Let’s pick up right where we left off last month in navigating the National Electrical Code to find needed requirements for overcurrent protection finds, which lands us in Article 450. Last time we saw that the overcurrent protection requirements in Article 240 aren’t going to cover the overcurrent protection for the transformer, so we must access requirements specific to transformers. As stated in the previous article, 240.4(F) tells us that the primary overcurrent protection device is not going to provide protection for the secondary conductors, and it even references us to 450.3. Another option is in 240.3 for specific application overcurrent protection requirements, which also sends us to Article 450.

This takes us to Table 450.3(B), which lists percentages of the full-load current ratings of transformers we can use to determine the maximum overcurrent protection device (OCPD) ratings for the protection of the transformer windings. Keep in mind that this table does not size protection for the conductors, but if we size the device to be used for the transformer, we can size our wire so that it serves the load and is protected by the OCPD. If the primary device is sized at 125% of the full load current rating of the transformer, there is no required secondary protection, and we could install any size overcurrent protection on the secondary side. If 125% creates issues with inrush current, though, we can increase the primary OCPD to 250%, but we are then required to have protection on the secondary side that is limited to a max of 125% of the secondary current rating. Sparing you the math for now, that limits us to 260A, but because this does not correspond to a standard size OCPD, we can round up to the next-higher standard size of 300A.

Overcurrent devices

However, the conductor will need to be protected by the overcurrent device we select. If we choose the maximum 300A, we will need to have 300A worth of conductor on the secondary side of the transformer, regardless of what load we supply. Conversely, if we supply a 200A main circuit breaker panelboard, the conductors will need to be protected from overcurrent by the 200A main circuit breaker and can be much smaller than if we use the maximum allowed. One way we can make the installation of a transformer easier, and often more cost-efficient, is to size the secondary conductors and OCPD to the load being served and verify that it is within the parameters of Table 450.3(B) requirements. Why provide larger wire and OCPDs than needed, right?

Now we need to decide on actual material to build with, which means a quick trip into Chapter 3 to visit Article 310 to size a conductor to run from the transformer to the first OCPD. Assuming we examined the load and found that we are supplying a 200A main circuit breaker panelboard from our transformer, we will select a conductor based on the OCPD in the panelboard. In Article 310, specifically Table 310.16 for conductor ampacities, we see that a 3/0 AWG copper THWN conductor has an ampacity of 200A. We could install this conductor to a 200A panelboard and have an installation where the conductor is protected by the OCPD, and the transformer secondary windings are also protected from overcurrent. Of course, this is the NEC, so nothing is that simple.

We still need to ensure that the 200A main circuit breaker in the panelboard can provide the overcurrent protection to the conductors like we had hoped. This brings us back to Article 240 for overcurrent protection of transformer secondary conductors. The general rule is that a conductor be protected from overcurrent in accordance with their ampacities as determined in Article 310. We established that our conductors are 200A and they land in a 200A OCPD. But can we put the OCPD at the end of the conductor, or must it go at the supply? A quick scan of the sections in Article 240 shows that 240.21 is “Location in Circuit” and might be a good place to look.

What rules?

The main rule in 240.21 is that the OCPD be located where the conductor gets its supply, which means we need a special rule to allow the OCPD at the end of the conductor to provide the protection. As it turns out, there are several instances where this is allowed, and they can be found in 240.21(A) through 240.21(H). For our example, we should look for an option that covers transformer secondary conductors. Section 240.21(C) covers exactly the requirements we are looking for. However, there are several options in this section to choose from. Which set of rules should we be following? That question requires a different approach to answer.

Section 240.21(C) is divided into different physical characteristic groupings. Some are based off conductor length and others on where they are installed. It helps to view these rules as conditions to be met for the OCPD protecting the conductor to be located somewhere other than the conductor supply. For instance, if the total conductor length is limited to 10 feet from transformer termination to main circuit breaker termination and the rest of the items in 240.21(C)(2) are true, then the OCPD can be at the termination point and still protect the conductors. However, it should be noted that this approach only provides overload protection and no short-circuit/ground-fault protection. However, the conditions listed for using this section help to ensure that the risk of short-circuit or ground-fault is minimized and that the primary OCPD will be able to protect the conductors should it happen.

If the conductors are unable to remain in that 10-foot length restriction, longer lengths can be used but are accompanied by more stringent conditions. For instance, to use the 25-foot rule, the conductors must have an ampacity of at least one-third of the primary OCPD rating adjusted for the transformer voltage ratio. It is also required that the conductors terminate directly into an OCPD, and they must be protected from damage by being installed in a raceway or other approved means. 

For our example installation, if the conductors are limited to 25 feet or less, installed in a raceway or other approved means to limit damage and terminate directly in the 200A main circuit breaker, they are considered protected.

This example is always a great exercise to show how the NEC is interconnected across all articles, and it is always fun to explore the various rabbit holes we get to follow in determining the proper installation requirements. It is examples like this that helped me fall in love with studying the NEC and all its fun little surprises. But the surprises in the Code are what keep us all on our toes and sharpen the skills needed to do the job the right way: safely. Next month’s article will wrap up this leg of navigation with a discussion on how the index can be used and some limitations that accompany this method. 

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

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