This article was posted 01/03/2007 and is most likely outdated.

NEC Questions and Answers – January 2007
 

 
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- NEC Questions and Answers – January 2007

January 3, 2007  

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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

 

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Comments
  • I'm coming out of the MDP and running about 100 feet to a new electrical room, and in the room I'm setting a new transformer. My drawings show a disconnect on the primary side of the transformer, do I really need one on the primary side?

    Philip
    Reply to this comment

  • I'm working at a surgery center, and I'm getting ready to install med. gas alarm wiring, my question is does it need to be in conduit? I'm looking in NEC 2005- Article 725.11 (a) and (b), and I believe it does. When I read Article 725.11 (a), when they say a direct fire or life hazard are they saying that the fire alarm wire needs to be in conduit as well? One more question, in the med. gas room, which is approximately 8 x 20, the room has oxygen, nitrogen, nitrous oxide, carbon dioxide, and medical air bottles. The manifold enclosures need 120 volt, and also med. gas alarm wiring. I have nothing on my drawings on how to wire this room, so my question is because of the gases, will I need to run rigid pipe and seal offs to hook up the equipment?

    Philip
    Reply to this comment

  • Mike, The question on raised flooring for computer rooms goes to NFPA 94 ADP systems on grounding for these facilities. This may not be a requirement, but it should be.

    David Rorer
    Reply to this comment

  • Change of email address.

    Joe Yazbek
    Reply to this comment

  • I have eight 3 1/2 in. pvc conduits leaving the transformer and coming into the bottom of the MDP. Can I arrange my conductors and isolated phase conductors? My local inspector said I probably can, but he would like a letter from the electrical engineer saying it's okay. I've looked at Article 303B1 exception and I believe you can arrange them as isolated phase conductors. Your comments would be greatly appreciated.

    Philip
    Reply to this comment

  • I need to feed a 3 phase 250A panel can I take 80% of 250A and size my wire according to 200A instead of 250A.

    Lou Garlobo
    Reply to this comment

  • Hi Mike, Regarding 25 hp, 208V three-phase fire pump motor branch circuit conductors. You have indicated 75 Deg C rated conductors. However, NEC 110.14 (C) (1) (a) requires to use 60 Deg C column for Temperature limitations. This would require the circuit conductors to be # 2. Unless the terminations are listed & identified for 75 Deg C terminations, circuits rated 100 A or less or conductors from #14 thru #1 shall use 60 Deg C rating column.

    Girish Tevar
    Reply to this comment

  • Mike,Kindly refer to last paragraph of answer to question7.I would like to inform that any induced current either by transformer action or by nearby lightning strike will be mostly confined within the system and will not flow through ground and surge protection may not help in this case.But since the current in direct lightning stroke tends to go to ground,properly rated surge protection may provide protection.

    T.M.Haja sahib
    Reply to this comment

  • I have a question on the temperature de-rating when sizing conductors. All of the tables in Article 310 have a temperature correction factor and are based on 30C/86F nominal. It can get above 86F even in Alaska. Most machinery is rated up to 40C/104F operation, though this wouldn't cut it in the summer in Phoenix or even in an attic in Alaska.

    Does the 125% rule cover this or should we be de-rating for temperature too?

    And is this table based on annual average temperature, or what it is right now?

    If the load is known to be full on continuous at its rated value, I worry about this as I've experienced warm wiring.

    In real operation, most loads do not even come close to the maximum plate ratings we go by, which are often the product of margin stacking. But when they do, or when someone plugs in too much stuff like the Dad in "A Christmas Story" things get hot quickly.

    Should we be doing additional de-rating?

    Matt
    Reply to this comment

  • RE:

    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)]. What about 645.5(E) Not required for IT equipment

    Terry Whitney
    Reply to this comment

  • Response to Q5 only partially covers overcurrent protection and feeder sizing/type for fire pumps. Further items to cover: 1) Since the code only allows for a maximum of one overcurrent protection device ahead of a fire pump, the installation is limited either to a separate service lateral via the utility company or a tap in the main switchboard ahead of the building main disconnect. In either case, the overcurrent protection device should be service rated and separated from the main switchboard. 2) In applications where the building is of highrise construction, the fire pump shall have an alternate means of service. This is generally accommodated by an emergency generator (since highrise construction would require a generator for life safety systems). Again, only one overcurrent protection device is allow upstream of the fire pump, which means the generator should have a minimum of two output breakers (typically more in highrise construction due to emergency and standby load separations). This overcurrent protection device is not required to carry the lock-rotor current as does the normal power feed. Fire pump controllers in highrise applications are generally furnished to include an ATS. 3) Feeders serving fire pumps should be routed outside of the building (generally means beneath ground floor slab-on-grade). Where this is not possible, the conductors routed within the building should be have a 2-hour fire rating, i.e. MI cable, R90 cable, etc. This is required for both the normal and emergency feeders. The exception to routing outside the building is within the main electrical room and the fire pump room.

    Brian Smith
    Reply to this comment

  • Response to Q6 should be clarified or qualified based on the application of the raised computer floor. Most raised computer floors in computer rooms are used to supply air to equipment for cooling purposes, which makes the underfloor space an air plenum. The PVC coating of standard sealtight flex would not be allowed in a plenum installation. They do make sealtight flex with a plenum rating that can be used in this application.

    Brian Smith
    Reply to this comment

  • Are service feeders allowed to pass though separate living units in a commercial apartment building?

    Mitchell Tolbert
    Reply to this comment


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