310.12 Dwelling Services and Feeders, P.I. Draft for 2023

Devin Hanes

Member
Location
United States
attached is the 1956 proposal, right side middle of third page.

“Also in connection with this table, the utility people pointed out that aluminum wire for services was being modified from the 84 per cent value previously given in the Code, that is 84 per cent of the copper carrying capacity, and while they were in agreement with that because the data was the result of a study by the sub-committee, they did want to point out that in a 3-wire single phase, with only one of the wires loaded, it would not be necessary to go to the higher values.
It was proposed that a note be made for 2 to 4/0, the allowable capacity, it would be one, the 84 percent value, and these values were ironed out by the Correlating Committee to values normally used by the Code so that for #2, 100 amperes; #1, 110 amperes; for #0, 120. That, too, was adopted by the Correlating Committee. There were minor editorial changes but those are the changes in substance."



also attached is 1953 NEC pages, page 7 note one shows how all aluminum was sized originally, 84%

also attached are 1956 NEC pages, page 4 table note is where this allowance was originally and it was for all single phase services not just residential


I appreciate the input and information
 

Attachments

Devin Hanes

Member
Location
United States
See 1993 cmp statement on rejection of allowing 120/208. Attached is a copy, here is a sample. you may also find Log#2616 substantiation interesting, it goes onto the second page.

middle of left side of first page attached
1993 ROP page 229 Log#2449
6-103-(Article 310 Notes to Ampacity Tables....
Proposal; allow 120/208 into this section (paraphrasing)
Panel action; Reject
Panel Statement on rejection;
In a three wire circuit consisting of two phase conductors and a neutral , the neutral carries approximately the same current as the phase conductors. The heat dissipation will not be the same on these circuits as on the three wire 120/240 volt single phase circuits. The ampacities of these tables are based only on these 3 wire single phase 120/240 volt circuits. See Note 10(b) of the Notes to the Ampacity Tables.
Vote On Panels Action; Unanimously Affmirmative , the panel unanimously rejected the proposal to allow 120/208 added to the allowance.


 

Attachments

Devin Hanes

Member
Location
United States
The 2017 vote was not unanimous, two panel members agree with me,
here are their 2017 comments on their negative votes
Negative with Comment
Huddleston, Jr., Robert L.
1. Section 220.61(C)(1), which (4) references in the FR, states: Prohibited Reductions. There shall be no reduction of the neutral or grounded conductor capacity applied to the amount in 220.61(C)(1) or portion of the amount in (C)(2) from that determined by the basic calculation: (1) any portion of the 3-wire circuit consisting of 2 ungrounded conductors and the neutral conductor of a 4-wire, wye-connected 3-phase system. 2. Section 310.15(B)(5)(b) states: In a 3-wire circuit consisting of two phase conductors and the neutral conductor of a 4-wire, 3-phase wye-connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors and shall be counted when applying the provisions of 310.15(B)(3)(a). 3. In addition, the ampacity adjustment for the grounded conductor allowed in single-phase 120/240 volt systems is based on the fact that when the two phase conductor currents are balanced, there is no current at all in the grounded conductor. If the two phase conductor currents are not identical, the grounded conductor only carries the imbalance, which is generally a very low amount of current. Because of this, the neutral is not really considered a current-carrying conductor and it does not produce heat in the raceway or cabling encasing the feeder. However, if the grounded conductor conducts significant current, as it would in a 3-wire 208/120 volt system, heat would be generated in this conductor. This should eliminate the reduction in sizing that FR 1504 allows.
Kent, Gerald W.
No technical substantiation was provide to allow for this change. In a 120/240v systems, heat is generated in effectively two conductors under full load. In a fully loaded 120/208v system, heat would be generated in all three conductors. Time honored tradition has shown the deductions work for 120/240v systems I believe due to the 'zero' effect of the neutral load that would not be present in 120/208v system.
 

Devin Hanes

Member
Location
United States
Yes it was recently added, but without evidence of a real world issue with the rule, it is my opinion, that the CMP will be unlikely to make a change. Part of the substantiation for the 2017 change was that this has been permitted by the Canadian Electrical Code without evidence of problems.
I appreciate the input, I will adjust the proposal to focus more on this, CEC did not have this table until 2015, before then they effectively just allowed a 1amp ampacity increase on #6 aluminum and 2/0 copper. part of their substantiation for the 2015 table was that the NEC allows it, which the NEC did not.

First attachment is when CEC added table in 2015

second attachment is a previous edition ampacity table, the #6 and 2/0 allowance where notes to the table, when you consider the next size up rule, they are effectively just giving you 1 amp more ampacity unless your actual load is above 50 or 175 amps respectively.

thanks again, I would appreciate it if you point out anything else you think I am overlooking.
 

Attachments

Devin Hanes

Member
Location
United States
Would anyone mind reviewing my latest revision before I submit it? I would appreciate any input or feedback good or bad. Thanks.

I've already posted in this thread the attachments referenced. The change is just removing 120/208 from the allowance. Below is the substantiation.


The additional allowance made in the 2017 edition for 120/208 3 wire services encourages a fire hazard. The reasoning behind the original allowance for the entire section appears to have been lost by some of the code making panel, which appears why 120/208 was added to the allowance. Also, the 2017 submitter gave bad information saying the CEC allows this, it does not, see CEC ampacity table and specifically it's note (attached), the CEC only increases 2/0cu and #6al by 1 amp for residential applications, it's only increasing by 1 amp because you have to consider the next-size-up rule, with 1 more amp you no longer land on a standard size ocpd so you can go the next size.

