T310.15(B)(6) or T310.16

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bphgravity

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Florida
I believe this has been covered before, however I can't seem to get the "search" feature to get very specific like it use to.

Lets say I have a 200A service on a single family dwelling. T310.15(B)(6) permits me to use 2/0 cu conductors for the service entrance and main power feeders. But lets say I use T310.16 instead and install 3/0 cu conductors. Does this represent an increase in the ungrounded conductor size and thus require an increase in the feeder EGC per 250.122(B)?
 

jwelectric

Senior Member
Location
North Carolina
Would the EGC of 250.122 be based on the overcurrent device protecting the feeders?
Would it matter which table you used to size the ungrounded conductors?
 

fc

Senior Member
Location
New Jersey
I would say no. My reason is table 310.15(B)(6) is allowing you to use 2/0 for your 200 amp service for single phase dwellings. Your not increasing your wire size for any other reason it's just because you want to use 3/0.
You still have a 200amp service and and 250.122 for you EGC would be # 6 AWG copper.
 

charlie b

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Lockport, IL
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No, for two reasons. First, if you pull a value from an applicable table, and use that value, then you are not "increasing" anything. Secondly, Table 310.15(B)(6) is worded in a permissive sense (conductors shall be permitted to be sized per this table). Nothing compels you to use that table.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
I'll third the no, and I agree with Charlie's reasoning. Table 310.15(B)(6) allows you to reduce the condutor size. If someone chooses to "Not reduce", that is not the same as "increase".

Steve
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
I'm not arguing against the no, but where would 250.122 apply?

If I fed a 60-amp load with #2 wire, would I still be able to use a #10 ground (per table 250.122)?

If not, what would I need to use and how did you come up with it?
 

George Stolz

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Location
Windsor, CO NEC: 2017
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Charlie's reasoning is spot on.
hardworkingstiff said:
If I fed a 60-amp load with #2 wire, would I still be able to use a #10 ground (per table 250.122)?

If not, what would I need to use and how did you come up with it?
No. See 250.122(B), and then wander back to Table 8 with me.

6 AWG = 26240 circular mils.
2 AWG = 66360 circular mils.
That's a 252.8% increase in size, which must be proportionally applied to the EGC.

10 AWG = 10380 circular mils.
10380 x 2.528 = 26250.6 circular mils.
4 AWG = 41740 circular mils. I reckon you could probably use a 6 AWG unless we were taking a test.

Somebody, check my thinking and math. :)
 

infinity

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New Jersey
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Back to the OP, I say no also. 250.122 would apply to the feeder based on the OCPD size. Service entrance conductors would have no EGC.
 

iwire

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Location
Massachusetts
I think we are oversimplifying the question. (Did I just say that? :shock: )

IMO Byran is asking a great question.

IMO 250.122(B) is vague when it comes to deciding what is a 'normal' size conductor for a given circuit size.

If I run a 100 amp feeder for a 100 amp load with THHN I might use 3 AWG copper.

If the specs asked for TW insulation I would have to run 1 AWG copper.

Did I increase the size of the circuit conductors as I could have used 3 AWG?

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.

Its a basic requirement no mention of why we increase the size of the circuit conductors only that we if we do we must increase the EGC.

I respectfully ask before snapping off a quick answer that you really think about what 250.122(B) actually says (and does not say) :?

To simply say that EGCs are only based on the OCPD is not true as 250.122(B) changes that basic rule.
 

bphgravity

Senior Member
Location
Florida
Thanks Bob! That was the point I was trying to make. The rule in 250.122(B) does not work correctly for small circuits but seems to make sense for larger circuits. Yet no appears it is not so clear even for larger circuits. I think the section is severely flawed. I think I understand the intent of the section, however it can be widely enforced. :(
 

jwelectric

Senior Member
Location
North Carolina
Bob
I think that you have made a good point and you have brought up a very good question.
I would like to ask this and use your question with a little twist.

