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Grounding vs Bonding: 250.66 Grounding Electrode Conductor—Size

February 10, 2006  

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PART III. GROUNDING ELECTRODE SYSTEM AND GROUNDING ELECTRODE CONDUCTOR  

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250.66 Grounding Electrode Conductor—Size.

Except for a ground rod electrode [250.66(A)], a concrete-encased electrode [250.66(B)], or a ground ring electrode [250.66(C)], the grounding electrode conductor must be sized based on the largest service-entrance conductor or equivalent area for parallel conductors in accordance with Table 250.66

Question: What size grounding electrode conductor is required for a 1,200A service that is supplied with three parallel sets of 600 kcmil conductors per phase? Figure 250–116

(a) 1 AWG     (b) 1/0 AWG    (c) 2/0 AWG     (d) 3/0 AWG

Answer: (d) 3/0 AWG

The equivalent area of three parallel 600 kcmil conductors is 1,800 kcmil per phase [Table 250.66].

FPN: Because the grounded neutral service conductor is required to serve as the low-impedance ground-fault current path back to the source, it must be sized no smaller than that shown in Table 250.66 [250.24(C)(1)]. Of course, it must be sized to carry the maximum unbalanced load as calculated by 220.61.
(A) Ground Rod. Where the grounding electrode conductor is connected to a ground rod, that portion of the grounding electrode conductor that is the sole connection to the ground rod isn’t required to be larger than 6 AWG copper. Figure 250–117

Author’s Comment: See 250.52(A)(5) for the installation requirements of a ground rod electrode.
(B) Concrete-Encased Grounding Electrode (Ufer Ground). Where the grounding electrode conductor is connected to a concrete-encased electrode, that portion of the grounding electrode conductor that is the sole connection to the concrete-encased electrode isn’t required to be larger than 4 AWG copper. Figure 250–118

Author’s Comment: See 250.52(A)(3) for the installation requirements of a concrete-encased electrode.
(C) Ground Ring. Where the grounding electrode conductor is connected to a ground ring, that portion of the conductor that is the sole connection to the ground ring isn’t required to be larger than the conductor used for the ground ring.

Author’s Comment: A ground ring encircling the building or structure in direct contact with earth must consist of not less than 20 ft of bare copper conductor not smaller than 2 AWG [250.52(A)(4)].



Figure 250–116
(Click on image to enlarge)


Figure 250–117
(Click on image to enlarge)


Figure 250–118
(Click on image to enlarge)

   

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Comments

  • Article 250.50 Grounding Electrode System. All grounding electrodes as described in 250.52 (A)(1)through(A)(6)that are present at each building or structure served shall be bonded together to form the grounding electrode system. Does this mean that a UFER Ground will be required whenever there is rebar 1/2" or larger in a foundation? Is it still present after the concrete has been poured? What is the best way to meet the requirements on single family homes?

    David B. Sheppard
    Reply to this comment

    • Reply from: Mike Holt   
    You need to get there 'before' they pour concrete.
    Reply to Mike Holt

    Reply from: Hung Ton   
    We have an network equipment with 660A, what size of ground-rod should be used, 6AWG? If it is OK to use 6AWG, can it be grounded into smaller size (12AWG) by using parallel?

    Best Regard, Hung Ton

    Home email: hung.ton@embarqmail.com

    Note: Pleae, don't post it to this newsletter. Send it by email.
    Reply to Hung Ton

  • My viewpoint of a Ufer ground is that for a service over 200 amps the length of metal that is in the footer should be increased to more than 20 feet and the wire size should be increased as well.

    So far I have inly installed 1 electrical service that used a Ufer ground because most of the work that I do does NOT involve residential new construction. In this case I was building a 400 amp residential sevice using two 200 amp service switches. To avoid using crimpers I used two #4 copper wires each 24 feet long in the footer running in opposite directions. This way I could bond the 2 grounding electrode conductors together using a clamshell terminal.

    Mike Cole, mc5w at earthlink dot net

    Michael R. Cole
    Reply to this comment

    • Reply from: Mike Holt   
    Michael, please don't take my response to be rude, but do you have anything to support your theory or do you just think it's a good idea?
    Reply to Mike Holt

  • I support the idea that a larger service size will have a lower supply impedance, therefore a higher available ground fault, however I doubt if there's anything official to back up increasing the GEC beyond 4AWG. The reasoning behind the 4AWG size is that the impedance of the UFER ground alone is high enough that the division of ground fault current between the UFER path and the neutral path is such that 4AWG is adequate. If the electrode length and GEC size are increased, thereby lowering the path's impedance, more of the fault will follow the UFER path. I wouldn't want that. I do see an advantage of more UFER capacity in the event that the neutral path is interrupted; then there'd be safety problem. To guard against that the size would probably need to be much greater.

    james wilson
    Reply to this comment

    • Reply from: Mike Holt   
    I've done the math, the impedance of the Ufer ground does not have any impact on the fault current to the source and it has no effective impact if the service neutral is open. So to close this topic, the purpose of the Grounding Electrode System is not to assist in clearing a ground, it's purpose is to ground metal parts to the earth to reduce transient overvoltage... but this is another topic.
    Reply to Mike Holt

    Reply from: James Wilson   
    The UFER ground is in parallel with the grounded conductor back to the source. In case of a ground fault a portion of that fault, although a small amount, will flow thru the UFER electrode back to the grounded source. If the UFER ground is increased in length, say from 20ft to 40ft, the resistance of the earth connection is reduced by 26%. That's using a formula from IEEE Std 142 "The Green Book" on grounding. This reduction would result in a slight increase in the return current thru the UFER. As I stated I see no advantage in increasing the capacity of the UFER path. There is nothing in the previous comments that implied that the grounding electrode, be it a UFER or any other approved type, is used to assist in clearing a ground fault. The NEC doesn't require that it be sized to guard against a neutral being open, which, if that occurred would be influenced by a lower UFER resistance.
    Reply to James Wilson

  • For a 1600 amp feed, you need to provide 3/0 grounding conductor. In the multi-conductor cables, stnadrad 500kcmil cables have only #2 grounding conductor. Do you need to provide a separate 3/0 grounding conductor run along the multi-conductor cables to satisfy article 250-66

    50HZ60
    Reply to this comment

    • Reply from: Mike Holt   
    The article was about the Grounding Electrode Conductor to the grounding electrode. This we use 250.66 for sizing. You are asking about the Equipment Grounding Conductor, which is another topic. The Equipment Grounding Conductor is sized in accordance with Table 250.122.
    Reply to Mike Holt

  • what is a U.L. listed grounding tool? isn't a crimper under a Klein pliers a U.L. approved tool?

    tom treacy
    Reply to this comment

    • Reply from: Mike Holt   
    I don't understand your question.
    Reply to Mike Holt

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