Grounding, Bonding, and Overcurrent Protection for Low Voltage Systems
Grounding and bonding
Article 250 supplies the general grounding and bonding rules for premises wiring. Low-voltage Class 2 and Class 3 circuits are often isolated from ground, but that does not mean grounding and bonding can be ignored. Metallic raceways and enclosures must be bonded where required. Communications systems have their own grounding requirements in Article 800, and coaxial and antenna systems have separate bonding rules. The grounding electrode system must be installed per 250.50, and equipment grounding conductors must be identified and continuous per 250.118.
Overcurrent protection
Class 2 and Class 3 circuits are limited by a listed power source, not by adding a fuse or breaker to an unlimited source. A listed Class 2 transformer or power supply defines the circuit class. Adding a 5-amp fuse to a 24-volt transformer secondary does not convert a Class 1 circuit into Class 2. Supplementary protectors may protect equipment, but they do not substitute for the listing and power limitation of the source.
Worked example
A 24-volt AC transformer supplies a thermostat circuit. The transformer is listed as a Class 2 power source, and the secondary is not bonded to ground. If a technician adds a fuse to the secondary and runs the conductors with 120-volt power conductors, the circuit does not become Class 1 and the separation rules still apply. The correct approach is to keep the Class 2 separation and use the listed power source.
Traps that cost marks
- Assuming low voltage cannot fault or create a hazard.
- Trying to reclassify a circuit by adding overcurrent protection.
- Leaving metallic raceways unbonded.
- Grounding a Class 2 secondary without a specific requirement or manufacturer instruction.