Sizing Conductors

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wirebender

Senior Member
It's easier to go the other way...

39A ? 125% = 48.8A
39A ? 0.7 ? 0.91 = 61.2A, greater than 48.8A
#8 THHN table ampacity is 55A < 61.2A... too small
#6 THHN table ampacity is 75A > 61.2A... smallest permitted.

"so simple like the jitterbug it plumb evaded me." Jimmy Buffet

Thanks, this helps a lot.
 

mlnk

Senior Member
210.19 .... "the minimum
branch-circuit conductor size, before the application of any
adjustment or correction factors, shall have an allowable ampacity
not less than the noncontinuous load plus 125 percent
of the continuous load."
I think this means we multiply 39 amps continuous load by 125% and get 48.8 amps...this is the load that we will now be working with. We do not go back to 39 amps.

If you derate the wire based on 90 degree ratings:
#8 = 55*.91*.7 = 35 amps
#6 = 75*.91*.7 = 47.8 amps
#4 = 95*.91*.7 = 60.5 amps
Therefore, you need to use #4 for the 48.8 amp load.
Then you check the 75 degree ampacity for terminal connections: OK @ 50 amps for #8 or larger wire

If you go at this from the other direction and use the derating to increase the load(by dividing by the derating)-and then go back to 39 amps, you have 39 amps divided by .91 and divided by .7 = 61.2 amps The result you get is: use #6 because it is rated 75 amps @ the 75 degree ampacity for terminal connections. (If you use 48.8 amps; you end up with #4)

High-lighted in red or not, it depends on your interpretation of 210.19 whether to use #4 or #6
 
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Smart $

Esteemed Member
Location
Ohio
...

High-lighted in red or not, it depends on your interpretation of 210.19 whether to use #4 or #6
After having posted the correct method to determine the minimum size conductor, and assuming you have read the entire thread, I guess all there is left to say is, I respectfully disagree with your approach. The consolation for me is that I know you will not be under sizing your conductors :p
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
210.19 .... "the minimum
branch-circuit conductor size, before the application of any
adjustment or correction factors, shall have an allowable ampacity
not less than the noncontinuous load plus 125 percent
of the continuous load."
I think this means we multiply 39 amps continuous load by 125% and get 48.8 amps...this is the load that we will now be working with. We do not go back to 39 amps.

After having posted the correct method to determine the minimum size conductor, and assuming you have read the entire thread, I guess all there is left to say is, I respectfully disagree with your approach.

I, like Smart$, will also have to respectfully disagree with your approach.

You do not fully post 210.19(A)(1)...

"Branch circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous load or any combination of continuous and non-continuous loads, the minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the non-continuous load plus 125% of the continuous load."

The first sentence says that the conductors must have an ampacity not less than the maximum load to be served. The maximum load is 39A. The circuit will never see more than 39A load.

The second sentence tells you minimum allowable conductors size. 39*1.25=48.75. The minimum conductor size is #8AWG (which has an ampacity of 50.)

#6 THHN has a starting ampacity of 75. Its adjusted ampacity is 75*0.91*0.7=47.78.

The adjusted ampacity of 47.78 is large enough to serve the load (39A) and the #6 is not smaller than the minimum conductor size, so #6 THHN would be acceptable.

Example D3(a) in Annex D gives a good example of this.
 

mlnk

Senior Member
I am not alone in my thinking: "Ungrounded conductors for branch circuits, feeders, and services are sized at a minimum of 125% of the continuous load before applying any adjustment factor [ 210.19(A)(1), 215.2(A)(1), and 230.42(A)]" .........Mike Holt I have had inspectors read it this way also - and they can force you toput in bigger wire no matter how much of an expert you are in English semantics.
 

Smart $

Esteemed Member
Location
Ohio
I am not alone in my thinking: "Ungrounded conductors for branch circuits, feeders, and services are sized at a minimum of 125% of the continuous load before applying any adjustment factor [ 210.19(A)(1), 215.2(A)(1), and 230.42(A)]" .........Mike Holt I have had inspectors read it this way also - and they can force you toput in bigger wire no matter how much of an expert you are in English semantics.
That's not the problem with your approach.

The problem is: you use the extra 25% continuous load factoring in the application of adjustment and correction factors, and also in determining terminal temperature limitations.

I understand you are not the only one.
 
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kenaslan

Senior Member
Location
Billings MT
I got to say #8 THHN CU. The only thing missing (not part of the problem) is voltage drop.

But it is too early in the morning. I have only had 1 pot of coffee so far.


