Engineering Judgment / Diversity

[ZSIM]

Greetings.

My mentor indicated that in a very old code cycle, the NEC permitted Engineering Judgement / Diversity to be considered in sizing cables and equipment.

Can anyone point me to the code year that changed? I am trying to see the language they used and what exactly was permitted to be done.

Thank you.

 

[bwat]

Are you referring to the “Engineering Supervision” in 310.15(C) for conductor ampacity? (2017 NEC)

 

[ZSIM]

Are you referring to the “Engineering Supervision” in 310.15(C) for conductor ampacity? (2017 NEC)

Bwat,

Thank you, but no. That section allows for modification of a cable ampacity using the Neher-McGrath Formula.

The section my mentor is referring to was (again.... well before my time) I'm guessing in 220.xx somewhere and allowed for an Engineer to use diversity (or something) other then all Total Connected for the determination of load on a feeder.

I've never seen the section, but its a "commonly understood" thing my office all seems to swear existed. None of them know the Article # and I've gone back to 1978 and I did not see it there. My problem is we keep referencing this section in conversations (sometimes with Clients) that it used to be allowed... but no one seems to know anything about it. If I am going to have to speak about the past, I would at least like to know it really existed and what it actually did / allow.

 

[ron]

Hokus pokus, especially when discussing NEC service or feeder calculations

 

[jim dungar]

There is some engineering judgement allowed for selecting >600V feeder conductors under 215.2(B)(3).

 

[Hv&Lv]

220.35?
I'm asked for a years worth of demand a lot for additions to existing houses here.
it takes me about three minutes to pull it up and send it.

 

[steve66]

There is also 220.60 - noncoincident loads.

"Where it is unlikely that two or more noncoincident loads will be in use simultaneously, it shall be permissible to use only the largest...."

How much mileage you get from this paragraph depends on how strictly you or the AHJ wants to define "unlikely".

 

[ZSIM]

Hokus pokus, especially when discussing NEC service or feeder calculations

At this Point I agree. However that means I will probably have to start some uncomfortable conversations and having them supply the information if they think they are right...

There is some engineering judgement allowed for selecting >600V feeder conductors under 215.2(B)(3).

That is true, but we deal primarily in low voltage and all the conversations are around low voltage. I do not believe this is it.

220.35?
I'm asked for a years worth of demand a lot for additions to existing houses here.
it takes me about three minutes to pull it up and send it.

This became 220.87, and we use this very frequently. This is not what they meant though. They are basically saying there was a section of code that said (paraphrased and using random numbers) "Using engineering judgment, the load you determined to be 700A can be designed as only 500A because the engineer has the experience is willing to place his seal on it."

There is also 220.60 - noncoincident loads.

"Where it is unlikely that two or more noncoincident loads will be in use simultaneously, it shall be permissible to use only the largest...."

How much mileage you get from this paragraph depends on how strictly you or the AHJ wants to define "unlikely".

Agree. Our AHJs require electrical or mechanical interlocks that prevent simultaneous operation. However this is not the code section, either.



So Far, Hokus Pokus seems most accurate.

 

[tortuga]

Ahh a code history question. I love an excuse to dust off the books.

Greetings.

My mentor indicated that in a very old code cycle, the NEC permitted Engineering Judgement / Diversity to be considered in sizing cables and equipment.

How old is very old or do you want to go all the way back?

 

[ZSIM]

I stopped at 1978 cycle.

But sometime around the time of the Big Bang would be an acceptable "way back" time frame if such an item existed in that code cycle.

 

[charlie b]

I cringe when I see the word, "diversity." The NEC does not define or use that word by itself. It does use the phrase "load diversity" in a context unrelated to your question. At my desk I have an engineering textbook that has a definition of "diversity factor," but that too is in a context unrelated to your question.

However, I think the question is entirely academic. Even if there had been a rule similar to your question, you can't use it anymore. You need to use the edition of the code book that has been adopted by your jurisdiction. For my home state of Washington, that would be the 2017 NEC. It does allow the use of certain "demand factors" that enable us to design feeders to handle a lower load than their total connected load. I half wonder if this is what your mentor is talking about.

