Understanding how to perform load calculations is an important part of an electrician’s professional career. Before obtaining a permit, some jurisdictions require paperwork showing load calculations for services and feeders. Article 220 of the National Electrical Code (NEC) provides requirements for calculating branch circuit, feeder and service loads. [220.1] While following the calculation procedures in Article 220 will provide the minimum-size branch circuit, feeder or service, it will not necessarily provide for future expansion.
Compliance with provisions in the NEC (and proper maintenance) will result in an installation that is essentially free from hazard but not necessarily efficient, convenient or adequate for good service or future expansion of electrical use. [90.1(B)] Although load calculations may not be a part of everyday life for most electricians, every electrician should have a good understanding of load-calculation procedures.
Article 220 contains load-calculation procedures for all types of occupancies, but they are not organized by types of occupancies. The organization of this article is in its scope (220.1). In previous editions, there were only four parts, now there are five. Part I (220.1 through 220.5) covers general requirements for calculation methods. In previous editions, most of the branch-circuit calculations were a part of the general requirements in the first part. In the 2005 edition, the first part was split into two parts.
Because of this, Parts II, III and IV were renumbered to Parts III, IV and V. Branch-circuit load calculations are now covered in Part II (220.10 through 220.16). Part III (220.40 through 220.61) covers feeder and service load calculations. Optional feeder and service load calculations are in Part IV (220.80 through 220.88). Since optional calculation methods are in Part IV, calculations in Parts I, II and III are sometimes referred to as standard calculation methods. For example, if a single-family dwelling is said to have been calculated by the standard method, it was calculated in accordance with the first three parts of Article 220. Part V (220.100 through 220.103) contains specifications for calculating farm loads. A new figure (220.1) showing the outline of Article 220 has been added to the 2005 edition (see Figure 1).
In the 2005 edition of the NEC, the term “compute” and its derivatives were changed to “calculate” and its derivatives. This change is referred to as a global change. This change not only occurred in Article 220, but throughout the entire Code book. The National Electrical Code Technical Correlating Committee submitted the change as a result of an effort by the NEC Usability Task Group to standardize the language throughout the NEC.
Another proposal submitted by the National Electrical Code Technical Correlating Committee was the reorganization of Article 220. Except for 220.1 (Scope), every section in Article 220 has been renumbered. While a few new sections have been added, the running order is the same for sections in this article. For example, in previous editions, the section pertaining to electric ranges and other cooking appliances for dwelling units was 220.19. Now, the same section is 220.55. Figure 2 is a cross reference showing the renumbering of sections from the 2002 edition to the 2005 edition of the National Electrical Code.
A great help with load calculations is provided in the book and many, if not most, electricians forget it is there. Annex D (in the back of the book) provides many examples of the calculation procedures stipulated in Article 220. The annexes are for informational purposes only and are not part of the requirements of the National Electrical Code. When there is a question as to how to do a certain type of load calculation, there is a chance something in one of the examples in Annex D could help. For example, a calculation is needed for 12 single-phase ranges supplied by a 208Y/120-volt, three-phase, 4-wire service. Although the wording in 220.55 specifies how to calculate the load for single-phase ranges supplied by a three-phase, 4-wire service (or feeder), there is room for misinterpretation. Example D5(a), in Annex D, shows a load calculation for 10 single-phase ranges supplied by a three-phase, 4-wire service. By studying the load calculation in example D5(a), the calculation procedure is clarified.
Some of the results from calculations in Article 220 are not stand-alone answers when calculating conductors and overcurrent protection. The results are calculated loads and must become part of additional calculations from requirements in other articles. For example, branch-circuit conductors and overcurrent protection are needed for a circuit feeding show-window lighting in a store building.
The length of the show window is eight feet. In accordance with 220.14(G)(2), at least 200 volt-amperes are required for each foot of show window. The calculated load for this show widow is 1,600 volt-amperes (8 x 200 = 1,600). Although there are no requirements in this section to multiply continuous loads by 125 percent, there are requirements in other sections of other articles.
First, it must be determined if show-window lighting in a store is a continuous load. The definition in Article 100 states a continuous load is a load where the maximum current is expected to continue for three hours or more. Since show-window lighting in a store will be energized for at least three hours, it is a continuous load.
Example D3, in Annex D, also confirms that show-window lighting is considered a continuous load (see Figure 3). In accordance with 210.19(A)(1) for branch-circuit conductors, the continuous load must be multiplied by 125 percent. The branch-circuit conductors must be rated to carry at least 2,000 volt-amperes (1,600 x 125 percent = 2,000). At 120 volts, the minimum conductor ampacity is 16.7 amperes (2,000 ÷ 120 = 16.7).
In accordance with 210.20(A), the branch-circuit overcurrent protection must also be calculated at 125 percent of the continuous load. The rating of the overcurrent protection must be at least 125 percent of the continuous load. The branch-circuit overcurrent protection must have a rating of at least 2,000 volt-amperes (1,600 x 125 percent = 2,000).
At a voltage of 120 volts, the overcurrent device must not be rated less than 16.7 amperes (2,000 ÷ 120 = 16.7). The next higher standard-size circuit breaker is 20 amperes. [240.6(A)] Without the consideration and application of other sections in other articles, the branch-circuit conductors and overcurrent protection would be sized wrong and be in violation of the Code (see 215.2(A)(1) for feeder conductors and 215.3 for feeder overcurrent protection).
Next month’s column continues the discussion of load calculations. EC
MILLER, owner of Lighthouse Educational Services, teaches classes and seminars on the electrical industry. He is the author of “Illustrated Guide to the National Electrical Code” and NFPA’s “Electrical Reference.” He can be reached at 615.333-3336, charles@charlesRmiller.com or www.charlesRmiller.com.