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

by Mark Lamendola

Based on the 2020 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). You can round calculations to the nearest whole ampere, and that includes rounding down. Not just up. 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 (all of which are non-dwelling).
  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. But if the building is designed to comply with an energy code adopted by the local AHJ, you can calculate the lighting load per that code if you meet the four conditions in 220.12(B). This latter approach can reduce materials cost considerably. Please note, Table 220.12 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 briefly described 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. The optional method is usually part of an electrical licensing exam.
     
  10. 220 Part V includes 220.100 through 220.103 and it covers computing farm loads.