By Mike Holt for EC&M Magazine

The most basic of all safety rules is that conductors must be protected against overcurrent in accordance with their ampacity [240.4]. In addition, conductor overcurrent protection must be provided in each ungrounded circuit conductor and it must be located at the point where the conductors receive their supply [240.21]. The NEC does permit conductors that are identified as “tap conductors” to have overcurrent protection set at a value that greatly exceeds the ampacity of the conductors. However, this is only permitted under specific conditions as contained in 240.21.

Before we get into the details of 240.21, we need to understand that a tap conductor is not permitted to supply another tap conductor. In other words, you cannot make a tap from a tap. So let’s get started.

Feeder Tap Rules
10-Foot Feeder Tap Rule [240.21(B)(1)]
Feeder tap conductors can be run not over 10 ft without overcurrent protection at the point they receive their supply, but they must be installed in accordance with the following requirements: Figure 1
(1) The ampacity of the tap conductor is:
1 Not less than the computed load in accordance with Article 220, and
2 Not less than the rating of the device supplied by the tap conductors or the overcurrent protective device at the termination of the tap conductors.
(2) The tap conductors do not extend beyond the equipment they supply.
(3) The tap conductors are installed in a raceway if they leave the enclosure.
(4) The tap conductors have an ampacity of no less than 10 percent of the ampacity of the overcurrent protection device from which the conductors are tapped.

25-Foot Feeder Tap Rule [240.21(B)(2)
Feeder tap conductors can be run not over 25 ft without overcurrent protection at the point they receive their supply, but they must be installed in accordance with the following requirements: Figure 2
(1) The ampacity of the tap conductors is not less than 1/3 the ampacity of the overcurrent protection device protecting the feeder.
(2) The tap conductors terminate in a single circuit breaker, or set of fuses having a rating no greater than the conductor ampacity as listed in Table 310.16.
(3) The tap conductors are suitably protected from physical damage or are enclosed in a raceway.

Outside Feeder Tap of Unlimited Length Rule [240.21(B)(5)
Outside feeder tap conductors can be of unlimited length without overcurrent protection at the point they receive their supply, but they must be installed in accordance with the following requirements: Figure 3
(1) The tap conductors shall be suitably protected from physical damage.
(2) The tap conductors shall terminate at a single circuit breaker or a single set of fuses that limit the load to the ampacity of the conductors. This single overcurrent device shall be permitted to supply any number of additional overcurrent devices on its load side.
(3) The overcurrent device for the tap conductors is an integral part of a disconnecting means or shall be located immediately adjacent thereto.
(4) The disconnect is located at a readily accessible location either outside the building or structure, or nearest the point of entry of the service conductors.

Secondary Tap Rules
10-Foot Secondary Tap Rule [240.21(C)(2)]
Secondary conductors can be run up to 10 ft without overcurrent protection, but they must be installed in accordance with the following requirements: Figure 4
(1) The ampacity of the tap conductor is:
Not less than the computed load in accordance with Article 220, and
Not less than the rating of the device supplied by the tap conductors or the overcurrent protective device at the termination of the tap conductors.
(2) The secondary conductors do not extend beyond the switchboard, panelboard, disconnecting means, or control devices they supply.
(3) The secondary conductors are enclosed in a raceway.

Author’s Comment: Lighting and appliance branch-circuit panelboards require overcurrent protection to be located on the secondary of the transformer in accordance with 408.16(A), (B) and (E).

Outside Secondary Taps of Unlimited Length [240. 21(C)(4)]
Outside secondary tap conductors can be of unlimited length without overcurrent protection at the point they receive their supply, but they must be installed in accordance with the following requirements: Figure 5
(1) The conductors are suitably protected from physical damage.
(2) The conductors terminate at a single circuit breaker or a single set of fuses that limit the load to the ampacity of the conductors. This single overcurrent device shall be permitted to supply any number of additional overcurrent devices on its load side.
(3) The overcurrent device for the conductors is an integral part of a disconnecting means or shall be located immediately adjacent thereto.
(4) The disconnecting means for the conductors must be installed at a readily accessible location complying with one of the following:
a. Outside of a building or structure
b. Inside, nearest the point of entrance of the conductors
c. Where installed in accordance with 230.6, nearest the point of entrance of the conductors

Author’s Comment: This rule requires outside secondary conductors from customer owned transformers to terminate in a single overcurrent protection device. However, outside secondary conductors from transformers under the exclusive control of the electric utility can terminate in up to six overcurrent protection devices in accordance with 230.71(A).

25-Foot Secondary Tap Rule [240.21(C)(6)]
Secondary conductors can be run not over 25 ft without overcurrent protection at the point they receive their supply, but they must be installed in accordance with the following requirements: Figure 6
(1) The secondary conductors have an ampacity that (when multiplied by the ratio of the secondary-to-primary voltage) is at least 1/3 of the rating of the overcurrent device protecting the primary of the transformer.
(2) The secondary conductors terminate in a single circuit breaker or set of fuses that have a rating not greater than the conductor ampacity.
(3) The secondary conductors are protected from physical damage.

Author’s Comment: The secondary conductor must be sized no smaller than 1/3 the ampacity of the primary protection device based on the secondary/primary ratio. For all practical purposes, this can be ignored, because the secondary conductors are typically sized at 100 percent of the primary protection device rating.

If you have any comments or feedback, please let me know, Mike@MikeHolt.com