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

[Devin Hanes]

I would appreciate any thoughts on the following Public Input I am working on, Thank you.


310.12 Single-Phase Dwelling Services and Feeders
Change
I am asking to remove the allowance for 120/208 Three-phase Services. And add some clarifying language to the table title consistent with the other tables. (I will have the proposed revised language in my next post)

Substantiation
The allowance made in the 2017 edition for 120/208 3 wire services allows and promotes a fire hazard. The Edison three-wire single phase 120/240 under normal operation will only ever be exposed to the equivalent temperature rise of two fully loaded conductors (two heaters), while a neutraled 3-wire service from a three-phase system will allow all three conductors(heaters) to be fully loaded, exposing the conductor group to the temperature rise of three conductors (50% more than the Edison 3-wire). Our 310.16 table is based on 3 conductors fully loaded in a conductor group, the Edison 3 wire effectively only has 2 loaded, and also benefits from the heat sink properties of the effectively third conductor not carrying current, further lowering the operating temperature of the conductor group.

The reasoning behind the original allowance for the entire section appears to have been lost by some of the code making panel an industry over the years, which is probably why 120/208 was mistakenly added to the original allowance. 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(2 heaters instead of 3), I will provide clear historical proof.

Please see historical highlights of this allowance below and attached (under additional proposed changes), to illustrate the reasoning behind the allowance of the section (expanded versions attached). (I will post attachments in future posts)

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
“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."


Historical panel comments on not using 3 phase are attached under (additional proposed changes)

1956 NEC
Table 1A, Page
(attached) you will note the allowance was for all single phase 3-wire edison services not just residential or even 120/240 (see the table notes for the allowance)

1956 AIEE (Became IEEE later) 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
"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.

The Submitter for the 17' change even shows these conductor current differences(heaters) in his attachment (attached to this input). You are allowing three heaters in an assembly instead of two, all heaters being of the same BTU output, while also eliminating the heat sink properties of the equivalence of a 0 current third conductor. The heating difference is due to three phase being 120 degrees out of phase while the edison 3 wire is 180 degrees out of phase resulting in neutral only carrying current if there is imbalance, the three phase neutral will carry 200 amps when both of the 208 legs are carrying 200 amps, it doesn't "cancel". The I^2R power loss equation shows you have three heaters compared to two equivalent, I^2R, (Current Squared X the Resistance of the Wire).

Also, If the conductors are large enough for the residential load, then the normal over-current protection device is also large enough, there is only bad reasoning to allow a larger OCPD on THE MOST SENSITIVE conductor group in the MOST VULNERABLE type of building construction. What I mean by that is service conductors only have OCPD, not upstream short circuit protection like other conductors, so if these are overloaded even just slightly just occasionally after a long period of time the result will be a fire, there's no safety left, it was eliminated with this allowance, you have no appropriate OCPD. And dwelling units are wood framed with a continual decline in fire resistance due to the materials used, houses burn much easier, faster and hotter when built out of the common materials used today compared to decades past.

If the conductors are large enough for the load then the normal OCPD is as well. I am sending a copy of common OCPD trip curves, the Square D 175Amp would take a minimum of 20.83 minutes to trip at 175 amps all the way up to 2.78 hours. At 218 amps its about 5.83 minutes to 2.78 hours. Why would a 175 amp residential service ever need to operate at 175 amps for longer than 20 minutes which is the minimum time it will take ocpd to open.

There’s well more than enough wiggle room in OCPD curves to not need this allowance, just use a smaller breaker, if the wire's big enough then the normal OCPD is as well. Also, the CEC allowance does not prove it to be safe, table 310.16 values are based on legitimate testing while the CEC allowance is not. The allowance does not mean the service will definitely result in fire it just unnecessarily removes protection, so the fact that we don't have reports of these services burning up everywhere is not a legitimate argument for fast-tracking this allowance past legitimate testing.

I will add the referenced attachments in my next posts, Thanks.

