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Grounding vs Bonding: 250.86 Other Enclosures

March 17, 2006  

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PART IV. GROUNDING (BONDING) OF ENCLOSURE, RACEWAY, AND SERVICE CABLE   Hi Res - Cable/DSL [1249Kb]
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250.86 Other Enclosures.

Metal raceways and enclosures containing electrical conductors that operate at over 50V [250.112(I)] must be grounded (bonded) to an effective ground-fault current path [250.4(A)(5)].

Author’s Comment: Metal raceways and metal enclosures containing circuit conductors from electrical supply sources that operate at 50V or less aren’t required to be grounded [250.86]. For example, metal boxes used with power-limited fire alarm circuits operating at 50V or less aren’t required to be grounded.
Exception 2: Short sections of metal raceways used for the support or physical protection of cables aren’t required to be grounded (bonded) to an effective ground-fault current path. Figure 250–123

Author’s Comment: NM cable on a wall of an unfinished basement installed, in a listed metal raceway, must be grounded to an effective ground-fault current path [334.15(C)].
Exception 3: Metal elbows having 18 in. or more of earth cover aren’t required to be grounded (bonded) to an effective ground-fault current path. Figure 250–124



Figure 250–123
(Click on image to enlarge)


Figure 250–124
(Click on image to enlarge)

     

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Comments

  • Follow-up, Does anyone have any additional information on this:

    Greetings, I\'m unable to locate the specific verbiage of a ground not being required for metallic pathways containing circuits whose voltage is under 50 volts. This would be very important to me -- can you please help me identity the specific wording, I didn\'t see it in 250.86.

    Thanks! William Wilson April 12 2006, 9:45 am EDTAdd your reply to this comment

    William Wilson

    • Reply from: Mike Holt   
    This is not an easy one to follow, but let me try... 250.86 states "Except as permitted by 250.112(I), metal enclosures and raceways for other than service conductors shall be grounded."

    This appears simple enought, then when you visit 250.112(I), it states "Power-Limited Remote-Control, Signaling, and Fire Alarm Circuits. Equipment supplied by Class 1 power-limited circuits and Class 1, Class 2, and Class 3 remote-control and signaling circuits, and by fire alarm circuits, shall be grounded where system grounding is required by Part II or Part VIII of this article."

    Now the key to your answer (metal raceways and enclosures not required to be grounded (actually bonded) is contained in Part II of Article 250, sepcifically 250.20(A).

    Reply from: William Wilson   
    Thats very interesting, thanks. Looks like from a life-safety perspective, indoor pathway for telecomm really doessn't have to be bonded if all it contains is voice/data cable. We would still want to see bonding/grounding for EMI issues, though.

    Thanks again, William Wilson

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