Conductor Sizing for Branch Circuits

Step 1: 110.14(C )(1)

When sizing conductors, the first things we need to look at are the terminals. We need to look at the temperature rating of the terminals so that we know which column to use on table 310.16, in order to size our conductors. Sometimes (most times!) as electricians, we don’t have that luxury. 

Section 110.14(C)(1) gives us the code requirement for electrical connections of circuits rated under 100 amps, and circuits rated over 100 amps. This section tells us that we can’t assume anything. This makes sense because if the terminals can’t take the heat then, well, you know – get out of the kitchen! 110.14(C)(1)(a) tells us that if we don’t know the temperature rating and the load on the equipment or device is 100 amps or less, we would use the 60-degree celsius column.

Very clean panel wiring in an interior subpanel done by u/evilone17 on reddit.com

Section 110.14(C)(1)(b) says that if we don’t know the terminal temperature rating of a load that is over 100 amps then we use the 75-degree column. NEVER ASSUME, especially on a Journeyman test!

Step 2: Continuous loads 

If a load or piece of equipment is expected to operate for three or more hours, the overcurrent protection device and the conductor rating have to be adjusted. 

Let’s say a load pulls 45 amps, and it operates for six hours. We take the 45 amps that the equipment pulls and multiply it by 125%. This puts us at 56.25 amps. Now, of course, the load still pulls 45 amps, but the longer the load runs, the more heat can build up on the conductors. Heat creates resistance, and resistance opposes current flow. This is why we have to up-size both the wire and the breaker. We would now go to the table found in code section 310.16 and size the wire. Since this is 100 amps or less and we do not know anything about the terminal temperature ratings for this specific piece of equipment, we need to use the 60-degree column. Looking at the 60-degree column of Table 310.16, we can see that 56.25 amps isn’t an option. We can’t round down to the 55 amps shown in column 60, so we have to round up to the next highest which is 70 amps. This would give us a 4 AWG conductor for this 45 amp load. 

Table 310.16 Ampacities for Insulated Conductors – Source: link.nfpa.org/

Now let’s size the breaker for this 45 amp load. Code article 240 covers overcurrent protection. Table 240.6 covers amp ratings for different breakers and fuses. If our load is 45 amps, and it runs for – in this case – 6 hrs, we multiply it by 125% which we know gives us 56.25 amps. Just like the sizing of the wire for this circuit, there is no breaker that is rated at 56.25 amps, so we have to use a 60 amp breaker.

What we just went over was for continuous loads. Article 100 gives us a definition for a continuous load as “A load where the maximum current is expected to continue for three hours or more”. If you have a load that is not continuous, then these rules would not apply. We would look at the terminal rating, or use article 110.14(C)(1), and look at the amperage of the load to be served. Then we would go to table 310.16 to size the wire, and table 240.6 to size the breaker. 

The codes for the continuous and non-continuous loads can be found in the following code sections:

  1. Branch circuits: 210.19 covers the requirements for branch circuit conductors, and 210.20 covers the requirements for overcurrent protection.
  2. Feeder: 215.2 covers the requirements for feeder conductors, and 215.20 covers the requirements for overcurrent protection.

Step 3: “The Glue” 240.4

Okay, so why do I call Article 240.4 the glue? Well, it’s the “glue” when it comes to conductor protection. Look at table 310.16. The first 5 wire sizes, 18 AWG- 10 AWG, all have an asterisk next to them. If we look at the bottom of the table we can see that the asterisk sends us to 240.4(D). Code section 240.4 sub-section (D) is titled, Small Conductors. This starts off by telling us that “Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed that required by (D)(1) through (D)(7) after correction factors for ambient temperature and number of conductors have been applied”. This code section goes on to say that we need to protect these conductors with a certain overcurrent protection device. Let’s take a look:

  1. 14 AWG Copper, 15 amperes
  2. 12 AWG Copper, 20 amperes
  3. 10 AWG Copper, 30 amperes

Now, it’s important to note that it says “Unless specifically permitted in 240.4(E) or (G)”. This is where we can get confused. Let’s look at 240(G).

Overcurrent Protection for Specific Conductor Applications: 240.4(G)

This code section says that “Overcurrent protection for the specific conductors shall be permitted to be provided”, and it refers you to table 240.4(G). This table gives us specific types of loads that need to be sized in their own manner. Some examples of these loads are Motors, article 430, Air-Conditioning, article 440, and Electrical Welders, article 630. 

This table gets overlooked all the time, and electricians end up sizing things like motors and air-conditioning circuits incorrectly. I know that sounds crazy, but it’s true. On a good note, they usually size the conductors larger than required, and when taught how to do it correctly they just say “code is a minimum standard.” This is true, but when taught that money can be saved in labor and material, all of a sudden the codebook is the way to go. 

4: Conclusion 

It is important to know how to size conductors and how to protect them. As electricians, we are trusted by the general public to ensure that our installations are safe. We all know the joys of helping to provide the comforts that electricity offers to people in their homes and businesses. However, if we go around and install electrical equipment and devices and don’t pay attention to things like temperature rating or breaker rating, we can cause major damage to property or worse, harm to people. Be safe, and stay educated as things in the industry are always changing. 

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