#2 or #6

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cjcrawfo

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All,

Tell me what you all think is correct for the given situation.

200amp Service 3-3/0 and a #6 ground from meter to panel. Should that not be a #2 ground as according to table 250.##?? (don't remember what the number is) referring to service grounds. All wire is copper. There is a never ending argument. Please include the proper table # for me too. Thanks in advance all!

Chris Crawford
Oak Ridge, TN

[ July 12, 2004, 03:39 PM: Message edited by: cjcrawfo ]
 
Re: #2 or #6

Gronding electrode conductors are sized per Table 250.66. Equipment grounding conductors are sized per Table 250.122

I can't tell by your description exactly what one you refering to. 250.66 requires a #4 for a 3/0 service however, it can be smaller for a ground rod connection. 250.122 allows a #6 based on the 200A OCD.

What exactly is this "ground" wire. Is it and EGC or GEC? You state meter to the panel. Do you mean from the meter to main disconnect, or to a panelboard after the service?
 
Re: #2 or #6

Chris I would like to answer but I do not understand the question.

200amp Service 3-3/0 and a #6 ground from meter to panel.
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Is there a service disconnect at the meter?

If there is a service disconnect at the meter then the wires between the service disconnect and the panel are feeders. In this case the equipment grounding conductor is sized by Table 250.122. That table tells us the EGC for a 200 amp OCPD is 6 copper.

If there is not service disconnect at the meter there should not be a separate 'ground' between the meter and the panel.
 
Re: #2 or #6

Sorry about the confusion Bob. There is a disconnect between the meter and panel. I finally got a hard copy of the NEC and now see how they are able to use the #6. It just seemed to be very small to me.

Bryan thank you for steering me in the right direction.

Thanks again gentlemen,

Chris Crawford
Oak Ridge, TN
 
Re: #2 or #6

No problem at all. :)

With the added info you gave it is a feeder and as such Table 250.122 applies.

You can always exceed the requirements but if the length is not very long the 6 AWG will be more than enough to trip the breaker if a ground fault happens.

Heres where it gets strange, change the 200 amp breaker to a 100 amp breaker but leave the 3/0 in place.

Now 250.122(B) would require a larger EGC. :p

Bob

[ July 12, 2004, 04:16 PM: Message edited by: iwire ]
 
Re: #2 or #6

Bob,

Not to drag this on but what is too long. 100 ft rule??
Thanks

Chris

[ July 12, 2004, 04:15 PM: Message edited by: cjcrawfo ]
 
Re: #2 or #6

There is no rule for NEC. If you were to increase the size of the feeder you would be required to increase the EGC.
 
Re: #2 or #6

I did some editing so take a second look.

There is no NEC rule that tells us when it is 'to long'. :confused:

However, when the ungrounded conductors are up sized for any reason, usually for voltage drop, 250.122(B) requires the EGC to be increased by the same proportion.

Increasing the wire size for voltage drop is not required it is strictly voluntary but once you do the EGC must be increased also.
 
Re: #2 or #6

Got ya!! Thanks again. I know there is no rule in the NEC but in the past I have heard that the wire size is increased one size for every 100 ft for a 200 amp service.
 
Re: #2 or #6

The difficult part is that the code doesn't tell you. 250.4(A)(5) tells us that we must have an "effective ground fault current path", but the NEC doesn't give you actual values here, it gives you a performance based concept. Notice also the note to table 250.122 that tells you that the table values might not satisfy 250.4(A)(5).
 
Re: #2 or #6

Bob brings up an interesting point. What if I don't increase the conductor sizes, but only reduce the OCD. For example, as Bob points out, lets say I have a 200A load that is reduced to a 100A load because equipment is removed or taken out of service. I then remove the 200A OCD and replace it with a 100A OCD. I have not increased my conductors. Does the section still apply?
 
Re: #2 or #6

Not IMO. when you reduced the OCPD you did not decrease the conductor size, therefore they are oversized. Note, I did not do any calculations, but shot from the lip.
 
Re: #2 or #6

Are we getting a little carried away with the "interpretation" of 250.122(B)? If we increased the EGC everytime we reduced the OCP, then mathmatically we could wind up with a branch circuit or feeder EGC which is larger than the Main Bonding Jumper in the service. Was this the intention of Code-Making Panel #5 when they wrote 250.122? If we use Table 250.66 and Table 310.16 to properly size conductors why did the NFPA give us an open-ended Table 250.122? Since EGCs should really be based on available fault current, then perhaps a fault current analysis should be done everytime we use Table 250.122?????