 

[tortuga]

Well mostly academic, however there a cases when no work is being done rather a building is sold and an insurer or underwriter is involved, then it comes down to the 'code in effect at the time built'.

I don't have it happen often but we were asked for an opinion on a large apartment building that had a unusual service arrangement.
The other time it comes up is on large renovations and an inspector finds something outside of the 'scope of work' that may not meet code. If it meets the code when it was installed it can stay.

I dont have my books right now but I imagine something could be found in the first code era 1897-1920, under 'Class C' - Inside work'

 

[ZSIM]

I cringe when I see the word, "diversity." The NEC does not define or use that word by itself. It does use the phrase "load diversity" in a context unrelated to your question. At my desk I have an engineering textbook that has a definition of "diversity factor," but that too is in a context unrelated to your question.

However, I think the question is entirely academic. Even if there had been a rule similar to your question, you can't use it anymore. You need to use the edition of the code book that has been adopted by your jurisdiction. For my home state of Washington, that would be the 2017 NEC. It does allow the use of certain "demand factors" that enable us to design feeders to handle a lower load than their total connected load. I half wonder if this is what your mentor is talking about.

Understood and fully agree. My only intent is to figure out what my mentor keeps referring too (because he is adamant it once existed). I have yet to find proof that actually was ever true, and I really do not want to keep repeating that statement to a client if its false and we really need to present a strong face for.

The issue at hand is we have a client who feels we are over-sizing the feeders. We design them per 220 and subsequently 430, but they often run at 1/3 to 1/2 of that designed load. It is coming to me to defend the sizing (which I have no issue with that) but the door was opened by saying "Engineer's used to be able to adjust things". Now am I trying to defend / explain code section I've never seen myself and they keep bringing refering to my Mentor statement.

(Little more background. There is a claim that we are not taking into account "diversity" in our design. As you mentioned, that term doesn't really exist in the NEC, but we do apply the demand factors the NEC allows, when it allows us to. When installing fifteen oversized and separate large Horsepower motors that not interlocked to prevent coincident operation, the code ties our hands per 430.24. We try to explain that even though your motor are not fully loaded and the process's natural diversity will likely show the peak to be 50%, to be code compliant it must be sized as if they are all running. la sigh)

 

[ZSIM]

Well mostly academic, however there a cases when no work is being done rather a building is sold and an insurer or underwriter is involved, then it comes down to the 'code in effect at the time built'.

I don't have it happen often but we were asked for an opinion on a large apartment building that had a unusual service arrangement.
The other time it comes up is on large renovations and an inspector finds something outside of the 'scope of work' that may not meet code. If it meets the code when it was installed it can stay.

I dont have my books right now but I imagine something could be found in the first code era 1897-1920, under 'Class C' - Inside work'

Interesting. That was 50 years before this building was built so I think we are OK there.

I guess I can see if NFPA 70 has some online PDFs dating that far back...

 

[charlie b]

My only intent is to figure out what my mentor keeps referring too (because he is adamant it once existed).

What did you say his role was? Is it "mentor," or does he have those two syllables backwards?

When installing fifteen oversized and separate large Horsepower motors that not interlocked to prevent coincident operation, the code ties our hands per 430.24.

Not quite. 430.24 Exception 3 tells us that we are allowed to disregard some of the motors, if there are interlocks. That is not the same as forbidding us to disregard some of the motors, unless we install interlocks. I think that 220.60 can still be brought into play. Indeed, this may be the article your (tor)mentor is talking about. It includes the word "unlikely," not the word "impossible," and it does not itself call for interlocks.

 

[ZSIM]

Not quite. 430.24 Exception 3 tells us that we are allowed to disregard some of the motors, if there are interlocks. That is not the same as forbidding us to disregard some of the motors, unless we install interlocks. I think that 220.60 can still be brought into play. Indeed, this may be the article your (tor)mentor is talking about. It includes the word "unlikely," not the word "impossible," and it does not itself call for interlocks.

