Incorporating demand factors into service feeder calculations is a key skill any electrical professional (including apprentices) must understand – both for working on the job and passing the all-important electrical licensing or journeyman exam. Let’s look at which scenarios require demand factor considerations, where to find the necessary NEC requirements, and how to work through a typical exam question related to demand factors and service feeder calculations.
When to use service feeder calculations
Service feeder calculations are typically completed during a construction project's design stage after the conception phase. These calculations are routinely completed by the electrical engineer, then reviewed by the master electrician during the bidding stage. The journeyman electrician assigned to the project must understand these calculations to identify errors or omissions that could cause delays and unforeseen expenses. In addition, if the building is a multi-family or mixed-use (or special occupancy) dwelling with electrical appliances in each unit, a demand factor must be applied to each portion of the electrical service calculation to determine the final calculated load.
The demand factor is the ratio of the maximum demand of a system (or part of a system) to the total connected load of a system or the part of the system under consideration. This definition can be found in Art. 100. Demand factors are usually used to calculate the service load for:
- Electric ranges: Table 220.55, Demand Factors and Loads for Household Electric Ranges, Wall-Mounted Ovens, Counter-Mounted Cooking Units, and Other Household Cooking Appliances over 1¾kW Rating
- Electric clothes dryers: Table 220.54, Demand Factors for Household Electric Clothes Dryers
- General lighting loads (this usually includes general-purpose, non-dedicated receptacles): Table 220.12, General Lighting Loads by Occupancy
- Mobile homes, manufactured homes, and mobile home parks: Table 550.31, Demand Factors for Services and Feeders
Reasoning behind applying demand factors.
Demand factors must be applied to our service feeder calculations to correctly size the main electrical service equipment, service conductors, and/or gear. If we didn't apply demand factors, we would grossly oversize our service conductors and equipment, leading to frivolous expenses, wasted labor, and inflated equipment costs. In addition, demand factors are necessary because it's unlikely that all equipment, appliances, or lighting will be used all at once in any building or facility. Instead, these items are typically used intermittently or randomly throughout days, weeks, and months, reducing the necessity of calculating each appliance's total load (or volt-ampere rating) or every light and receptacle.
For example, I live in an apartment complex with 320 units, all furnished with electric clothes dryers. It's highly unlikely that all 320 residents will ever use their clothes dryers simultaneously, so we apply a demand factor to the total calculated load of all the electric clothes dryers, which I like to call a “discount factor.” We only use a portion of the nameplate rating of the appliance or what the NEC specifies we should calculate, whichever is greater.
Calculation example
Here is an example of a question you would typically see on an electrical licensing or journeyman exam.
Question: A 4,500W electric clothes dryer will be installed in each unit of an 11-unit apartment complex. For calculating the service load for the 11-unit apartment complex, the calculated load to be used for all the clothes dryers is ________ VA?
A. 28,850
B. 47,970
C. 55,000
D. 205,000
Step 1. Locate the Code section that covers electrical clothes dryers in Art. 220 [Branch-Circuit, Feeder, and Service Load Calculations]. The fastest way to do this is to reference the index at the rear of the NEC. Search for clothes dryers, and you'll find:
- Clothes dryers
- Calculations for, 220.54
- Feeder demand factors, Table 220.54, 220.82(B)
Remember, whenever you calculate any portion of a service with a bulk quantity of appliances or units, typically, there's a table, formula, or Article (section and part) covering reducing the load (heating and air conditioning is an exception). Examples of formulas are located at the rear of the NEC in Annex D.
Step 2. Section 220.54 for electric clothes dryers within dwelling unit(s) instructs us that “the load for household electric clothes dryers in a dwelling unit(s) shall be either 5,000W (VA) or the nameplate rating, whichever is larger, for each dryer served.” We are then permitted and directed to use the demand factors in Table 220.54. We use this table because “where two or more single-phase dryers are supplied by a 3-phase, 4-wire feeder or service, the total load shall be calculated based on twice the maximum number connected between any two phases.”
Step 3. Here's the math based on what we find in Sec. 220.54 and its associated demand factor Table.
1. The number of electric clothes dryers served is 11.
2. The Wattage of each dryer is 5,000. Remember Sec. 220.54’s verbiage regarding electric clothes dryers within dwelling unit(s). In our question, the rating of each electric clothes dryer is only 4,500W; that's lower than what the Code requires us to use for the demand factor, so we must use 5,000W as our rated load.
3. 11 dryers x 5,000W/dryer = 55,000W
4. Now apply the demand factor to determine how much load should be used in the service calculation.
55,000W x 47% = 25,850W (Answer A)
So, as you can see, we only need 47% percent of the total calculated load, which would be an additional load of 108A to a single-phase service.
25,850W ÷ 240V = 107.71A
This would be an additional load of 72A to a 3-phase service.
1.732 x 208V = 360.26
25,850W ÷ 360.26V = 71.80A
Follow my column for more practical how-to articles and tips for applying electric basics concepts in the field. Past columns include Everyday Instructions for Electricians, Introduction to Commercial Service Calculations, Introduction to Residential Service Calculations, The Apprentice’s Guide to Non-Metallic Sheathed Cable, The Apprentice’s Guide to NEC Chapter 4, The Apprentice’s Guide to Art. 300, the Apprentice’s Guide to Art. 640, The Apprentices Guide to Special Equipment, The Apprentice’s Intro Guide to Motors, and The Apprentice’s Guide to Service Entrance Cable. For more information on why a structured approach is so important to navigating the NEC and how to put its requirements into practice in real-world settings, read “The NEC for Newbies.”
Harold De Loach, a master electrician and electrical trainer/instructor, is the founder of The Academy of Industrial Arts (www.taia-school.com) in Philadelphia. With more than 30 years of experience in the field, he writes regular exclusive content for the E-Train and can be reached at [email protected].
