This month continues the in-depth journey through common requirements found in the National Electrical Code involving the backbone of electrical systems: branch circuits and feeders. 

Last month’s article looked at an example of sizing feeder conductors based on NEC Section 215.4. If you remember, this was based on two key concepts: 

1. The conductors must be able to handle 125% of the continuous load plus 100% of any noncontinuous load.
2. The conductors must be able to handle the total load after we apply any adjustment or correction factors based on the conditions of use.

We performed calculations to determine which of the two methods required the larger conductor size, and that became the minimum. This month’s article changes gears to discuss overcurrent protection of these conductors.

Conductor ampacity reprise

So, what is the most important factor to think about when sizing overcurrent protection? Of course, it is the ampacity of the conductor we are trying to protect. After all, Section 240.4 states that conductors are to be provided with overcurrent protection in accordance with their ampacities. But as we have seen time and again with the NEC, this is a generalized statement, and alternatives always seem tied to the general rule. 

Before diving too deeply into Article 240 and all the nuances of overcurrent protection, let’s go back to Article 215 and see what is there first. Section 215.5 gives us our marching orders for overcurrent protection of feeder conductors. It pushes toward Part 1 of Article 240 but gives some direction, such as one of the conditions for conductor ampacity. Essentially, we are going to ensure that the chosen overcurrent protective device (OCPD) is sized to handle 125% of the continuous load plus any remaining balance of the load that is noncontinuous. This will be the minimum size OCPD based on the supplied load, and we will simply need to ensure the OCPD rating is equal to or less than the ampacity of the feeder conductors. That ensures the OCPD has enough capacity to allow the load to operate but will also limit the current flowing on the conductors so the heat produced in the conductor will not damage the insulation. 

Looking back at last month’s article, we had 99A of continuous and 81A of noncontinuous load. Factoring in the 125% for a continuous load gave us a total of 204.75A. So, the minimum OCPD rating must exceed 204.75A. Section 240.6 lists the standard size OCPD ratings in Table 240.6(A), and the next-highest standard size OCPD is a 225A device. 

At the conclusion of the conductor sizing conversation, we determined that we would be using a 250-kcmil copper conductor with XHHW-2 insulation. From Table 310.16, we can see the ampacity of the conductor is 255A in the 75°C column, which is correct to use since the conductor will be connected to 75°C terminations at the OCPD. However, 255A far exceeds the 225A rating of the OCPD, and we can rest assured the OCPD will prevent the conductor from being damaged by overcurrent events.

This effort is to make sure that the OCPD enables the load to operate and protect the conductor. This is not an overly complicated process when we think about it in those terms. 

But to channel my inner late-night infomercial personality—wait, there’s more! Now let’s look at the more complicated OCPD rules that often are applied to feeders. And right out of the gate, we will jump over to Section 240.21 and the feeder tap rules.

Feeder tap rules

In general, the location in the circuit where we will find feeder overcurrent protection is the point where the feeder gets the supply. That keeps everything nice and simple, but sometimes it is advantageous to take a different approach. 

Let’s imagine the example feeder is one of many similar distribution panelboards in a large industrial plant, and rather than run multiple feeders to supply each distribution panelboard, we would like to run a single large feeder to where these panelboards are located and splice off to supply the individual panelboards. This might be beneficial for several reasons, but it would create a situation where the OCPD at the feeder supply would exceed the ampacity of the conductors feeding the 225A panelboard, and it would seem the conductors are no longer protected from overcurrent the way 240.4 states it must be. 

Overcurrent protection

But all is not what it might seem. To understand this, we need to look at what exactly we are trying to protect conductors from. First, the conductor must be protected from overcurrent events that happen during short-circuits and ground-fault events. Second, the conductor must be protected from overloading conditions. In the example of a large feeder supplying feeder taps to the individual panelboards, let’s put some numbers in to illustrate how the required protection can still be achieved. 

For this, we will use an 800A OCPD to feed the large feeder and run 800A worth of wire to the tap box. From the tap box, the 250-kcmil conductors will run to a panelboard and terminate in a 225A main circuit breaker. Should a short-circuit or ground-fault occur, the amount of amperes flowing in the feeder typically is in the range of thousands to tens of thousands. The fault current is likely going to be high enough to still be in the instantaneous range of the 800A OCPD, or very close to it, and, therefore, the 250-kcmil conductors are still protected from fault currents. 

But what about overload conditions? The ampacity of the 250-kcmil conductors is roughly 195A based on conditions of use. If an overload were to reach 300A, it would far exceed the ampacity of the conductors and would never trip the 800A OCPD supplying the system. 

Remember that in between the supplied loads and the 250-kcmil conductors is a 225A OCPD, which will trip on a 300A overcurrent. So, in this feeder tap situation, we have an OCPD upstream that will protect from short-circuits and ground-faults, and we have an OCPD downstream that will limit the load the conductors carry and protect them from overloading that way. It is this combination of OCPDs that provides the required conductor overcurrent protection.

This is just a brief look into some nuances that exist in feeder overcurrent protection. Obviously, we need to look at the specifics of the feeder tap rules to get into how this combined protection for the wire is achieved. 

Next month’s article will take a break from the discussion on feeders and branch circuits to take a special look at how the NEC specifically addresses electrical safety. 

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

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