Understood, however we do not have the knowledge to determine which of the 15 will be operational and which won't be at any given time.

The designer of the motor system is also not willing to speculate in that matter either so any problem falls on us alone.

(We usually reserve 220.60 for HVAC and similar. A process system like this, that is up to some guy deciding what he wants to make today, I do think qualifies for this section. Have you had luck applying it though in that manner?)

 

[GeorgeB]

We try to explain that even though your motor are not fully loaded and the process's natural diversity will likely show the peak to be 50%, ...

I'll interject here that loading and current IS NOT a linear relationship. It is not unusual for an unloaded (completely) motor to draw about 40% of full load current, and current to be APPROXIMATELY linear from that 40% to 100%, so ABOUT 70% current at 50% load.

You don't get as much advantage as you might expect.

 

[tortuga]

OK I am back from the books.
Before the 1928 NEC added rule 613 demand factor tables, there were no prescriptive methods for 'diversity' sizing what we now call services and feeders.
There were rules on sizing branch circuits but not feeders. And there were no 'demand factors'.
However nothing would have prevented a PE from sizing a feeder smaller than the connected load as long as the feeder conductors were properly protected.
The NEC did require a feeder be replaced if the load was found to be too great.

The 1929 supplement added the definition 'demand factor' to the NEC.

In many cases conductors of a feeder or main circuit supplying a group of motors need not have a carrying capacity equal to the sum of the full load current ratings of the motors supplied. A diversity factor may be permitted by the authority having jurisdiction of these regulations to be used in determining the carrying capacity of these feeders or main circuits, the value of this factor depending on the size and number of the motors supplied and the character of the load.

Although the idea only applied to motors.

Service Switch rating
The service switch shall, unless a demand factor has been granted, have a rating sufficient to carry the total connected load, and have a rating not less than the rating of the cutout base or the setting of the circuit breaker in series with it.

In 1937 the code is rearranged and article 220 is born:

Article 220--Feeders.
In contrast with the direction of the rules adopted for Article 210 as to
adequacy of wiring layouts is the further recognition extended in this article
to the diversity of use of appliances, other than motors and the resulting
appropriate economies in the amount of copper permanently installed as
feeder conductors.
It is appropriate to make special mention of the extensive
authoritative data covering the ratio of "demand" or use to current rating of
connected load secured and presented by the Electric Light and Power Group.
Consideration of this data resulted after full discussion in material decreases
in the demand factors specified for installations of domestic electric ranges
rated at 3.5 kilowatts or larger

By 1940 the code is first requiring the loads to be computed:

The conductors of feeders shall not be smaller than as specified in Tables 1 and 2, chap 37, for the computed load feeder loads.

So between 1928 and 1940 engineers and electrician designers had some wiggle room with the AHJ. Before 1928 code did not address it.

 

[ZSIM]

OK I am back from the books.
Before the 1928 NEC added rule 613 demand factor tables, there were no prescriptive methods for 'diversity' sizing what we now call services and feeders.
There were rules on sizing branch circuits but not feeders. And there were no 'demand factors'.
However nothing would have prevented a PE from sizing a feeder smaller than the connected load as long as the feeder conductors were properly protected.
The NEC did require a feeder be replaced if the load was found to be too great.

The 1929 supplement added the definition 'demand factor' to the NEC.


Although the idea only applied to motors.



In 1937 the code is rearranged and article 220 is born:


By 1940 the code is first requiring the loads to be computed:


So between 1928 and 1940 engineers and electrician designers had some wiggle room with the AHJ. Before 1928 code did not address it.

Interesting and thank you for diving into the history books! So the moral of the story is basically 90 years ago the code kinda sort of let an engineer do as they please by not actually stating what to do, but for the past 80 years has been pretty solidly defined.

Can probably put a "Myth Busted" on this one.

 

[anthonysolino]

appears to be again "this is how we do it ,been doing like this for years" I always then ask show me where it says , 9 times out of 10 they don't even have a code book of any sort to show me