Date: 201704
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Flight conditions: Mixed
Light: Daylight
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Cabin Lighting
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural MEL / CDL
Detector: Person Observer
When detected: In-flight
Result: General Release Refused / Aircraft Not Accepted
Contributing factors: MEL; Aircraft
Primary problem: MEL

Narrative: FAA on jumpseat to ZZZ. Halfway through the flight FAA Inspector asked to look at logbook. He found MEL 33-2-XX Cabin Interior Lighting was; in his opinion; misapplied. We were getting close to TOD so we decided we would discuss on ground. I sent ACARS for Maintenance to meet us at gate. On the ground I was asked if I thought that MEL applied. FAA Inspector thought it did not because it was not the lights; they were on full bright. FAA Inspector pointed out it was the switch; not the lights. I countered; to no avail; that the switch is part of the lighting system covered by the MEL and all MEL items A through E are being complied with. He said it was the switch and that is was not covered. Then he asked me to make the decision on the spot if I thought the MEL was right or wrong. Hello rock; meet hard place. I have the perfect textbook answer. I don't make those calls on my own. I am tasked with calling Dispatch and getting Maintenance Control in the loop as well. And that's what I did.ZZZ Maintenance showed up. FAA Inspector; Maintenance; and I walk to the back of the aircraft; as FAA Inspector wanted to see the switch and the lights. Maintenance was unable to dim the lights with the switch. FAA Inspector took pictures of switch and lights illuminated. We called Dispatch and Maintenance Control. Maintenance Control said it was the correct MEL; but would try to get it fixed. FAA Inspector let me know he was staying right there with me so he could document whether I took off with the aircraft with a no-go item. Maintenance said they couldn't fix it but that it was good to go as is.I headed up to the top of the jetway to tell the Agent we were holding off boarding only to find the Agent had everyone lined up and was about to start boarding! The Agent asked me to stand in her place and tell the Passengers we were not boarding because she thought they would be less likely to take their frustration out on the Captain. It worked. Passengers were satisfied. If I was not willing to fly the aircraft; they didn't want to get on it either.I then got a call from ZZZ Operations on the jetway phone asking me to explain why I'm wasn't boarding. I gave them a general thumbnail. I made a call to Dispatch and asked for Chief Pilot on Call patch. I gave him the thumbnail of the situation and let him know at this point I was not taking this aircraft. He agreed 100 percent! THANK YOU; THANK YOU; THANK YOU; for backing me. Chief also knew FAA Inspector was sitting right there on the aircraft with me. Thanks to whoever gave him the heads up and not putting me on the spot. At this point we were given a swap. Just to make sure; FAA Inspector stayed with us until we left the aircraft with Maintenance. We got a new aircraft and uneventful flight.Change the MEL to include the switch in the description. Maintenance Control was very sure this was the proper MEL. At this point; I just don't know.


Second narrative: [Report narrative contained no additional information.]


Synopsis: B737 flight crew reported that the MEL for the cabin lights was misapplied.