How it works & the NEC rules behind it
Under NEC §240.21, overcurrent protection is generally required at the point where a conductor receives its supply. The Feeder Tap Rules (§240.21(B)) provide critical exceptions allowing smaller gauge conductors to be tapped from larger feeders without overcurrent protection at the tap point.
The Two Primary Feeder Tap Rules:
- 1. The 10-Foot Tap Rule (§240.21(B)(1)):
- Tap conductor length must not exceed 10 feet (3.0 m).
- Tap conductor ampacity must be not less than:
(a) The combined calculated load,
(b) The rating of the device supplied by the tap, and
(c) Not less than 1/10th (10%) of the rating of the upstream overcurrent protective device. - Tap conductors must be enclosed in a raceway.
- 2. The 25-Foot Tap Rule (§240.21(B)(2)):
- Tap conductor length must not exceed 25 feet (7.5 m).
- Tap conductor ampacity must be not less than 1/3rd (33.3%) of the rating of the upstream overcurrent device.
- Tap conductors must terminate in a single circuit breaker or set of fuses that limits current to the ampacity of the tap conductor.
- Tap conductors must be protected from physical damage.
- 3. Taps from Taps Prohibited: A tap conductor is never permitted to supply another tap conductor ("no tapping a tap").