CSLB C-10 Electrical ContractorEquipment and InstallationHard
A 400-ampere feeder OCPD supplies tap conductors run in conduit for 8 feet before terminating in a single 40-ampere panelboard. Applying the 10-foot tap rule of NEC 240.21(B)(1), what is the minimum ampacity required for the tap conductors?
- A400 amperes
- B20 amperes
- C40 amperes
- D100 amperes
Show answer & explanationAnswer & explanation
Correct answer: C. 40 amperes
The 10-foot tap rule requires tap conductor ampacity to be not less than 1/10 of the feeder OCPD rating (400A x 0.10 = 40A) and not less than the rating of the device it supplies (40A panel). Both conditions point to a minimum of 40A, and the 8-foot tap length satisfies the 10-foot maximum requirement.
Why the other options are wrong
- A. 400A would require full-size conductors, defeating the purpose of the tap rule.
- B. 20A does not meet the 1/10 rule or the panelboard's 40A rating.
- D. 100A is far more than required and is not derived from the applicable rule.
10-Foot Tap Rule (240.21(B)(1))
Allows tap conductors up to 10 feet long, run in a raceway, to be sized smaller than the feeder they tap from, provided they meet minimum ampacity and termination requirements.
- Tap ampacity must be at least 1/10 of feeder OCPD rating
- Tap length limited to 10 feet, enclosed in a raceway
- Must terminate in a single OCPD not exceeding the tap conductor's ampacity
Memory trick: Ten feet, ten percent — the tap rule's simple scent.