 

[infinity]

Are saying that the 3 wire feeder or service entrance conductors from a 208Y/120 volt system are unsafe because of the theoretical possibility that each conductor can carry the same amount of current simultaneously?

 

[don_resqcapt19]

Given that the actual dwelling unit load is typically less than 50% of the load as calculated in accordance with Article 220, I don't think there is any issue here.

 

[Devin Hanes]

Are saying that the 3 wire feeder or service entrance conductors from a 208Y/120 volt system are unsafe because of the theoretical possibility that each conductor can carry the same amount of current simultaneously?

I greatly appreciate your time and review, thank you.

I wouldn't word it the way you did, I'm not saying they are unsafe because all three wires carry the same current, i'm saying you can't overrate them like you are allowed to do with the edison 3-wire conductors. its a matter of temperature rise and dissipation of the conductor group, similar to how you have to de-rate when you have more conductors in a conduit.

I don't believe theoretical is the best word to use, even if you consider power factor the neutral current will be close to the other two conductors because the phases are 120 degrees out of phase where a 3-wire edison is 180 degrees

if you have a 200 amp breaker, three phase 3 wire with neutral, and both phases running 185 amps the neutral will also be 185 amps, unless you have some power factor shifting your phases, then they will adjust accordingly but usually still close. But with a 3-wire edison only two wires will have 185 amps, 2 heaters compared to 3. Our 310.16 standard ampacity table is based on 3 or more conductors(heaters) in a conductor group, this allowance was because a 3-wire edison effectively will only ever have two fully loaded conductors(heaters), that's not the case with a three phase neutral. The 1956 AIEEE study I quoted was the study for our de-rating table (more than 3 conductors in a conductor group), they tested conductor heating diversity (some wires carrying and some not)


here is a thread where the OP describes just that and is questioning why the neutral has full load, the fellow members give resources to explain.

How to calculate the unbalanced neutral current in a 3 phase circuit

I was teaching a class in sharing the grounded conductor in a single phase circuit. We only have 3 phase power in our class. I built a simple circuit with 2 equal loads on 2 seperate phases with a shared grounded conductor. I figured that the current in the grounded conductor in a three phase...

forums.mikeholt.com

attached is the attachment from the 2017 submitter of the public input that was accepted to allow the 120/208 three-phase service to be added to the allowance, and even his diagram shows all three wires carrying the same amount of current in the 3 phase 3 wire with neutral. I got this off the NFPA website just now.

Thank you, i hope you respond again.

 

[Devin Hanes]

Given that the actual dwelling unit load is typically less than 50% of the load as calculated in accordance with Article 220, I don't think there is any issue here.

Thank you, I appreciate the input. Just because I know my original post was long i would like to make sure you noticed i am just asking to remove the three phase allowance, not the single phase allowance, i believe the single phase allowance is fine.

I agree the load calculations are overly conservative, the hang up i have with this section is you are unnecessarily using a larger over-current device. If your load is 50%, your breaker at the standard level without this allowance is fine as well, you don't need the larger breaker this section allows. why should you use a larger breaker than normal for a given conductor when you will only ever run 50%? The smaller breaker will appropriately protect against overload if for some odd instance the current approaches that level and like you said it should never get close to it anyway, so why do you need a larger one? It seams like you are unnecessarily removing a safety for no gain. We are also usually using inverse time breakers like i describe in my original post where i gave some trip times which are well over the handle rating and allow overload for a good length of time even with the smaller breaker.

Also, the growing popularity in electric vehicle charging is increasing that 50% dramatically in some cases.

Thanks again, i would appreciate any further input. Also, I have been reading the forum for years and have learned a lot from both you and infinity so thank you very much for that as well.

 

[don_resqcapt19]

Thank you, I appreciate the input. Just because I know my original post was long i would like to make sure you noticed i am just asking to remove the three phase allowance, not the single phase allowance, i believe the single phase allowance is fine.

