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

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 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. NEC 220.3 has Table 220.3(A), which 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. 220.3(B)(8)clears up some mystery regarding the 180VA requirements for fixed multioutlet assemblies. The NEC isn’t a design manual, and so doesn’t 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. 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.
    
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 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.
    
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 Electrician’s 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.