After last month’s interlude, this article returns to the series taking an in-depth look at the National Electrical Code (NEC) requirements involving the main infrastructure of an electrical system: branch circuits and feeders. 

The simple approach is where the overcurrent protective device (OCPD) is at the point where the conductor receives its supply, but we looked at feeder taps where there is a combination approach of using the upstream and downstream devices to provide the necessary protection. 

Now we will look at the specifics of the feeder tap rules to get a better understanding of how the combination approach still protects the conductors.

Feeder tap rules

The feeder tap rules are broken up into five conditions with different rules:

1. Tap conductors not more than 10 feet long
2. Tap conductors not more than 25 feet long
3. Tap conductors supplying a transformer where the combined total length of the primary and secondary conductor is not more than 25 feet
4. Tap conductors more than 25 feet long in high-bay manufacturing buildings
5. Tap conductors outside of buildings or structures with no length limitations

The most-used tap conductor rules are the first two conditions in the above list. It is important to note that these conditions apply to the total length of conductor, not just physical distance. This means that if the tap conductors leave the tap box and enter the panelboard but use 5–6 feet to wrap around and terminate in the main circuit breaker, the length of the conductor in the tap box and panelboard count toward the total length calculation. The 10-foot and 25-foot restrictions are from lug to lug measured along the length of the conductor. 

If the tap conductors remain short enough to qualify for the 10-foot tap rule, the requirements are the least restrictive. First, the ampacity of the tap conductors must be not less than the combined loads supplied. This is how we determined the feeder size, so this should not be an issue. But the ampacity must also not be less than the rating of equipment containing OCPDs or the rating of the OCPD in which the tap conductors terminate. For our example, this won’t be a problem, as they are landing in a 225A OCPD. However, the way the 10-foot rule is written, 10-foot tap conductors can land in a “main lug only” panelboard, and the tap conductors need to have an ampacity that exceeds the rating of the MLO panel. This accounts for what the panelboard can supply versus what we calculated. 

Next, 10-foot tap conductors must not extend past the equipment they are supplying. Therefore, we cannot run tap conductors to an MLO panelboard with lugs on the other side of the bus and extend the tap into another panelboard side-by-side with this one. If that is required due to needing extra space for circuit breakers, then we must land in a main circuit breaker to provide overcurrent protection of the conductors that extend to the second panelboard. Also, it is important that tap conductors are protected from physical damage from the tap enclosure all the way to the equipment they supply.

Finally, for the combination of the upstream and downstream OCPDs to effectively protect the tap conductors, there are limits to sizing the upstream device. For installations where the tap conductors leave the enclosure where the tap is made, the ampacity of 10-foot tap conductors must not be less than one-tenth of the upstream OCPD rating. In our example, this means  if we’re using a feeder OCPD upstream of 800A, any tap that qualifies under the 10-foot rule must have an ampacity of at least 80A.

Now, let’s examine the rules that apply to tap conductors limited to 25 feet in length. Here the NEC makes things a little less complicated. First, it eliminates the option for the tap conductors to feed an MLO panelboard or other equipment containing OCPDs, and it requires the tap conductors to terminate in a single circuit breaker or set of fuses that will limit the load current to the ampacity of the tap conductors or less. So, if we can’t limit our tap conductors to less than 10 feet, we will need to terminate in an OCPD that will provide the overload protection the tap conductors need. 

There is also some consideration to the ratio of the upstream OCPD to the ampacity of the tap conductors. The limited length of the 10-foot tap conductors minimizes the risk of a short-circuit or ground-fault in the tap conductors, but the longer length of the 25-foot tap conductors means there is more of a chance. Therefore, the ratio of tap conductor ampacity to upstream OCPD rating is more restrictive at one-third instead of one-tenth of the rating. This means the smallest tap conductor that uses the 25-foot rule must at least have an ampacity of 266.67A. 

This puts us in a little bit of a bind here, since the 250-kcmil conductors only have an ampacity of 255A in the 75°C column and even less when factoring in conditions of use. This means we will need to look at possible ways to limit the number of current-carrying conductors in the same raceway and reassess the ambient temperature where these conductors will be installed. At a minimum, we will need to increase the size of tap conductors to 300 kcmil. So, keeping the tap conductors to the shortest length possible is a good idea.

Other conditions

Let’s look at the other conditions for tap conductors that are less common in practice, but in certain instances are helpful to electrical system designers. Take, for instance, the high-bay manufacturing tap rule. For installations with high ceilings and open spaces typically found in manufacturing facilities, it would be very difficult to limit the length of a tap conductor to less than 25 feet if the ceiling is 35 feet high. But there are significant restrictions to be able to use this rule, such as conditions of maintenance and supervision ensuring only qualified people will service the system. 

They also give restrictions on horizontal and total length. Remember, this is about high-bay installations here and not just an open permission slip to run tap conductors all over the facility. Therefore, the tap conductors are only permitted to run a maximum of 25 feet horizontally before dropping down to the equipment supplied but, at no time are the tap conductors permitted to be longer than 100 feet total, and the tap must be made no less than 30 feet from the floor.

Another less common use of this rule is for tap conductors outside of a building. Here there is no restriction on length, but the conductors must be protected from physical damage and land in a disconnect either before they enter the building or immediately after they enter, or as the NEC states, nearest the point of entrance. 

This unlimited length is permitted because the tap conductors are not installed in a building that could be damaged by overloaded or damaged conductors. If conductors are overheating and installed underground, it might cause an unplanned outage, but remember the purpose of the NEC is to protect people and property. This doesn’t necessarily mean the installation will be efficient or ideal. It means the installation will be safe. 

Next month’s article will look at feeder neutral conductor rules and possible reductions in size permitted by Article 215. Until next time, stay safe and remember to always test before you touch!

e-Hazard Management LLC