It appears the panel believes this allowance is due to the diversity of the loads served, when it is actually because of load diversity of the conductors. The three-wire single phase 120/240 under normal operation will only ever be able to see the equivalent of two fully loaded conductors, while a single phase service from a three-phase system will allow all three conductors to be fully loaded because they are vectorially 120 degrees apart rather than 180 like the Edison/Split-Phase. Table 310.15 is based on 3 conductors fully loaded in a conductor group, the Edison 3 wire is limited by physics to only ever see the equivalent of two fully loaded conductors, while it also benefits from the heat sink properties of the third conductor. The Submitter for the 17' change even shows these conductor current differences in his attachment. Thermal contribution of conductors is base on I2R, you are allowing three heaters in an assembly instead of two, all heaters all being of the same individual BTU output, while also eliminating the heat sink properties of the third conductor.

If you don't want to double check my temperature rise calculations you can look at table 400.5(A)(2) 60degree c row, column E is 2 conductors while column F is 3 conductors, they have the same ampacities +/-1or2 amperes used in 310.15(3wire) and 310.12(2wire) respectively.

Please see historical genesis of this allowance below, to illustrate the reasoning behind the allowance of the section. Attached are originals.

Before 1956 there was no special allowance for the service conductors.
1956
NFPA Proceedings of the sixth annual meeting June 4-8, 1956
Page 52 and 53
Report of Electrical Correlating committee(attached)

“Also in connection with this table, the utility people pointed out that aluminum wire for services was being modified from the 84 per cent value previously given in the Code, that is 84 per cent of the copper carrying capacity, and while they were in agreement with that because the data was the result of a study by the sub-committee, they did want to point out that in a 3-wire single phase, with only one of the wires loaded, it would not be necessary to go to the higher values.
It was proposed that a note be made for 2 to 4/0, the allowable capacity, it would be one, the 84 percent value, and these values were ironed out by the Correlating Committee to values normally used by the Code so that for #2, 100 amperes; #1, 110 amperes; for #0, 120. That, too, was adopted by the Correlating Committee. There were minor editorial changes but those are the changes in substance."


1956 NEC
Table 1A Page(attached)
shows in the note the allowance applied to all Edison feeders and services not just residential, it was later restricted to residential in 78'.

1956 IEEE (AIEE at the time) Paper "The Heating and Mechanical Effects of Installing Insulated Conductors in Steel Raceways" By Brandon, Kline, Geiges and Paradise. https://ieeexplore.ieee.org/document/4499473 (3 of 12 pages attached)
"Purpose
The purpose of the investigation was to
compile information, observations, and
test data which could be used for analyzing
the following:
1. The applicable, current-carrying capacity
reduction factors for more than nine
conductors.
2. The effect of load diversity in reference
to the current-carrying capacity reduction
factors and conductor-operating temperatures."

This entire article is available online from IEEE, I am sending a copy of the test findings chart that illustrates the use of the term "load diversity" as not meaning the actual load being served, but the number of conductors conducting. The chart is titled Table VIII. Diversity Tests, column titles are Wire Type, Wire Size AWG, No. of Wires, No. Conducting, Operating Temperature C, Amperes, and Volts. The first row of data is #6 AWG, 12 wires, 4 conducting, operating at 60C, 59.5Amperes and 3.5 Volts. This testing was used for our De-rating table.
 

suemarkp

Senior Member
Location
Kent, WA
Occupation
Engineer
You could also point to article 400 for cord ampacities where there are two columns -- one for 2 carrying conductors and one for three. What is the difference between the two columns -- about 81 to 86%.... Too bad that table stops at 2 AWG, but its the same idea.
 

Devin Hanes

Member
Location
United States
You could also point to article 400 for cord ampacities where there are two columns -- one for 2 carrying conductors and one for three. What is the difference between the two columns -- about 81 to 86%.... Too bad that table stops at 2 AWG, but its the same idea.
Thanks, that's what my third paragraph is talking about, turn one more page in your code book and you will see it actually goes up to 1,000Kcmil. The values in the dwelling table(310.12) are within 1 or 2 amps of the 2 wire 60c column(E) and 310.15(3-wire)table is within 1 or 2 amps of the 3 wire 60c column(F). I refereed to this in the third paragraph, but I will try to make it more clear.
I appreciate the feedback
 

mikeames

Senior Member
Location
Germantown MD
Occupation
Teacher - Electrician - 2017 NEC
In theory its 100% logical and I cant disagree with your request for change. From a reality standpoint I disagree for the same reasons other shave stated.
1. Highly unlikely and almost impossible to run a 3 phase residential service to 100% capacity with no neutral current for 20 plus min.
2. If that did happen the hazard of igniting wood frame would still be difficult. The lowest ignition point of wood I could find was 250 deg F or 121C. http://marioloureiro.net/ciencia/ignicao_vegt/wood_ign.pdf How much current will it take to get the service conductors that high.

In my opinion it was be hard to do on purpose no less under normal conditions. I think the cost of changes outweigh the safety increase, if there is any at all.
 
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