I install a 100 amp circuit and use #3 THW conductors just because this is what I have on the truck. I also only have a #8 TW conductor to install for the Equipment Grounding Conductor.

Do I pass inspection?
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
I think you pass Mike. #3 is good for 100A, which requires a #8. I don't think the fact that one has a "H" and the other doesn't makes any difference. The ground wire shouldn't normally have any current flow, so it shouldn't be hot.

I do agree with Bob's point to some extent. I think 250.122B should say "increased in size above table 310.16. Would that make anyone happy - probably not :lol:

But I also think 250.122B is a whole lot better than the 2002 which said increased in size "due to voltage drop". Funny how many reasons someone can come up with to increase a wire size other than voltage drop.

Steve
 

Johnmcca

Senior Member
Bob,
There is no real 'normal' wire per se. We commonly use THHN so it seems normal, but your point about a spec'd insulation and the resultant change in circular mils to accomodate the amp load is comparing limes to oranges. They both have about the same ampacity and 250.122(B) is first concerned with the ampacity then the increase in conductor size for what ever reason.
Your example you put the wire size ahead of the ampacity, which for the service entrance would be correct under 250.66 but after the first OCPD the feeder, IMHO would then be governed by 250.122(B) and the wire size based on the insulation spec then corrected for vd or temp

John
 

charlie b

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Lockport, IL
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Retired Electrical Engineer
georgestolz said:
Somebody, check my thinking and math.
I concur with your math, and my only comment on your thinking is that I wouldn't have mentioned #6 as an option. I would have simply said that a #4 EGC is needed.
 

charlie b

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Location
Lockport, IL
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Retired Electrical Engineer
jwelectric said:
Do I pass inspection?
Mike, I don't understand the purpose of your example. Using #3 THW for phase conductors and #8 for the EGC is a perfect fit. It doesn't bring any of the "upsizing" issues into play. :?:
 

charlie b

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Lockport, IL
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Retired Electrical Engineer
iwire said:
If I run a 100 amp feeder for a 100 amp load with THHN I might use 3 AWG copper. If the specs asked for TW insulation I would have to run 1 AWG copper. Did I increase the size of the circuit conductors as I could have used 3 AWG?
You did not. A #1 TW is not an "increase" from a #3 THHN. The two are identical in character, as they are each the smallest conductor size that has sufficient ampacity for a 100 amp load.

iwire said:
IMO 250.122(B) is vague when it comes to deciding what is a 'normal' size conductor for a given circuit size.
I don't think it is vague. (I agreed with you on something recently, and I have to make up for that transgression. :lol: :lol: )

It is not a question of what is "normal." Nor is it a matter of choices (i.e., choosing to use THHN versus TW, or choosing to use Table 310.16 versus Table 310.15(B)(6)). IMO the process is clear:
  • Step 1: Determine the load.
  • Step 2: Determine which NEC article governs the ampacity of conductors for the planned installation.
  • Step 3: Determine from that article the minimum size for ungrounded conductors.
  • Step 4: Determine from Table 250.122 the minimum size for the EGC.
  • Step 5: If you select an ungrounded conductor size that is larger than the size determined from Step 3, then you need to "biggie size" the EGC
 

bphgravity

Senior Member
Location
Florida
Thanks for that Charlie. You have a great way of explaining things. Perhaps you could write a book annotating the NEC rules into steps like this one. That would be helpful. :D
 

chevyx92

Senior Member
Location
VA BCH, VA
Lets say I have a 200A service on a single family dwelling. T310.15(B)(6) permits me to use 2/0 cu conductors for the service entrance and main power feeders. But lets say I use T310.16 instead and install 3/0 cu conductors. Does this represent an increase in the ungrounded conductor size and thus require an increase in the feeder EGC per 250.122(B)?
From what I'm reading from the OP, he's talking about the service entrance or main power feeds as he says. That being the case then it doesn't matter if you use 310.15(B)6 or 310.16 , which ever wire size you choose, use MUST size your GEC per table 250.66.
 
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