If its not broke, your not trying.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Well I would really like to hear Charles Millers interpretation but I don't know if that is going to happen. BTW, I am in agreement with Smart and others
 
The correct answer is 6 AWG. Before I cover the answer, I want to talk about the wording of the Code section.
Here is the wording from 210.19(A)(1):
(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

There is no question about the first sentence; it is very understandable. The problem is with the second sentence. This sentence is very confusing, especially the part that says "before the application of any adjustment or correction factors". It seems to be saying to multiply continuous loads by 125 percent and then apply the additional correction factors for ambient temperature and the adjustment factors for more than three current-carrying conductors. But this is not what it is saying. This is not just my opinion. david luchini was exactly right when he said "Example D3(a) in Annex D gives a good example of this." In this example, the calculations are separate calculations. Also, as an instructor teaching NFPA seminars, this is how we teach it. I spend quite a lot of time explaining this section, because what it sounds like is not what it is saying.

The continuous load calculation is separate. The continuous load aspect can be calculated with 110.14(C), especially when determining the minimum size conductor at the overcurrrent device termination. The correction factor and adjustment factor (sometimes referred to as the "conditions of use") is another calculation. Perform this calculation with the actual load, not with the extra 25 percent for continuous loads.

Besides the two separate calculations, we must also consider the size of the overcurrent protection. I thought it was great when david luchini (post #13) changed the number of conductors to make a point. I thought this was great because the change that david made is exactly one of my examples that will be in the September issue.

To satisfy 110.14(C) and 210.19(A)(1), multiply the load by 125 percent (39 ? 125% = 48.75 = 49). Select an 8 AWG conductor out of the 75?C column. Now see if that 8 AWG can carry 39 ampere after applying the correction and adjustment factors. Because the conductors are THHN, it is permitted to use the 90?C column (55 ? 0.91 ? 0.70 = 35). Since the load is 39 amperes, an 8 AWG is not permitted. Perform the same calculation using a 6 AWG conductor (75 ? 0.91 ? 0.70 = 47.775 = 48). Although the continuous load is 49 amperes, the conductors are only required to have a rating of the actual load of 39 amperes. Therefore, 6 AWG THHN copper conductors are permitted to supply this branch circuit
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I spend quite a lot of time explaining this section, because what it sounds like is not what it is saying.

Sounds to me like a good reason for re-wording the code.

Should have been:

"Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size shall have an allowable ampacity (before the application of any adjustment or correction factors) not less than the noncontinuous load plus 125 percent of the continuous load. "
 
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I mentioned earlier that I do not like the wording in 210.19(A)(1). It is obvious by the replies to this one question that this section is difficult to understand. Although I have a great job writing and teaching the NEC, I don't like the Code being difficult to understand. Because of this, I submitted a proposal to change the wording of this section. For this section, my proposal was accepted in principal and I like the change that was made. Here is the draft copy of this section:

210.19(A)(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Conductors shall be sized to carry not less than the larger of (a) or (b). [ROP 2?131]
(a) Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load. [ROP 2?131]
(b) The minimum branch-circuit conductor size shall have an allowable ampacity not less than the maximum load to be served after the application of any adjustment or correction factors. [ROP 2?131]

I think the new wording is easy to understand and is what the Code is trying to say. Although it was accepted at the proposal stage, it may be rejected in the comment stage. This is what the Technical Correlating Committee (TCC) said about the proposal:

TCC Action: It was the action of the Correlating Committee that this proposal be reconsidered and correlated with the actions taken on Proposals 2-201 and 2-202. The Correlating Committee directs that the panel clarify whether the 125 percent is applied before or after the correction factors for consistency. The Correlating Committee also directs that this proposal be submitted to Code-Making Panel 6 for comment. This action will be considered as a public comment.

I submitted a similar proposals for feeder conductors in 215.2(A)(1) and, like the one for branch-circuit conductors, it was accepted in principal. I also submitted one for service conductors in 230.42 and it was rejected. Here is the response from the committee:

Panel Statement: The current text is clear that conditions of use must be considered in addition to continuous loading.

Even their statement is not clear. Do they mean to calculate the conditions of use along with the continuous loads or perform the calculations separate?

I don't care if they accept my wording; I just want the references to be clear and understandable. I would like to know other thoughts on this.
 
Ummm... where does 110.14(C) say to do that???

To satisfy 110.14(C) and 210.19(A)(1),

Section 110.14(C) is only for terminations. Section 210.19(A)(1) provides the wording for continuous loads. After multiplying the load by 125 percent, select a conductor from the column as determined by 110.14(C). This is also the way the calculation is performed in Example D3(a). You could select a conductor from the 90?C column (using this example), but then you will have to turn around and select the conductor from the 75?C column when determining the minimum size conductor at the overcurrrent device termination.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Charles, I like the change you made to 210.19 as that was close to what I was saying earlier but this captures the meaning as I understand it.
 
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