How to Size a Branch-Circuit

A question electricians are often asked is, ‘how do you size a circuit?’. To most, this seems like it would be a simple question to answer; however, there is more to it than you would think. It would be fair to say that even a high percentage of electricians get confused on this subject. We should respond back to that question with one of our own: ‘What type of circuit are you asking about?’. A lot of electricians seem to think that every circuit should be sized to 80 percent of the breaker, but though this might hold true for a curtain type of circuit, it isn’t true for all of them. The National Electrical Code book is where electricians go to size circuits – hope you have yours ready, because we’re going to get to the bottom of this. Let’s get started!

Article 220 of the National Electrical Code book is the article we would look up in order to calculate certain types of loads such as feeders, services and branch circuits. This text is going to focus on branch circuits. A branch circuit is the circuit that is found on the ‘load side’ of the overcurrent protection device, or OCPD. I know what you might be thinking: ‘What does that even mean?’. The branch circuit is the circuit that connects to the last breaker or fuse. I was once told that branch circuits are 90 percent of what electricians work on or install, but I’m not sure where that statistic comes from. The electrical trade is vast and there are many different types of installations that electricians have to work on, however I can see why it might seem that 90 percent of the work we do is on the branch circuit. All receptacles (plugs) and light circuits are terminated directly to branch circuits. 

Alright, let’s dive into the code book. Part II of Article 220 covers branch circuits. Section 220.14 gives the calculations for specific loads. For example, Section 220.14(I), Receptacle Outlets, tells us that except as covered in Section 220.14(J) and (K), receptacles, or plugs, must be sized not less than 180 volt amps per yoke. Section 220.14(J) is the calculation used for dwelling units, and 220.14(K) is for office buildings. The 180 volt amp requirement is for receptacles in any type of occupancy other than the ones found in dwellings and offices. This is what is known as ‘sizing a load’. 

Article 220 also covers demand factors; these can be thought of as a kind of multiplier. Table 220.44 gives the demand factors for receptacles other than those found in dwellings. You would multiply a load for a non-dwelling receptacle by a given percentage, as shown in this table. This given percentage gets the load closer to the size of its actual use. If we want to size a specific load like this, then we would use Article 220.

To size the actual branch circuit, we would go to Article 210. This is titled Branch Circuits, and gives the associated codes that pertain to this specific type of circuit. So if we go to Article 220 to size our circuit, we would then go to Article 210 to find the specific rule for a branch circuit. Part II of Article 210 is titled Branch-Circuit Rating. This is where we would go to find the requirement for sizing all the specific parts pertaining to a branch circuit. Section 210.18 tells us that the OCPD (the breaker or fuse) is what the circuit should be based on, or sized to. For example, if a load pulls 19 amps and is terminated to a 20 amp breaker, and the circuit conductors are 10-AWG copper which is rated for 30 amps, the circuit is considered a 20 amp circuit.

Section 210.19 is specifically talking about the conductors. 210.19(A)(1) says that the conductors in a branch circuit must be able to carry 100 percent of the noncontinuous load, plus 125 percent of a continuous load. Article 100 gives the definition of a continuous load as a load that is expected to operate for three or more hours. Section 210.20 tells us that the same holds true for the OCPD that protects the circuit.

When sizing both the breaker and the conductors, we can’t size them smaller than the load to be served. Section 210.21 covers the rating of the actual devices such as receptacles and cord-and-plugs. There are two different tables: one for cord-and-plugs in section 210.21(B)(2), and one for the receptacle devices that can be found in section 210.21(B)(3).

We need 125 percent for a continuous load because the longer equipment is operating, the more heat will build up. Heat will create more resistance, and resistance opposes current flow. This can cause a massive amount of strain on the conductors and could cause damage to the equipment and its terminals – or worse, it could cause a fire. The majority of electricians out there, when asked what percentage a circuit can be loaded up to, will tell you that it is 80 percent. This can confuse a lot of younger electricians, so let me explain: 80 percent is the reciprocal, or inverse, of 125 percent. This means that the equipment being served is 80 percent of the conductors and breaker, and that the breaker and conductors are 125 percent of the equipment. 

Let’s go to code sections 210.22 & 210.23, which are both titled Permissible Loads. Section 210.22 covers permissible loads for individual branch circuits, and 210.23 covers permissible loads for multiple-outlet branch circuits. Let’s take a look.

Section 210.22 says that an individual branch circuit can supply any load for which it is rated. That load can’t be bigger than the branch circuit rating, however. The NEC Handbook gives an example of electric vehicle supply equipment being supplied by an individual branch circuit. In the field, electricians call an individual circuit a ‘dedicated circuit’. This particular code is referring to a dedicated circuit, and is saying it can be sized up to the total rating of the circuit. We learned earlier that a circuit’s rating is dependent upon the OCPD. If a piece of equipment pulls 20 amps, it can be protected by a breaker that is a 20-amp rated breaker.

Section 210.23 covers Permissible Loads, Multiple-Outlet Branch-Circuits. This section is talking about circuits with two or more outlets or receptacles. According to Section 210.23(A), 15 and 20 amp circuits are allowed to have lighting and receptacles on the same circuit. Section 210.23(A) is broken into two subsections. (1) talks about circuits that have two or more cord-and-plug pieces of equipment that are not fastened in place. These circuits cannot exceed 80 percent of the branch circuit rating. This means that if you have a circuit on a 20 amp breaker, it can’t be loaded up to more than 16 amps. Again, this is for a circuit with more than one outlet or receptacle that is cord-and-plug connected. 

Section 210.23(A)(2) is for circuits that are cord-and-plug connected with two or more outlets or receptacles, with a piece of equipment that is fastened in place. Here, the equipment cannot exceed 50 percent of the circuit rating. A good example of this would be a Central Vac that is fastened in place in a garage. If connected to the 20 amp circuit in the garage that has other receptacles, the Central Vac could not exceed 10 amps.

So to summarize, if a circuit has one piece of equipment on it, it may be loaded up to its full capacity. If it’s a shared cord-and-plug connected circuit that isn’t fastened in place, it can’t exceed 80 percent of the breaker. If it’s a shared cord-and-plug connected circuit that is fastened in place, then no piece of equipment connected to that circuit can exceed 50 percent of the rating of the breaker.

There’s definitely a lot more that goes into sizing a circuit than a lot of people realize. This is one of the main reasons people have such a hard time with their Journeyman exams. The best way to get really good at navigating the code book is to be in it all the time. I hope this article helps you in your electrical journey – stay safe, and stay relevant!