I agree the load calculations are overly conservative, the hang up i have with this section is you are unnecessarily using a larger over-current device. If your load is 50%, your breaker at the standard level without this allowance is fine as well, you don't need the larger breaker this section allows. why should you use a larger breaker than normal for a given conductor when you will only ever run 50%? The smaller breaker will appropriately protect against overload if for some odd instance the current approaches that level and like you said it should never get close to it anyway, so why do you need a larger one? It seams like you are unnecessarily removing a safety for no gain. We are also usually using inverse time breakers like i describe in my original post where i gave some trip times which are well over the handle rating and allow overload for a good length of time even with the smaller breaker.

Also, the growing popularity in electric vehicle charging is increasing that 50% dramatically in some cases.

Thanks again, i would appreciate any further input. Also, I have been reading the forum for years and have learned a lot from both you and infinity so thank you very much for that as well.

The service size is not a technical issue, it is a marketing issue.

 

[tom baker]

Deleted as was already covered

 

[infinity]

I greatly appreciate your time and review, thank you.

I wouldn't word it the way you did, I'm not saying they are unsafe because all three wires carry the same current, i'm saying you can't overrate them like you are allowed to do with the edison 3-wire conductors. its a matter of temperature rise and dissipation of the conductor group, similar to how you have to de-rate when you have more conductors in a conduit.

I don't believe theoretical is the best word to use, even if you consider power factor the neutral current will be close to the other two conductors because the phases are 120 degrees out of phase where a 3-wire edison is 180 degrees

I used the word "theoretical" because it assumes that the two phase conductors will be loaded at or near their maximum current to generate the maximum conductor heating. As Don stated that theoretical load is not going to happen due to the generously high load calculation values in Article 220.

 

[Devin Hanes]

Deleted as was already covered

Thank you, I am trying to figure out how to shorten things up. The fear is that the CMP believes the allowance is due to load diversity instead of conductor group diversity, that's why I have the background information. I was wanting to use this forum as a Litmus test to see what I actually need in the proposal and what I can get rid of. I greatly appreciate the input.

 

[Devin Hanes]

I used the word "theoretical" because it assumes that the two phase conductors will be loaded at or near their maximum current to generate the maximum conductor heating. As Don stated that theoretical load is not going to happen due to the generously high load calculation values in Article 220.

Thanks, I figured that was where you were going but it read different to me.

To the high load calculation value, do you believe that would be more appropriately corrected in 220 rather than allowing a larger breaker on the conductors? effectively using the same size wires in the end but a size smaller overcurrent device in many situations. It's just not right to me to use a larger breaker when even in both of your arguments i believe clearly states for all practical purposes is unnecessary and the smaller breaker will work fine and offer tighter protection.

it just seems like two wrongs are being used to make a right, if the smaller wire is big enough then the appropriate/standard size overcurrent device is large enough as well, especially considering the use of inverse time circuit breakers and their time current curves i already provided that allow operation above handle rating for very long times.

I'm not suggesting it would be easy to change 220, but two wrongs don't make a right.

Thanks again, I do appreciate the input.

 

[winnie]

I'd suggest that the issue is not in 310.12 but rather in the way ampacity is assigned, and the same value used for 2 or 3 conductors in a single conduit or cable.

IMHO residences are an example of a situation where the instantaneous load may be much greater than the average load, and where the conductors may be considered protected by the combination of the (oversized) breaker and the load itself. There are many situations in code where the amapacity of the conductors is smaller than the trip rating of the breaker protecting them.

However Devin Hanes is quite correct that the conductors in a 'single phase' 120/208 situation are more heavily loaded than the conductors in a single phase 120/240V situation at the same calculated circuit amps. As he says, there are 'three heaters' (3 current carrying conductors) on the 120/208 situation rather than only 2. The fact that ampacity tables assign the same values for 2 or 3 conductors in a conduit is clearly and approximation which must be incorrect, presumably conservative for the 2 conductor case rather than an overload in the 3 conductor case.

-Jon

 

[Devin Hanes]

The service size is not a technical issue, it is a marketing issue.

I think i know what you are getting at, the customers wanting 200 amp right?