[ July 12, 2004, 05:38 PM: Message edited by: taylorp ]
 
Re: #2 or #6

In my opinion 250.122(B), as currently written, applies anytime you use an ungrounded conductor that is larger than what would be required based on Table 310.16 and the rating of the OCPD. This section needs some work, because in many cases a smaller OCPD requires a larger EGC, than the larger OCPD requires. In the previous cycle this section only applied when you increased the ungrounded conductor size to correct for voltage drop, however that wording became an enforcement issue as the inspector has no way of proving why an ovesized conductor was used. The change for 2002 was an atempt to make this section more enforcable, but it still needs some work.
Don
 
Re: #2 or #6

When I did the calculations for the 200 amp OCP to a 100 amp OCP change, leaving the 3/0s I come out needing a 2 AWG EGC, 29 cmils away from a 3 AWG. ;)

Bob

[ July 12, 2004, 06:22 PM: Message edited by: iwire ]
 
Re: #2 or #6

Originally posted by bphgravity: I have a 200A load that is reduced to a 100A load because equipment is removed or taken out of service. I then remove the 200A OCD and replace it with a 100A OCD.
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I do not understand why you would want to do that. The OCD is there to protect the conductors. It had been sized for 200 amps worth of conductors, and the conductors have not changed. Also, you did not say that you replaced the sub-panel, but just took load off the sub-panel. So why would you spend any extra money to replace the OCD?
I have not increased my conductors. Does the section still apply?
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I think the section does apply, but it does not require you to change anything. That section tells us to increase the EGC commensurate with the increase in size for the phase conductors. In this scenario, the EGC was already sized to match the 200 amp conductors. Putting in a smaller OCD does not alter the conductors. True, they are now oversized for a 100 amp load. But the EGC is sized for a 200 amp load (or more precisely for a 200 amp OCD).
 
Re: #2 or #6

I have been searching on the net for some inverse time curves for various breakers, but I haven't had much luck. When you read 250.122(B) and look at the table, it basically says that no matter what, for a 15, 20 or 30 amp OCPD, a full sized EGC must be used. I am wondering if perhaps this is becasuse these small breakers are slower responding on their inverse time characteristics? I e-mailed a freind of mine with Cutler-Hammer requesting some data, and am waiting for his reply. I'll post whatever I can find.
 
Re: #2 or #6

As long as I have been in this industry, there has been the unwritten rule of upsizing the phase conductors for every 100 feet of run. I would think that it is not always necessary, and there may be times when it is. The only way to find out is do the math. That is why you see this done, no one wants to do the work :D

Pierre
 
Re: #2 or #6

Originally posted by iwire:
When I did the calculations for the 200 amp OCP to a 100 amp OCP change, leaving the 3/0s I come out needing a 2 AWG EGC, 29 cmils away from a 3 AWG.
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Like I said I was shooting from the lip based on logic. If the EGC is sufficient to operate a 200-amp breaker, it is more than sufficient to operate a 100-amp breaker under the conditions stated. Although by the letter of the code, which in this case is illogical, you would have to upsize the EGC. Needs work
 
Re: #2 or #6

Originally posted by dereckbc:
Like I said I was shooting from the lip based on logic.
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I understood that and I agree it is not logical. :)

For me as an installer logic has to be put aside and I must meet the letter of the code. The reason I harp on this section is the fact that overlooking this section can be very costly when the inspector says fix it.

Originally posted by Charlie B:
I think the section does apply, but it does not require you to change anything. That section tells us to increase the EGC commensurate with the increase in size for the phase conductors. In this scenario, the EGC was already sized to match the 200 amp conductors. Putting in a smaller OCD does not alter the conductors. True, they are now oversized for a 100 amp load. But the EGC is sized for a 200 amp load (or more precisely for a 200 amp OCD).
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Charlie I follow your logic but I do not agree that leaving the 6 AWG EGC in this case would be allowed.

250.122(B) Increased in Size. Where ungrounded conductors are increased in size, equipment grounding conductors, where installed, shall be increased in size proportionately according to circular mil area of the ungrounded conductors.
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I do not see any room for interpretation here, it tells us "increased in size proportionately" not simply match the EGC to the max OCP allowed by the ungrounded conductors.

The site lighting we run ends up with the same size EGC as the ungrounded conductors. This many times means 6, 4, or 2 AWG EGCs on 20 or 30 amp breakers. :p

Unless they change the wording of 250.122(B) I do not see anyway around it.

Bob
 
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