NEC Questions and Answers – January 2007

By Mike Holt for EC&M Magazine

Q1. How deep must I bury USE cable?
A1. When cables or raceways are run underground, they must have a minimum “cover” in accordance with Table 300.5. Note 1 to Table 300.5 defines “Cover” as the distance from the top of the underground cable or raceway to the surface of finish grade. According to this table, Type USE cable must have 24” of cover, unless the installation is a residential 15 or 20A branch circuit rated 120V or less with GFCI protection.

Q2. Can low-voltage Class 2 thermostat wire be installed in the same raceway with power conductors for the a/c compressor?
A2. No, 725.55 specifies that Class 2 cables are not permitted to be placed in any enclosure, raceway, or cable with conductors of electric light or power.

However, Exception No. 2 to 725.52 allows Class 2 circuits to be reclassified as a Class 1 circuit if the Class 2 equipment markings are eliminated and the circuit is installed in a Chapter 3 wiring method [725.55(D)(2)(b)]. Reclassifying the circuit allows the Class 1 circuit to be installed with functionally associated power circuits in accordance with 725.26(B)(1). Figure 725-30

Q3. Can Type MC cable be installed in a cable tray?
A3. Yes. See Table 392.3(A).

Q4. Can I use a cable tray in a commercial occupancy?
Q4. Yes, cable tray installations aren’t limited to industrial establishments [392.3].

Q5. How do I size the circuit conductors and protection device for a 25 hp, 208V three-phase fire pump motor?
A5. The fire pump motor circuit overcurrent protective device must be sized to carry indefinitely the sum of the locked-rotor current of the fire pump [695.4(B)(1)]. According to Table 430.251(B), the locked-rotor current of a 25 hp, 208V, three-phase motor is 404A. This requires a 450A protection device, in accordance with 240.6(A).

Conductors supplying a fire pump motor must must have an ampere rating of not less than 125 percent of the fire pump motor full-load current (FLC), as listed in Table 430.248 or 430.250. The FLC of a 25 hp, 208V, three-phase motor is 74.8A, so the conductors must be sized no smaller than 93.5A (74.8A x 1.25). According to Table 310.16, we can use a 3 AWG rated 100A at 75°C [110.14(C)(1)].

Careful, the branch-circuit conductors for a fire pump motor must be sized so that the voltage at the line terminals of the controller, when the motor starts (locked-rotor current), does not drop more than 15 percent below the controller’s rated voltage. In addition, the voltage at the motor terminals must not drop more than 5 percent below the voltage rating of the motor when the motor operates at 115 percent of the fire pump full-load current rating.

Q6. Can liquidtight flexible metal conduit be installed under the raised floor of a computer room and if yes, must it be secured?
A6. Branch-circuit supply conductors for information technology equipment can be installed in liquidtight flexible metal conduit located under a raised floor. And yes, the raceway must be securely fastened in place [645.5(D)(2)].

Q7. Can I ground a satellite system to the metal parts of electrical equipment?
A7. No. The antenna mast [810.15] and antenna discharge unit [810.20(C)] must be grounded to one of the following:

• Building or structure grounding electrode system [250.50]
• Interior metal water piping system, within 5 ft from its point of entrance [250.52(A)(1)]

• Accessible means external to the building, as covered in 250.94

• Metallic service raceway
• Service equipment enclosure
• Grounding electrode conductor or the grounding electrode conductor metal enclosure

Grounding the lead-in antenna cables and the mast helps prevent voltage surges caused by static discharge or nearby lightning strikes from reaching the center conductor of the lead-in coaxial cable. Nothing can prevent damage from a direct lightning strike. But grounding with proper surge protection can help reduce damage to satellite and other equipment from nearby lightning strikes.

Q8. What are the grounding requirements for the metal standards that support a raise floor system in computer room?
A8. None