Welcome back to this guide on how to effectively find your way through NFPA 70, the National Electrical Code. I already discussed how to find the requirements that guide our installations in the Code. Catch up on part 1 and part 2 at ECmag.com. We have now gone through the process of locating the section we need when the question is straightforward. Here, we look into what it takes to find the information for more difficult questions.
As with most installations, it always seems like the NEC has a few hidden gems to uncover. These allow us to shine as well-trained electrical professionals if we know how to use them. Take the example from part 2: “What size conductors do I need on the secondary of a 75 kVA transformer?” This question involves two obvious areas of investigation: conductors and transformers. There are hidden concerns because we also need to ensure the conductors are protected from overcurrent in accordance with their ampacity. Let’s start with conductors, the main subject of the question.
Where to start
First, we need to understand if these conductors are branch circuit (Article 210), feeder (Article 215) or service conductors (Article 230). We’ll assume that the installation is one of the most common configurations: a 75 kVA, three-phase, dry-type, 480V delta to 208Y/120V transformer installed indoors, supplied from a motor control center in the main electrical room and supplying a panelboard for receptacle outlets and other 208Y/120V items in the building.
A good starting point is to see how the NEC defines these conductors in Article 100.
- Branch circuit: The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).
- Feeder: All circuit conductors between the service equipment, the source of a separately derived system or other power supply source and the final branch-circuit overcurrent device.
- Service conductors: The conductors from the service point to the service disconnecting means.
We can rule out service conductors since the secondary conductors on this transformer are downstream from the service disconnecting means. We can also rule out being branch circuit conductors, as these are feeding a panelboard and not the final outlet where power is taken to supply utilization equipment. Therefore, we are dealing with feeder conductors because they rest in between the service equipment and the final branch-circuit overcurrent protective device.
We want to know what size conductors are needed, which sounds a lot like a “planning or layout” issue. These answers are found in Chapter 2.
Three ways to find information
So, what article deals with feeder conductors? We have three options: thumb through Chapter 2 until we find feeders, go to the table of contents and find the article titled “Feeders” or look up feeders in the index. This introduces the table of contents and the index. Both navigational tools will help us arrive at the correct place, Article 215, to find the requirements for feeder conductors.
Opening the book to Article 215, we can see that Section 215.2 deals with feeder conductor size. This section states that the conductors must be sized in one of two ways, and both require that we know the load being supplied. But a panelboard isn’t a load, per se. So, we might know the actual calculated load per Article 220, or this might just be a panelboard installed so that as we need to install 208V and 120V circuits, there is a panelboard to supply them. Both scenarios are tricky and, frankly, more than we’ll cover here. But remember those hidden gems from earlier? Section 215.3 states that feeder conductors will be protected against overcurrent in accordance with part 1 of Article 240. Let’s go see what we can find there.
In Article 240, we find two important sections right away. Section 240.3 lists other articles where we will find overcurrent protection based on specific equipment involved, such as transformers. Section 240.4 describes the requirements for overcurrent protection of the conductors. The general rule is that conductors are protected from overcurrent in accordance with their ampacity unless otherwise permitted by 240.4(B) through (H).
Section 240.4(F) states that transformer secondary conductors are not considered protected by the primary overcurrent protective device. This means they need to be protected from overcurrent by a device installed on the transformer’s secondary side. We’ll learn about transformer overcurrent protection in Article 450 next month.
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About The Author
Vigstol is an electrical safety consultant for E-Hazard, a provider of electrical safety consulting and training services. He is also the co-host of E-Hazard’s electrical safety podcast “Plugged Into Safety.” For more information, check out www.e-hazard.com.