Stumped By the Code? Branch Circuit Requirements for Central Vacuums
All questions and answers are based on the 2017 NEC.
Q. What are the NEC branch-circuit requirements for central vacuums?
A. (A) Circuit Loading. Listed central vacuum systems must be on a separate circuit if the rating of the equipment exceeds 50% of the ampere rating of the circuit [Sec. 210.23(A)(2) and Sec. 422.15] (see Figure).
Sec. 210.23(A)(2) specifies that equipment fastened in place, other than luminaires, must not be rated more than 50% of the branch-circuit ampere rating if this circuit supplies both luminaires and receptacles. Due to this requirement, a separate 15A circuit is required for a central vacuum receptacle outlet if the rating of the central vacuum exceeds 7.50A. A separate 20A circuit is required for a central vacuum receptacle outlet if the rating of the central vacuum exceeds 10A, but not 16A [Sec. 210.23(A)(2)].
Q. How many conductors or cables are permitted in a cable tray by the NEC?
A. The number of multiconductor cables, rated 2,000V or less, permitted in a single cable tray must not exceed the requirements of Sec. 392.22(A). The conductor sizes herein apply to both aluminum and copper conductors. Where dividers are used, fill calculations apply to each divided section of the cable tray.
If ladder or ventilated trough cable trays contain multiconductor power or lighting cables, the maximum number of cables must conform to the following [Sec. 392.22(A)(1)]:
- If all of the cables are 4/0 AWG and larger, the sum of the diameters of all cables must not exceed the cable tray width, and the cables must be installed in a single layer [Sec. 392.22(A)(1)(a)]
Cable trays must be supported in accordance with the manufacturer’s installation instructions [Sec. 392.30(A)].
Cables and conductors must be secured to and supported by the cable tray system in accordance with Sec. 392.30(B)(1), (2), and (3) as applicable.
Cables installed vertically must be securely fastened to transverse members of the cable tray [Sec. 392.30(B)(1)].
Supports must be provided to prevent stress on cables where they enter raceways or other enclosures from cable tray systems [Sec. 392.30(B)(2)].
A box is not required where cables or conductors exit a bushed raceway used for the support or protection of the conductors [Sec. 392.46].
Q. What are the NEC branch circuit overcurrent protection requirements for appliance circuits?
A. (A) Branch Circuits. Branch-circuit conductors must have overcurrent protection in accordance with Sec. 240.4, and the overcurrent protection device rating must not exceed the rating marked on the appliance [Sec. 422.11].
(E) Nonmotor Appliances. The appliance overcurrent protection device must:
(1) Not exceed the rating marked on the appliance.
(2) Not exceed 20A if the overcurrent protection device rating isn’t marked, and the appliance is rated 13.30A or less, or
(3) Not exceed 150% of the appliance rated current if the overcurrent protection device rating isn’t marked, and the appliance is rated over 13.30A. Where 150% of the appliance rating doesn’t correspond to a standard overcurrent protection device ampere rating listed in Sec. 240.6(A), the next higher standard rating is permitted.
Let’s run through an example scenario based on these requirements.
Example: What’s the maximum size overcurrent protection device for a 4,500W, 240V water heater?
Solution: Protection Size = 4,500W ÷ 240V = 18.75A × 150% = 28A
Answer: 28A doesn’t correspond to a standard ampere rating, so use 30A, which is the next size up [Sec. 240.6(A)].
These materials are provided to us by Mike Holt Enterprises in Leesburg, Fla. To view Code training materials offered by this company, visit www.mikeholt.com/code.
