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National Electrical Code Top Ten Tips: Article 220 -- Branch Circuit, Feeder, and Service Calculations

by Mark Lamendola

Based on the 2017 NEC

Please note, we do quote from copyrighted material. While the NFPA does allow such quotes, it does so only for the purposes of education regarding the National Electrical Code. This article is not a substitute for the NEC.

These are the 10 NEC Article 220 items we deem most important, based on the pervasiveness of confusion and the potential costs of same.

  1. 220.5(B). "Where computations result in a fraction of an ampere that is less than 0.5, such fractions shall be permitted to be dropped." At first glance, you may wonder what the code panel was smoking. However, you are just calculating here, not selecting components—which are available only in standard sizes. The various safety factors in the Code already account for enough headroom that dropping a fraction of an amp won’t matter.
     
  2. 220.12 addresses how to account for lighting loads for specific occupancies.
  3. Table 220.12 shows you the general lighting loads, by occupancy. This is useful if you don’t have a specific lighting design and need to plan total load. Some people automatically add 20% to this, while others use the NEC figures because they plan to install efficient lighting. Please note, this table does not apply to determining the number of branch circuits for general illumination. Also, demand factors don’t apply to circuits where the entire lighting is likely to be used at one time—for example, in operating rooms, ballrooms, or dining rooms.  
  4. To understand how to apply this Article correctly refer to examples in Annex D. It’s important to note that the computed load of a feeder or service cannot be less than the sum of computed loads on the branch circuits. You do not select the feeder or service breakers based on the sum of the branch breaker ratings. You select them based on the total load supplied by that feeder or service. That total load must account for demand factors and diversity factors (for example, you don’t run your air conditioner and heater at the same time, and you wouldn’t be using all 19 welding receptacles in a small shop at one time) and other items detailed elsewhere in Article 220.
     
  5. Table 220.3 refers to other Articles for specific-purpose calculations.
     
  6. Table 220.44 gives a simple rule for applying demand factors to receptacles that are not in dwellings. Basically, you assign a 100% demand factor to the first 10 KVA or less. For KVA beyond 10, you can apply a 50% demand factor. This is subject to certain requirements, as spelled out in 220.44.
     
  7. 220.52 through 220.56 cover appliance calculations, which are often required knowledge for obtaining a Master Electrician’s License.
     
  8. 220.60 is the famous "Noncoincident Loads" clause. "Where it is unlikely that two or more coincidental loads will be in use simultaneously," you can use just the largest load to compute the total load of a feeder. In Article 430 (motors), this is a major consideration. Ignoring this results in expensively over-engineered systems.
     
  9. 220 Part IV includes 220.80 through 220.88 and it details optional calculations.
     
  10. 220 Part V includes 220.100 through 220.103 and it covers computing farm loads.

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How the NEC is arranged

  1. The first four Chapters of the NEC apply to all installations.
  2. Article 90 precedes Chapter One, and establishes the authority of the NEC.
  3. Article 80 follows the body of the NEC; it exists as Annex H. It provides the requirements for administration.
  4. Chapters 5, 6, and 7 are the "special" chapters, covering special: occupancies, equipment, and conditions (in that order).
  5. Chapter 8 provides the requirements for communications systems.
  6. Chapter 9 provides tables.
  7. The appendices provide mostly reference information.
  8. Appendix D contains examples that every NEC user should study.

Try your NEC moxy:

  • Do you know the difference between bonding and grounding? Hint: Look in the NEC, Article 100.
  • Does the NEC refer to grounding incorrectly in any of its articles? Yes! So be careful to apply the Article 100 definitions. Don't ground where you should bond.
  • When doing motor load calculations, which Article covers hermetic motors? Answer: While Article 440 covers the application of hermetic motors, it does so only by amending Article 430 because hermetic motors are a special case of motors. For motor load calculations, refer to Article 430.
  • Does the NEC provide a voltage drop requirement? Yes! It does so in a special case, which is Article 648 Sensitive Electronic Equipment. But for general applications, it does not provide a requirement; it merely provides a recommendation in a couple of FPNs.
  • Take our Code Quizzes.

Remember other applicable codes, rules, standards, and references:

  • OSHA's electrical worker safety rules.
  • IEEE standards.
  • NETA standards.
  • NFPA standards.
  • International Codes (if applicable to the installation).
  • State Codes (if the state has them).
  • Local ordinances and permit requirements.
  • Local fire codes.
  • Manufacturer requirements or guidelines.
  • Customer security requirements.
  • Industry standards.
  • Your company's own internal standards, practices, and procedures.
  • Engineering drawing notes.

 

 

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