Question: I know you have addressed the practice of using 15 amp rated conductors for switch legs in residential 20 amp lighting circuits. I still have electricians saying it meets code. Will you try again? Thanks, Don Stephens
Answer: Thanks for your comment Don. I’m glad you said the installers are saying it meets Code. That makes it easier to answer. Section 210.20(B) requires conductors to be protected in accordance with 240.4(D). Section 240.4(D) requires that overcurrent protection for 14 AWG shall not exceed 15 amperes. So obviously it doesn't meet the requirements of the Code. I believe I understand what some people are getting at. Table 310.16 shows an ampacity of 20-amperes for 14 AWG conductors. But there's a little asterisk at the bottom of the table, which sends you to 240.4(D) where you are limited to 15-amperes. If you told me that installers are saying that the Code rules don’t make sense, then that's more difficult to answer. Certainly the load on the switch leg will never exceed the capabilities of the 14 AWG. However, if there is a neutral conductor present in the switch outlet box (and it would be considered an outlet box if the conductors are present to feed a utilization equipment), then it would be possible to connect to the supply to the switch and to the neutral and supply an additional load. This is a fairly remote possibility and I really don’t understand why a conductor that is capable of carrying 20 amperes can't be used on a circuit with 20-ampere overcurrent protection. Of course, under a ground-fault or short-circuit condition, the conductor would be subjected to the time/current opening time of a 20-ampere overcurrent device. But then again, if the conductor is rated 20-amperes on Table 310.16, there should be no problem.
Section: 210.20(B), 240.4(D), Table 310.16