Simple historical or scientific explainations for NEC codes

[deculver3]

I am a new AHJ with 35 years of electronics experience and very little electrical background. Many of the NEC codes are clear to me, but I lack the knowledge to figure out the reason for some of the rules. Instead of following the book as it is written, I am the kind of person who would like to understand WHY the code has certain rules. In our library we have the latest NFPA 70, NFPA 70 Handbook and the 2 volume set of Mike Holt's Understanding the NEC, but they don't answer my questions. I would like to have answers to common questions I get asked when a I fail a device and the customer asks why the code says that. Basically, what is the history for some of these rules? Some are obvious but some are a little tough to put into words.

If there was just a guide that added a few words to point you in the right direction or to jog your memory for why a rule was added to the code.

A great example is today's graphic of the day.

If questioned by a customer, I could answer why I think NEC 70, 210(A)(1) says "Receptacles shall be installed such that no point measured horizontally along the floor line of any wall space is more than 1.8 m (6 ft) from a receptacle outlet". I may be correct, I may be wrong out of ignorance or forgetfulness, or I may be correct but leave out a perfectly logical detail as to the history behind such a rule. Also, sometimes a rule applies in a residential setting but not in a warehouse or laboratory setting. I would like to be able to explain to customers instead of just saying that "Because the code says so."

Can you suggest a book or two to add to our library?

 

[charlie b]

You may not always be able to find the reason for the rules. Some have good explanations that may or may not have been the reason the rule was first put into place. Your example fits into that category. The reason you want receptacles no more than 12 feet apart (i.e., so that the spot in the middle is no more than 6 feet from a receptacle to the left or to the right) is likely to be that most lamps and other appliances have supply cords that are 6 feet long.

Welcome to the forum.

 

[jap]

You may not always be able to find the reason for the rules. Some have good explanations that may or may not have been the reason the rule was first put into place. Your example fits into that category. The reason you want receptacles no more than 12 feet apart (i.e., so that the spot in the middle is no more than 6 feet from a receptacle to the left or to the right) is likely to be that most lamps and other appliances have supply cords that are 6 feet long.

Welcome to the forum.

Just to expand on that just a little more, the major intent of the spacing of the receptacle outlets would be to limit the need to have to use an extension cord to plug things in.

JAP>

 

[roger]

I am a new AHJ with 35 years of electronics experience and very little electrical background. Many of the NEC codes are clear to me, but I lack the knowledge to figure out the reason for some of the rules. Instead of following the book as it is written, I am the kind of person who would like to understand WHY the code has certain rules. In our library we have the latest NFPA 70, NFPA 70 Handbook and the 2 volume set of Mike Holt's Understanding the NEC, but they don't answer my questions. I would like to have answers to common questions I get asked when a I fail a device and the customer asks why the code says that. Basically, what is the history for some of these rules? Some are obvious but some are a little tough to put into words.

If there was just a guide that added a few words to point you in the right direction or to jog your memory for why a rule was added to the code.

A great example is today's graphic of the day.
View attachment 19150
If questioned by a customer, I could answer why I think NEC 70, 210(A)(1) says "Receptacles shall be installed such that no point measured horizontally along the floor line of any wall space is more than 1.8 m (6 ft) from a receptacle outlet". I may be correct, I may be wrong out of ignorance or forgetfulness, or I may be correct but leave out a perfectly logical detail as to the history behind such a rule. Also, sometimes a rule applies in a residential setting but not in a warehouse or laboratory setting. I would like to be able to explain to customers instead of just saying that "Because the code says so."

Can you suggest a book or two to add to our library?

You should go to the NFPA website and start researching the ROP's (PI's now). Find the article section you are wanting back up for and look it up in previous cycles. Many times you will be able to track something down through the acceptance or rejection commentary from the particular CMP.

Roger

 

[jumper]

You should go to the NFPA website and start researching the ROP's (PI's now). Find the article section you are wanting back up for and look it up in previous cycles. Many times you will be able to track something down through the acceptance or rejection commentary from the particular CMP.

Roger

+1

I do that constantly.

 

[480sparky]

The rule we now know today started out in the 1937 NEC (Rule 2110), stating "a sufficient number, but not more than 10 feet from any point along the wall".

In 1940, the '10-foot' wording was changed to "no more than 20 feet apart".

In 1956 (now Rule 2124b), the 20-foot reference was altered to 12 feet.

In 1959 (renumbered to 210-22(b)), the language was rewritten to the current "no point along a wall more than 6 feet from a receptacle". It also included the current language of require a receptacle in a wall 2 feet or longer.

 

[MasterTheNEC]

What i can tell you is that graphic is incorrect, in the fact it states they must be placed in these configurations.

While you utilize 210.52(A)(1)-(A)(4) to determine the actual number, the rule is similar to the intent of how it is done in motels and hotels. the actual placement is for the owner or designer of the building to decide. In speaking with CMP 2 it was supposed to be crystal clear that the intent of the reference to 210.52(A) was simply to gain a count and not to determine actual placement. Sure, you can place them as shown but that is not what the rule is actually saying.

In the 2020 NEC it will be cleared up and said a little better I promise you.

 

[MasterTheNEC]

I also have a comment about researching PI's from the 2014 NEC cycle and moving forward. Since the CMP's are now given the latitude to "write" code at the table versus simply reviewing PI's that are submitted the language that you will see in the panel statements will never fully reflect the conversation that takes place at the actual table. What I mean is, the PI research will be helpful and I encourage it just know that in many cases unless it is accepted as written it will be translated into a "First Revision" at this point and will be made up of many inputs at that point.

I agree.....research the PI's just wanted you all to know how it is now changed in the acceptance process and how the CMP's now own the committee they sit on and can rewrite, change, amend or delete anything they want.