National Electrical Code (NEC) general requirements and definitions
What the exam tests
This domain covers NEC Article 90, Article 100, and Article 110. It asks whether an installation is code-minimum, whether equipment is approved and listed, and whether the installer followed listing instructions. Low-voltage work is not exempt: Class 2 and communications circuits still need listed power supplies, suitable cables, and accessible equipment.
Core rules
- The NEC is minimum; the AHJ interprets and approves. Article 90.4 permits the AHJ to grant exceptions only with equivalent safety.
- Listed means evaluated by a recognized testing laboratory; approved means acceptable to the AHJ. They are not synonyms.
- Section 110.3(B) requires equipment to be installed per its listing and instructions.
- Working space at electrical equipment is generally 3 ft deep, 30 in wide, and 6.5 ft high for 0-150 V to ground; higher voltages and conditions can require more.
- Equipment must be accessible for examination and repair without removing building finish.
Worked example
A 24 V Class 2 power supply is mounted above a 120/208 V panel in a closet. The panel needs 3 ft clear depth, 30 in clear width, and 6.5 ft clear height under 110.26. The low-voltage supply cannot intrude into that space, even though it is only 24 V. The installer must relocate it or provide the required clearance and dedicated equipment space.
Traps
Do not assume low voltage means no rules. Do not replace a listed wall transformer with an unlisted supply. Do not treat approved and listed as identical. Do not ignore manufacturer instructions for clearance, orientation, or enclosure.