Welcome back to the discussion on developing a better understanding of the National Electrical Code. This month we will skip ahead a little in Article 210 to discuss how branch circuits are rated and how to decide what size branch circuit needs to be installed. We will also cover how to properly size branch circuit conductors that will adequately serve the equipment connected to it while making sure these conductors are properly protected from overcurrent by the branch circuit overcurrent protective device (OCPD).

Editor's Note: Read part 1 and part 2 of this series if you need to catch up.

Remember, the NEC requires us to identify branch circuit conductors, and the branch circuit will consist of ungrounded and grounded conductors, often with an equipment grounding conductor (EGC) routed with the circuit conductors. (See my article series starting with “Connecting up to Code” in the March 2024 issue of ELECTRICAL CONTRACTOR for more information on properly sizing the EGC.)

The NEC and branch circuit sizes

Let’s focus on how the NEC rates or defines the size of a branch circuit. To find this information, head to Section 210.18. Branch circuits recognized by Article 210 will be rated based on the ampere rating or maximum adjustable setting of the OCPD. This is fairly common sense. If I install a 20A circuit breaker for a branch circuit, the circuit is a 20A branch circuit.

An individual branch circuit can be rated for whatever the circuit calls for based on what is supplied. Article 100 reminds us that an individual branch circuit is one that is installed for the purpose of supplying a single utilization equipment. Section 210.18 only specifies certain ratings for branch circuits if the circuit is not an individual one. The ratings permitted to supply multi­outlet branch circuits are listed as 10A, 15A, 20A, 30A, 40A and 50A, with an exception that permits multioutlet branch circuits rated greater than 50A to nonlighting loads in locations where it can be certain that only qualified people will service the equipment. 

To determine the rating of the branch circuit, we need to figure out what size OCPD is needed based on the supplied equipment.

To determine the proper size of the OCPD, let’s jump ahead to Section 210.20. First, to determine the size of the OCPD, we need to know if the equipment we will be supplying will be continuous or not. Again, this takes us back to Article 100 to consult the definition of a continuous load. Simply put, a continuous load is expected to draw maximum current for three hours or more. There are also articles pertaining to specific equipment, such as those in Chapter 4 that don’t leave the determination of a continuous load up to the installer or authority having jurisdiction. 

Several sections throughout the NEC flat-out state that specific equipment types are considered a continuous load, i.e., fixed outdoor electric deicing and snow-melting equipment. This is done to ensure that the OCPD and conductors are sized properly to minimize the chance of overheating and potential issues. 

OCPD ratings

The basic rule with sizing the OCPD is that, at a minimum, the OCPD must be rated at not less than the noncontinuous load on the circuit plus 125% of the continuous load. As always, this guidance is followed by a “what if?” statement in the form of an exception that permits the use of OCPDs listed for operation at 100% of their rating to not require the sizing include 125% for the continuous load. 

It is important to note that there are several places in the NEC that state the OCPD must have an ampere rating not less than 125% of the equipment’s ampere rating. A prime example is storage-type water heaters with a capacity under 120 gallons. 

Once we determine the minimum ampere rating of the OCPD based on the load we are serving with the branch circuit, if known, we must remember that this is the minimum rating and that the OCPD can have a rating greater than this value. However, there are a couple of items to note. First, don’t forget that conductors are required to be protected by this OCPD in accordance with their ampacity and, therefore, if we select an OCPD larger than the minimum, we might need to use larger conductors in our circuit than required to ensure the conductors are protected. 

There are exceptions to the conductor overcurrent protection rule in 240.4, such as for motor circuit conductors where the OCPD is sized for short-circuit and ground-fault protection and not overload. The second thing to keep in mind here is that if there is a specific requirement for a maximum permitted OCPD that can supply the equipment, then this is the value that must not be exceeded. Motors are a good example. Where a maximum OCPD size will be determined, see Table 240.3 for specific types of equipment with their own OCPD rules.

Branch circuit conductor sizing

Now that we have our minimum size OCPD calculated, we can determine the minimum size branch circuit conductor required based on the load to be served. The minimum ampacity and size requirements for branch circuit conductors can be found in Section 210.19. In general, we want to start by looking at 210.19(A), where it states that branch circuit conductors will need an ampacity not less than the larger ampacity determined by either (A)(1) or (A)(2). 

We should be cautious here, as it can be very tempting to merge these two ampacity conditions into one. Don’t worry if you do, as the result will typically be larger wire than required; this mistake is often referred to as “double derating,” even though it isn’t derating the conductor at all. This isn’t really a safety issue, but an economical one, and if you are bidding on a job, it might be the reason you miss out. 

The first condition in 210.19(A)(1) is similar to the statement that we already discussed for sizing the OCPD: the conductor must have an ampacity sufficient for the noncontinuous load plus 125% of the continuous load. This ensures the conductor is adequate to not overheat under continuous loading conditions. Keep in mind that 201.19 also requires the termination temperature requirements of 110.14(C) to be met. 

The second condition for conductor ampacity is that the conductor must be able to serve the load after the application of any applicable adjustment or correction factors required by Section 310.15. Each of these conditions will specify a minimum conductor ampacity required, and your job is to compare these values and select the larger one as the minimum ampacity requirement for the branch circuit conductors.

Next month’s article will continue this discussion in determining the size branch circuit we need, including an example that ties in selecting the OCPD and conductor sizes while making sure all requirements are met.

Until next time, stay safe and remember to always test before you touch! 

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