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ER rating, cables must pass additional crush and impact tests that certify they are rugged enough to be run without conduit to protect them from damage. With these types of cordsets established under NFPA 79, they can now be used to more quickly and easily distribute power throughout a network via feeder lines and taps. The configuration of those feeder conductors (or trunk line) and branch circuits (or taps) is defined under the NEC Tap Rule. NEC Tap Rule Overall, the NEC Tap Rule requires overcurrent protection to be present where the conductors receive their supply, but several sub-rules define how a trunk line can be tapped to energize a load without the need for overcurrent protection at the tap point itself. This makes it much easier to install passive distributors without additional fuses or breakers to tap power off the trunk line for individual load devices. For typical power distribution networks, where the tap lines are generally under 25 A, two defined sub-rules are the most relevant: the 10-foot (3-meter) and 25-foot (7.5-meter) rules. In both cases, overcurrent protection only needs to be located at the termination point of the tap line, meaning it can be located at the load device. This leaves the travel distance from the tap point to the load device free of the need to implement additional components. Specifically for the 10-foot rule, the amperage of the tap conductors must be no less than 1/10 of the amperage rating for the trunk line conductors. For example, a trunk line with conductors rated up to 40 A must have tap lines with conductors rated for a minimum of 4 A. In this case, many sensor-actuator cordsets will meet the 4 A minimum rating, but more typically, one would see the deployment of 7/8- inch or M12 power form factors rated above 10 A (Figure 1). For longer taps, there is the 25-foot rule. Any tap lines between 10 and 25 feet would now be required to carry conductors rated for at least one-third of the amperage of the trunk line conductors.
Figure 1: Example of trunk-and-drop where the feeder line and tap line are both rated to the same current.
Tap Rule in action As an example, let’s look at a trunk line rated for 30 A per conductor (Figure 2). Here, each of the tap line conductors would need to be rated for at least 10 A. Typical sensor- actuator cordsets would be off the table, and we would be looking more at power cordsets in the 7/8-inch and M12 power form factors that generally run at least 10 A, but can go up to 16 A or more depending on the applied UL test standards.
Figure 2: Example of trunk-and-drop where the feeder line is rated up to 30 A, while the tap line is smaller, with a capacity of at least 10 A.
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