250.24(B)(1)

[ryan_618]

I plan on sending the replies to this thread to a couple of engineers in my area that I am in disagreement with, so plese respond appropriatley, thanks.

240.24(B) states that a grounded conductor must be ran from the supply to the service equipment if the utility company provides a grounded(nuetral) conductor to your premises. 250.24(B)(1) tells us how to size such conductor, which is in accordance with 250.66.

It is my contention that if the service entrance conductors are 500Kcmil, the grounded conductor is to be a 1/0 AWG, in accordance with the table. An engineer's rebuttal to this (by the way, I respect the man's opinions, so don't think otherwise) is that going by the text of section 250.66, if the grounding electrode system consists of a ground ring, ground rod and concrete encased electrode, 2 AWG would be the largest required conductor, citing 250.66(A)(B)and (C). I disagree with this logic becasue 250.24(B)(1) directs you to the table 250.66, and not the text of section 250.66.

The scenario is this: A 400 amp service is installed, and the service is all motor loads (line to line). 500Kcmil phase conductors have been pulled from the CT can to the service equipment, along with a 2 AWG grounded conductor. I asked for a 1/0, citing the above section. The biggest problem that I have is that a 2 AWG condcutor has twice the impedence value of a 1/0 AWG condcutor. Now, on the supply side of the service, I would consider the impedance value of the grounded conductor to be even more important than on the load side of the service, due to it having a greater task in initiating an overcurrent protective device.


Please share your thoughts with me on this, if you have a minute. Thanks in advance.

 

[iwire]

Re: 250.24(B)(1)

Ryan IMO you are correct.

This conductor back to the transformer must be large enough to withstand all the fault current that the transformer can provide.

The reduced size conductors that are allowed for ground rings, rods and concrete encased electrodes are never going to have that level of current imposed on them, the connection back to the transformer through the dirt will provide to much resistance to allow that much ampacity on those grounding electrode conductors.

IMO you should require the installation of the 1/0 back to the transformer.

Good Luck let us know what happens.

[ May 07, 2004, 06:32 AM: Message edited by: iwire ]

 

[bphgravity]

Re: 250.24(B)(1)

This is taken directly from Mike Holt's newsletter on grounding and bonding and can be found on page 158 of his "Understandng the National Electrical Code."

(1) Minimum Size Grounded (neutral) Conductor. The grounded (neutral) conductor from the electric utility must be sized so that it can safely carry the maximum ground-fault current likely to be imposed on it [110.10] from where a ground-fault may occur in accordance with Table 250.66, based on the total area of the largest ungrounded (hot) conductor. In addition, the grounded (neutral) conductors must have the capacity to carry the maximum unbalanced neutral current in accordance with 220.22.

Question. What is the minimum size service grounded (neutral) conductor, if the service ungrounded (hot) conductors are 500 kcmil and the maximum unbalanced load is 100 A?
(a) 3 AWG (b) 2 AWG (c) 1 AWG (d) 1/0 AWG

Answer. (d) 1/0 AWG
Step 1. Ensure the grounded (neutral) conductor has the capacity to carry fault current.
1/0 AWG, Table 250.66 based on 500 kcmil
Step 2. Ensure the grounded (neutral) conductor has the to carry the maximum unbalanced neutral current of 100 A. 3 AWG has an ampacity of 100 A, Table 310.16
Note: 250.24B1 (routing and sizing) states that the neutral conductor is not allowed to be smaller than the grounding electrode conductor at your service.

 

[charlie b]

Re: 250.24(B)(1)

Ryan: Please help me understand the situation, for I do not often get involved in the utility side of the service equipment. Am I right in saying that you are discussion the grounded conductor that runs from the utility transformer to the service equipment? Or are you talking about the conductor that runs from the ground bus and neutral bus within the service equipment (i.e., these two busses having been bonded together) to the grounding electrode that is required for the service equipment?

If you are talking about the first item, I would not give that conductor the name ?grounding electrode conductor.? Therefore, the text of 250.66 (A) through (C) would not apply. The article that does apply to this conductor is 250.24(B)(1), as you mentioned. That takes you to the table in 250.66 merely for the sake of convenience, so that the NEC writers do not have to place a duplicate of that table into article 250.24. Essentially, I am agreeing with you.

If you are talking about the second item, that conductor is a ?grounding electrode conductor.? To learn about sizing that conductor, you start with 250.66, and therefore the text of 250.66 (A) through (C) does apply. However, I must be missing something here as well. The largest conductor called for in these three sub-paragraphs is #4 copper, not #2. And I am sure you are not discussing aluminum, since you talked about 500 MCM for a 400 amp service. Could you clear up this mystery for me?

 

[ryan_618]

Re: 250.24(B)(1)

Hi Charlie, and thanks for your reply. Situation #1 is the scenario. I probably made this thread more confucing than it need be by saying 2 AWG. That is the condcutor size they installed, but you are right about the 4 AWG if section 250.66(A)(B)(C) applied.

 

[eprice]

Re: 250.24(B)(1)

Ryan,

I agree with your interpretation for two reasons.

1) As you say, 250.24(B)(1) leads to table 250.66, not Section 250.66.

2) 250.66(A), (B), and (C) are exceptions to the general rule that apply to GECs, not Grounded Conductors, and I can not see how the type of grounding electrodes on a system will affect the current handling capacity needed for the grounded conductor.

 

[a.wayne3@verizon.net]

Re: 250.24(B)(1)

This has been a previous thread.what it comes down to is simple. the size of the EGC has to be sized to the maximun sized conductor that can be installed for that amp rating,so 250.66 takes control.500=1/0.