I think i see where you are coming form, the way i look at that is if the code is changed the way i am proposing, the customer is the one wanting the 200 amp service and the customer would be the one footing the bill for the larger wire. the contractor makes more on their markup, the customer got their 200 amp service, if its code then all contractors are on same playing field right? Also, I believe the installation is safer, but I understand if you don't believe that. And the contractor can still offer the few amp smaller service for a price reduction as well.

Just to confirm, you know i am only asking to remove the 3 phase language and not the single phase language right?

thanks, I don't think i thought of that and i do remember customers wanting a 200 amp service. In my opinion although I admit it is a stretch, you're not giving them a X amp service for a 3 phase 3 wire with neutral if you are using smaller conductors than you would using 310.16, the wire is the weakest link/the limiting factor, if it isn't good for 200 amperes i don't believe they are getting what they are asking for, like i said i know its a stretch but that is my opinion.

I greatly appreciate the input

 

[Devin Hanes]

I'd suggest that the issue is not in 310.12 but rather in the way ampacity is assigned, and the same value used for 2 or 3 conductors in a single conduit or cable.

IMHO residences are an example of a situation where the instantaneous load may be much greater than the average load, and where the conductors may be considered protected by the combination of the (oversized) breaker and the load itself. There are many situations in code where the amapacity of the conductors is smaller than the trip rating of the breaker protecting them.

However Devin Hanes is quite correct that the conductors in a 'single phase' 120/208 situation are more heavily loaded than the conductors in a single phase 120/240V situation at the same calculated circuit amps. As he says, there are 'three heaters' (3 current carrying conductors) on the 120/208 situation rather than only 2. The fact that ampacity tables assign the same values for 2 or 3 conductors in a conduit is clearly and approximation which must be incorrect, presumably conservative for the 2 conductor case rather than an overload in the 3 conductor case.

-Jon

Thank you very much for the input i greatly appreciate it. To the point of high instantaneous loads i'm sure you are familiar with time current curves of the inverse time circuit breakers we typically use for these installations, I don't see for example using a 175 instead of a 200 breaker causing nuisance trips when you consider these curves
from my original post
"the Square D 175Amp would take a minimum of 20.83 minutes to trip at 175 amps all the way up to 2.78 hours. At 218 amps its about 5.83 minutes to 2.78 hours. Why would a 175 amp residential service ever need to operate at 175 amps for longer than 20 minutes which is the minimum time it will take ocpd to open. "

I agree you shouldn't have to worry about overloading the service conductors when a load calculation was done for a dwelling, "protection by loads served". but what does it hurt to use the standard sized breaker for the service? and why go a size larger? I'm not asking to add a breaker, there's already one there just not appropriately sized in my opinion. in many instances i'm not even asking for larger wire just a smaller breaker, that i am assuming everyone else that has posted also believes will hold and be below the required 80%(breaker continuous loading).

I am guessing you are referring to sections such as 240.4(G) (Air conditioning, refrigeration equipment, motors, etc.) that covers most of what i believe you are referring to, many of those have overcurrent protection by means other than the branch circuit overcurrent protection (overload relays in motor starter or in motor, part of equipment served, etc.), and many of those have upstream short circuit protection unlike our service conductors. Service conductors do not have short circuit or ground fault protection like other conductors, so when they are overloaded and melt through upstream of the overcurrent device there is no other safety like what i believe you are referring to has.

Also as i stated in my original post I believe the dwelling service conductors may be the most dangerous conductors in the NEC as they only have downstream overload protection and they are installed on kindling, houses today built with osb, vinyl siding, standard carpet and furniture, etc. burn much faster and easier than even plywood and aluminum siding let alone building materials used before them. i really am concerned for safety, hopefully incorrectly but i am. I don't believe there will be fires popping up everywhere but if there is even one because of this that's too many.

anymore input is greatly appreciated. As i said about the others too i have read the forum for years and am familiar with your name, i'm sure I've learned a lot from you as well and thank you for that.

