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.
- 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.
- 220.12 addresses how to account for lighting loads for specific occupancies (all of which are non-dwelling).
- 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.
- 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.
- Table 220.3 refers to other Articles for specific-purpose calculations.
- 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.
- 220.52 through 220.56 cover appliance calculations, which
are often required knowledge for obtaining a Master Electrician’s
- 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
- 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.
- 220 Part V includes 220.100 through 220.103 and it covers
computing farm loads.