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

by Mark Lamendola

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. NEC 220.2(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 componentsówhich 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 wonít matter.
     
  2. NEC 220.3 has Table 220.3(A), which shows you the general lighting loads, by occupancy. This is useful if you donít 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. 220.3(B)(8)clears up some mystery regarding the 180VA requirements for fixed multioutlet assemblies. The NEC isnít a design manual, and so doesnít try to anticipate every permutation. Thus, 220.3(B)(11) instructs us to use 180VA for loads not covered in 220.3(B)(1).
     
  3. 220.10 gives general instructions on feeders and services. The FPN refers to examples in Annex D. Itís 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 donít run your air conditioner and heater at the same time, and you wouldnít be using all 19 welding receptacles in a small shop at one time) and other items detailed elsewhere in Article 220.
     
  4. NEC Table 220.11 provides the demand factors for lighting loads in various types of occupancies. Please note, this table does not apply to determining the number of branch circuits for general illumination. Also, demand factors donít apply to circuits where the entire lighting is likely to be used at one timeófor example, in operating rooms, ballrooms, or dining rooms.
     
  5. NEC 220.12 addresses how to account for loads when planning show windows and/or track lighting.
     
  6. NEC Table 220.13 gives a very 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 NEC 220.13.
     
  7. NEC 220.16 through NEC 220.20 cover appliance calculations, which are often required knowledge for obtaining a Master Electricianís License.
     
  8. NEC 220.21 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. NEC 220 Part III includes NEC 220.30 through NEC 220.36 and it details optional calculations.
     
  10. NEC 220 Part IV includes NEC 220.40 and NEC 22041 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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