 

[synchro]

Are saying that the 3 wire feeder or service entrance conductors from a 208Y/120 volt system are unsafe because of the theoretical possibility that each conductor can carry the same amount of current simultaneously?

I wouldn't word it the way you did, I'm not saying they are unsafe because all three wires carry the same current, i'm saying you can't overrate them like you are allowed to do with the edison 3-wire conductors. its a matter of temperature rise and dissipation of the conductor group, similar to how you have to de-rate when you have more conductors in a conduit.

I don't believe theoretical is the best word to use, even if you consider power factor the neutral current will be close to the other two conductors because the phases are 120 degrees out of phase where a 3-wire edison is 180 degrees

I used the word "theoretical" because it assumes that the two phase conductors will be loaded at or near their maximum current to generate the maximum conductor heating. As Don stated that theoretical load is not going to happen due to the generously high load calculation values in Article 220.

When Rob (infinity) mentioned "theoretical" I was also thinking that L-L (line-to-line) loads should be considered in this discussion because they don't contribute any neutral current even in 120/208. With 120/208 the current on the neutral conductor will be the same as the phase conductor current only if there are: 1.) equal L-N load currents on the two phases and no L-L loads, or 2.) L-N loading on only one of the phases, no L-N loads on the other phase, and no L-L loads. Otherwise the neutral current will be lower than the highest of the line currents.
The proliferation of EV charging loads is certainly important because these persist for long durations and therefore may significantly increase conductor temperatures. However, any large EV charging loads will be L-L instead of L-N. Also, I've no data to back it up but the deployment of high efficiency LED, CFL, etc. lighting is likely reducing the percentage of L-N loading as a percentage of the total load in many cases. So I think both of these trends would result in a reduced percentage of neutral conductor current in 120/208 services and feeders.
I'm not saying that I'm opposed to the proposal by Devin, just that the issues I mentioned above should be considered because as a minimum they will inevitably come up in later discussions.

 

[wwhitney]

If the 0.83 factor for dwelling unit service and main feeder ampacity were 100% due to the benefits of having only 2 CCCs rather than 3 CCCs, then why have a special rule for dwelling unit services and main feeders? Why not just have a 1.2 multiplier for all cases of only 2 CCCs?

So I conclude that part of the reason for the rule is the load diversity (aka NEC overestimation of actual load) of dwelling unit services and main feeders. Now the question is how much of the 0.83 factor is attributable to load diversity, and how much to having only 2 CCCs? Because with 3-wire 120/208Y dwelling unit services and main feeders, we have the same load diversity, but have 3 CCCs. So if it's almost all load diversity, there's no reason not to extend the 0.83 factor to those 3-wire 120/208 dwelling unit services and main feeders.

Cheers, Wayne

P.S. Not very serious suggestions:

Maybe split the difference and propose a 0.92 factor for 3-wire 120/208Y dwelling unit services and main feeders? : - )

Maybe also introduce a 150% factor, instead of the usual 125% factor, for EVSE loads in dwelling units when computing service and main feeder demands? This would counteract the 0.83 factor, since an EVSE load is one dwelling unit load the NEC doesn't systematically overestimate. : - )

 

[don_resqcapt19]

I don't see this PI going anywhere unless you can cite actual real world damage to conductors because of the existing code permissions.

 

[tom baker]

But didn't the NEC just change to allowance for 120/208, in 2017? There may not be many installations yet.

 

[synchro]

Given the present capability of physical and electrical modeling with computer simulation, it's disappointing if the common scenario posted by the OP has not been simulated by academia or industry.
Preferably this would include "Monte Carlo" simulations using thousands of specific scenarios to identify any "outliers" that could require more conservative requirements and with the probabilities that they would occur. These results could then be used along with historical experience to establish rules using less guesswork, so that hopefully they'd be less "fluid" from one code version to the next. Maybe that would be expecting too much?

 

[don_resqcapt19]

But didn't the NEC just change to allowance for 120/208, in 2017? There may not be many installations yet.

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.

 

[mbrooke]

I think people forget how conservative 310.16 really is.