should be 100%. Refer to the panel schedule in Table 2 for the correct version for this project scenario; in this case, all loads are factored at 100% and demand load equals connected load.
NEC areas of emphasis
Aside from the 125% misconception, engineers also sometimes may make the mistake of assuming that they can determine demand factors on their own. Instead, they should follow the NEC guidelines closely. Below are some especially useful sections to focus on.
Refer to Sec. 220.40 regarding the calculated load of a feeder or service. This Section of the Code indicates that the load for a feeder or service shall not be less than the sum of the associated branch circuits after any applicable permissible demand factors are applied. The NEC does allow demand factors for less than 100% for several scenarios.
It can be reasonably assumed not all occupants in dwelling units will use loads at the same time; therefore, the NEC has several allowances for a reduction of demand load when calculating the service size for a dwelling unit. Per Sec. 220.41, the minimum unit load, which includes most of the receptacles and lights, should not be less than 3VA/SF. This can then be reduced as applicable per Table 220.45.
The demand for motors and fixed heating should be 100% unless there is an exception that satisfies Sec. 220.51 and/or Sec. 430.26, and the AHJ has granted permission for this. An example of a possible application of Sec. 220.51 and/or Sec. 430.26 is electric heaters and/or motors that do not all operate at the same time. In this case, the AHJ may grant permission to use the largest of the non-coincidental loads for consideration.
Another exception cited within Sec. 430.26 is the ability to use historical data from an existing facility when factoring in motor loads for a new facility that is similar to the existing facility. Appliance loads in a dwelling unit may have a 75% demand factor applied for the situation of four or more appliances, per Sec. 220.53.
Table 220.54 shall be used for applying demand factors for electric clothes dryers. This demand factor is more applicable when considering several dwelling units as the demand factor for one to four electric clothes dryers is 100%.
Section IV of Art. 220 also offers an alternative approach for sizing service and feeders for dwelling units; this Section is entitled “The Optional Feeder and Service Load Calculations.” This approach allows for a more bulk-type demand factor, which can be applied for general receptacle/lighting loads, appliances, and motors; however, there are certain requirements that the engineer should take note of before choosing the optional calculation method. Some examples of these requirements are that multifamily units must have electric cooking and that dwelling units are equipped with electric space heating, air conditioning, or both. Table 220.84(B) can be referenced for the optional load demand factors related to three or more multifamily dwelling units.
For multiple elevators served by the same feeder, there is a demand factor that can be used in Table 620.14. These demand factors are based on a 50% duty cycle — meaning half time on, half time off.
For situations where an engineer is required to determine whether existing distribution equipment has adequate capacity to accommodate new loads resulting from a renovation and/or addition, refer to Sec. 220.87. Actual load usage for the existing equipment should be determined either by reviewing electric utility bills for peak load over the course of a year, or an electrician can perform a 30-day load study — and the peak load from this study can be used. Once the peak load is found, it should be multiplied by 125%; this plus the new load should not exceed the ampacity of the associated feeder or rating of the service.
Receptacles shall be calculated at not less than 180VA for each single or multiple receptacle on a yoke unless it is in a dwelling unit or office. The receptacle load for dwelling units is combined with the general lighting load 3VA/SF and outlined in Sec. 220.41. Note: This does not include receptacles for specific usage such as fixed-in-place appliances. For feeder and service load calculations in office buildings, Sec. 220.43 should be referenced regarding receptacle loads.
For non-dwelling units, Table 220.56 can be used to apply a demand factor to kitchen equipment. This table is especially useful for large commercial kitchens or restaurants because a demand factor of 65% can be applied to six or more units of kitchen equipment.
Section 220.60 describes a situation in which the smaller of two loads may be disregarded in calculating the total load for a feeder or service if it and the other larger load are not likely to run at the same time. Note: This scenario should be carefully considered — the exception applies to two items that are related to each other through switching and control.
There is an Informational Note below Sec. 430.26 that indicates how demand factors determined for the design of new facilities can be validated against actual historical experience from other similar installations. An example of this would be a new manufacturing facility, which could be based on one in a similar environment with similar equipment usage and operating hours as well as a similar size of facility. Note: AHJ permission would be required to implement this approach for a new facility.
The bottom line
Several ramifications can occur if demand factors are used incorrectly in an electrical design. First, the distribution equipment may be either oversized or undersized. There are negative cost implications for the equipment being oversized as well as an unclear depiction of available spare capacity. For example, if a 125% demand factor is incorrectly applied to each of the loads, it may appear there is no more space to add additional loads in the future. If the equipment is oversized, then the owner may be paying more for the wire and distribution equipment than what is actually needed. Furthermore, there are safety considerations if the equipment is undersized. If an overload occurs, the associated breaker may trip; if it does not, however, then the wire may overheat and cause a fire.
To conclude, it is imperative that engineers avoid adding 125% to the load on panels and/or services and that they take time to carefully decipher the various code sections regarding allowable demand factors. As indicated throughout this article, engineers need to assess demand factors for electrical design as covered in the NEC. It is a comprehensive guide and assists them with providing a minimum standard of care for their projects.
