Conductor sizing

Status
Not open for further replies.
H

hhsting

Senior Member
Location
Glen bunie, md, us
Occupation
Junior plan reviewer
I have 2 sets of 600 kcmil service conductor into 480/277 three phase 800A main service panelboard P at Bldg A. The panelboard has 200A feeder breaker with 3/0 AWG phase conductors feeding separate Bldg B. NEC 2014 section 250.32 requires grounding electrode system at Bldg B.

My service entrance conductors are 2 sets of 600 kcmil and my feeder conductors are 3/0 AWG. Grounding electrode conductor are sized based on service entrance conductor but for Bldg B it’s confusing.

Question the grounding electrode conductors size at Bldg B should be based on 3/0 AWG or based on 2 sets of 600kcmil?


Sent from my iPhone using Tapatalk
 
Good question, 250.32(A) sends you to Part lll of Article 250 which sends you to 250.66. If you check those sections what do you come up with?
 
Table 250.66 Note 2
2. Where there are no service-entrance conductors, the grounding electrode conductor size shall be determined by the equivalent size of the largest service-entrance conductor required for the load to be served.
Click to expand...

I don't think you have to use the up-stream buildings service entrance conductors for sizing. I don't think it's worded as clear as it should be though. There are no service entrance conductors at the building but there are service entrance conductors in the system.

250.66 Size of Alternating-Current Grounding Electrode Conductor. The size of the grounding electrode conductor at the service, at each building or structure where supplied by a feeder(s) ....
Click to expand...
This and reading everything together makes me think you don't use the up-stream service. And the common sense test, what if you had an outbuilding with a 50 amp feeder? But this, 250.32 and the table note are not as clear to me on your type of situation as I would like, now that you questioned it.
 
Last edited:
I agree it's not very clear. Note 2 to T250.66 says this but who knows what that means? Maybe this would apply to a feeder since there are no SEC's but wouldn't it be based on the smallest SEC for the load served in this case 200 amps not the largest?

2. Where there are no service-entrance conductors, the grounding
electrode conductor size shall be determined by the equivalent size of
the largest service-entrance conductor required for the load to be
served.
Click to expand...
 
The rule should apply separately to each building or structure. Otherwise, a shed could require a 200a GEC for a 20a feeder.
 
LarryFine said:
The rule should apply separately to each building or structure. Otherwise, a shed could require a 200a GEC for a 20a feeder.
Click to expand...
I agree, in this case the GEC should be based on the feeder size of #3/0 but I can't seem to find that wording in the NEC. :unsure:
 
LarryFine said:
Read the second quote in post #4 again.
Click to expand...

Yes I did and I noted it in my first post but that 250.66 sends you Table 250.66 which makes no mention of feeders except to say in note #2 that:

Note 2. Where there are no service-entrance conductors, the grounding electrode conductor size shall be determined by the equivalent size of the largest service-entrance conductor required for the load to be served.
Click to expand...

So should we assume that the #3/0 feeder to the separate structure meets note #2?


250.66 Size of Alternating-Current Grounding Electrode
Conductor. The size of the grounding electrode conductor at
the service, at each building or structure where supplied by a
feeder(s) or branch circuit(s), or at a separately derived system
of a grounded or ungrounded ac system shall not be less than
given in Table 250.66, except as permitted in 250.66(A)
through (C).
Click to expand...
 
This, whittled down from your own quote:
The size of the grounding electrode conductor . . . at each building . . .
where supplied by a feeder . . . shall not be less than given in Table 250.66
Click to expand...


There is no reason for a building's GEC to be sized larger than that required for its feeder.
 
LarryFine said:
This, whittled down from your own quote:



There is no reason for a building's GEC to be sized larger than that required for its feeder.
Click to expand...

I completely agree and I would always say that the GEC for a separate structure is based on the conductors size used in the feeder. IMO the table should state that directly (for use with feeders) because as the OP has stated it can be confusing.
 
LarryFine said:
Read the second quote in post #4 again.
Click to expand...

I am confused, I interpret that as requiring the GEC at the remote building to be the SAME size as the service's, like Rob in post #9. You think it is the other way? 250.66 gives you a list of locations, service, remote building, SDS shall be sized per T250.66. It is sized based on the service conductor size. (or derived conductor size per note a) IMO note number 2 is not there to indicate that case of feeder, it is there to address services that do not have service entrance conductors. I dont know what the intent is here, but if it is that a separate building's GEC be sized based on the feeder, they should add "feeder" into note a.

This is interesting though. IF you had asked me before this thread came up, I would say its sized based on the feeder. This has never came up in my career as I cant recall ever having an electrode at a remote building other than just rods.
 
What do you think after reading it as I pared it down?

The size of the grounding electrode conductor . . . at each building . . .
where supplied by a feeder . . . shall not be less than given in Table 250.66 . . .

In my opinion, a "for that building" is implied by "at each building."
 
LarryFine said:
What do you think after reading it as I pared it down?

The size of the grounding electrode conductor . . . at each building . . .
where supplied by a feeder . . . shall not be less than given in Table 250.66 . . .

In my opinion, a "for that building" is implied by "at each building."
Click to expand...

Yes but you look at the table, it says to use SEC size. "Feeder" is absent from note "a". I see nothing there that says to take the feeder size and apply it to the table.
 
electrofelon said:
Yes but you look at the table, it says to use SEC size. "Feeder" is absent from note "a". I see nothing there that says to take the feeder size and apply it to the table.
Click to expand...
That was my interpretation as well.

Anyone want to take a stab at explaining Note 2?
Note 2. Where there are no service-entrance conductors, the grounding electrode conductor size shall be determined by the equivalent size of the largest service-entrance conductor required for the load to be served.
Click to expand...
 
infinity said:
That was my interpretation as well.

Anyone want to take a stab at explaining Note 2?
Click to expand...

I believe you would have no SEC where the underground service conductors land on an exterior service disconnect. There could not be a self contained meter so it would be metered at the spades of the transformer. So basically, any larger underground service with exterior service equipment. So then see what conductor would be required for the load calc and find your GEC size based on T250.66. I dont know why you wouldnt just use the size of the "underground service conductor". Perhaps they are trying to cover a situation where the service point is at the terminals of the service equipment (would that ever happen?) where there would then be no SEC and no underground service conductors?
 
I find the part of that sentence that states the "equivalent size of the largest service-entrance conductor required for the load to be served" to be confusing. Why the largest, wouldn't calculating using the smallest conductor required be sufficient? And how do you calculate the largest SEC required?
 
infinity said:
I find the part of that sentence that states the "equivalent size of the largest service-entrance conductor required for the load to be served" to be confusing. Why the largest, wouldn't calculating using the smallest conductor required be sufficient? And how do you calculate the largest SEC required?
Click to expand...
Similar wording exists elsewhere in the code IIRC. I guess one phase could have larger OCPD and conductors, in theory. I know high leg Delta's would often have different sized service conductors.
 
Status
Not open for further replies.
Top