Installing a compliant electrical system can be tough. Reading and understanding the National Electric Code can be tougher. Understanding how the Code book is organized should help. Learning how to navigate around the NEC is crucial to being a successful electrician.
Often electricians will find rules in the Code book and use or abuse the rules found and try to fit a square peg in a round hole. The Code book is set up to establish general rules that can be modified or amended in other Articles and Sections to allow for a particular installation, special occupancy, or a specific wiring method.
As you may know, there are eight Chapters within the Code, and each Chapter is divided into Articles. Within each Article are Parts and then each Part contain Sections which are typically the rules we follow.
There are mandatory rules that are characterized by the words “shall” or “shall not.” When rules contain this language it must be followed or complied with. For example, in 210.8(A) you will find rules for GFCI protection and one rule states that “GFCI protection shall be provided” for receptacles installed at various locations in dwellings.
There are also permissive rules which are rules that are allowed but not required. These rules are characterized by the words “shall be permitted” or “shall not be required.” For example, in section 250.66(A), the rule states, “the grounding electrode conductor shall not be required to be larger than 6 AWG copper……”. This allows a larger conductor while still having the minimum size established.
In Article 90 you can find section 90.3 Code Arrangement. There you can see a figure (Figure 90.3) that illustrates the layout of the chapters and how they apply to other chapters. Chapters 1 through 4 apply generally to all electrical installations. While Chapter 5 are rules for special occupancies, Chapter 6 contain rules for special equipment and Chapter 7 contain rules for special conditions. All seven chapters may supplement or modify chapters 1 though 7. How is that possible?
Recall the requirements found in 310.15(C) that requires adjustments to the ampacities of conductors found in Table 310.16. If the raceway contains more than three current carrying conductors, then you must adjust the Table ampacity values of the conductors within that raceway.
If you install those same conductors within a metallic wireway, then the requirements are modified. They are found in Article 376. Section 376.22 states “the adjustment factors of 310.15(C)….. shall apply only where the number of current carrying conductors exceed 30 at any cross section of the wireway”. Be aware that this rule is not incremental. It is all or nothing. No adjustments at 30 conductors, but at 31, then you are at a 40 percent adjustment factor. 40% by just installing one more conductor that carries current.
However, if you take that same metal wireway and install it at an area used for presentation of a performing arts theater or for lighting of a stage, then the rules are amended again. The requirements for this type of occupancy is found in Article 520. In this occupancy, all of those rules do not apply. Section 520.6 still requires 20% fill through any cross section of the wireway but waives the rules of 376.22 for making adjustments to the conductors.
Another tip would be to read and understand the Scope. This is located in the beginning of each Article at section ***.1. By reading the scope you will be assured that you are looking at the correct article for your application. For example, many of the rules located in Article 230 which are for Services can also be found, word for word, in Article 225. Article 225 contain the requirements for outside branch circuits and feeders. For example, 225.18 and 230.23(B) require the exact same clearances for overhead conductors, except one is for service drops and the other is for feeders located outside of the building or structure.
Do not look at Tables and make installations without understanding how to apply the Table. I know some electricians (including me) may take Table 310.12(A) and misapply it to fit their installation. For example, You are sizing a 120/240-volt, single-phase, feeder rated at 200 amps fed from a 400-amp service. Looking at the Table you might select 4/0 AWG aluminum from the Table which is calculated at 83 percent of the rating of the overcurrent device for those feeders. The title of the table states “Service or Feeder” so your good! Right? Wrong! This rule only applies if it is the entire load of an individual dwelling unit. Because this is a feeder to only part of the dwelling, then this rule cannot be applied. Additionally, you must make adjustments and corrections if the installation requires it. This information is not found in the Table but is spelled out nicely in the text of that section.
Look at the following myths and see if you were taught something different or still believe these to be true or false.
- 320.40, Metal Clad Type MC cable does not require an anti-short bushing at the end of the armor before the connector is installed. There is not a section 330.40. See NEMA Bulletin, No. 90
- The aluminum bonding strip associated with Armored Cable, type AC cable, (BX) is not permitted to be used and connected to an equipment grounding conductor. It can be cut off at the point of termination. There is no direction found in the Code (320.100) but NEMA states the best practices are to bend it back over the anti-short bushing or to wrap it in the convolutions of the armor for and short distance and then install the connector. See NEMA Bulletin No. 91 for more information.
- Additional receptacles installed on the general lighting loads of dwellings do not require additional circuits or calculations at 180 VA each. These are convenience receptacles and are part of the 3VA/sq. ft. calculation found in 220.42 and 210.11.
- Up to two wall mounted ovens, and one counter mounted cooking unit of a residential kitchen is permitted to be installed on a same individual branch circuit. 220.55, Note 4 of Table 220.55
- Building steel is not considered a grounding electrode. 250.53(A)(2) and 250.68(C)(2)
- Flexible cables and fixture wires are not subject to the ampacity values located in Table 310.16. These conductors fall under 400.5 and 402.5. See 240.5
“I don’t know, that’s just the way we have always done it.”
Feel free to email me and share some others that I may have missed.
My recommendation to help avoid mistakes when applying the requirements of the NEC is to stay in it and stay current. Read it often and reach out to others who do understand it and get the right interpretation for your installation. Ask your inspector. Open an inquiring dialog with your AHJ’s, superintendents, foreman and engineers. Build a network of mentors to help you. You may not be able to Google everything. Technology can help you, but you must have the foundation. You must be able to navigate the NEC on your own to succeed in your craft. Understanding the Code and applying the specific rules are the biggest obstacle we can face when installing any electrical system. The results of your mistakes could be catastrophic.