The 2020 NEC made surge protection mandatory for the first time — a new Article 242 replaced the old surge-arrester rules, and dwelling-unit services now require a listed device as a baseline, not a recommendation. GFCI protection was extended to 250-volt receptacles in required locations (2017 only covered 125-volt), and coverage expanded to all basements, finished or not. AFCI protection was extended to kitchens and laundry areas. A new rule also required outdoor emergency disconnects for one- and two-family dwellings, changing how services get planned from the start.

This is the edition where studying either direction costs you. Prep built on the 2017 code leaves you missing every one of these additions. They're new requirements, not clarifications, so there's no "close enough." Prep built on the 2023 code teaches you rules that go further than what's required yet, like the expanded surge protection scope for multifamily and healthcare occupancies — content that's simply not on a T-series exam.

MCL's T-series prep is built exclusively on the 2020 NEC. Not a version blended from the edition before or after — just what your exam tests.