Jim Dollard has an extensive background in codes and standards. Send questions about the National Electrical Code (NEC) to Jim at [email protected]. Answers are based on the 2020 NEC.

Getting started with the NEC

I’m a junior electrical engineer working at a design firm and came across Electrical Contractor magazine. I am new to the Code and have been learning it slowly. Could you tell me how and what chapters to learn first? Looking for a little guidance!

To the new user, the NEC may seem difficult to navigate, but the Code is an extremely well-organized document. I suggest that you focus first on reviewing the arrangement of the NEC in Section 90.3 while reviewing the table of contents at the same time. That will help you to become familiar with the overall structure of the NEC . Focus also on Article 90, the “Introduction to the NEC .” Each of the Code’s nine chapters is broad in scope. Individual articles are listed in each chapter to fulfill the chapter’s scope.

It is also important to understand the structure of the NEC . The style manual is a free download and is useful in helping to understand the structure. Articles are separated into parts where necessary due to size or logical separation of requirements. Then there are sections that may contain multiple levels of subdivisions, exceptions, informational notes and more. In addition, it is very important to review the definitions in Article 100 and the xxx.2 sections in many articles. As we say in Philly, if you can’t talk the talk, you can’t walk the walk!

Stacked neutrals?

Are we still permitted to stack two neutrals under one terminal screw in residential panelboards? I just got back into the business and was told that is no longer permitted.

No. See Section 408.41 that requires each grounded conductor (neutral) in a panelboard be terminated in an individual terminal that is not also used for another conductor. There is an exception for grounded conductors of parallel circuits, provided the terminal is identified for connection of more than one conductor. See also the general rule in 110.14(A), which requires that terminals for more than one conductor be identified for that purpose.

Ampacity of Type NM cable

In a commercial occupancy where Type NM cable is permitted and used, can I protect No. 8 copper at 50A? The reduction to 60°C applies only in dwelling units, right?

No. The requirements in 334.80 mandating that the permitted ampacity not exceed that of a 60°C-rated conductor is not limited to dwelling units or to any type of occupancy. An 8 AWG copper conductor in Type NM cable will be limited to the 60°C ampacity in Table 310.16 of 40 A, which is a standard size overcurrent protective device in Section 240.6. Type NM cable is constructed with 90°C conductor insulation as required by Section 334.112. Note that 334.80 does permit the 90°C conductor insulation rating to be used for ampacity adjustment and correction calculations. However, the final ampacity rating cannot exceed the 60°C rating.

Mini split cable

We started installing mini split cable for air conditioners in a six-floor, 30-unit apartment building. The listed cable that we purchased was marked as mini split cable, TC-ER-JP. The NEC was even referenced on the cable reel. The inspector informed us early on that it was not permitted. Why can’t we use it?

The cable that you are referencing is addressed in Article 336, “Power and Control Tray Cable Type TC.” “ER” (exposed run)means that this cable meets the required crush and impact tests. “JP” permits this cable to be pulled through joists. Type TC cable is required to be listed in 336.6. This wiring method is commonly used in industrial settings and installed in cable trays. There is prescriptive permission to use Type TC-ER-JP cable in one- and two-family dwelling units in list item 336.10(9). This permitted use would not apply to a multifamily dwelling.

The permitted used in 336.10(9) requires that Type TC-ER-JP be installed indoors in accordance with the requirements of Part II of Article 334, “Type NM Cable.” It must be installed outdoors in accordance with Part II of Article 340, “Type UF Cable.” The original proposals to permit TC-ER-JP in one- and two-family dwelling units was for installations of optional standby generators. In one- and two-family dwelling units, it could be used for mini split units. However, when using this wiring method for mini splits, the limitation to an ampacity rating of 60°C applies because the cable must be installed in accordance with 334.80 and 340.80. There is an exception to the 60°C ampacity rating requirements following 336.10(9) for TC-ER-JP used to connect a generator and associated equipment, provided all of the terminals are rated 75°C or higher.

Voltage drop

To the best of my knowledge, the NEC contains information on voltage drop in the articles for feeders and branch circuits, but no requirements. When our department asks an engineer for voltage-drop calculations to the fire pump motor in accordance with Section 695.7, many are surprised that it is a requirement. Why don’t we have a minimum voltage drop for everything?

The NEC contains minimum installation requirements. We all need to keep in mind that, when we are in strict compliance with the NEC , we are quite literally doing the least we can do. The introduction in 90.1(B), which addresses the adequacy of the NEC, explains that: (1) the Code contains requirements that are considered necessary for safety; (2) if you comply and properly maintain systems and equipment, it should be free from electrical hazards; and (3) even when you comply with minimum installation requirements, the installation may not be efficient, convenient or adequate for good service or future expansion of electrical use. The NEC is not a design guide or manual; it is the gold standard for minimum installation requirements.

You are correct that the NEC does provide the user with information on voltage drop to achieve reasonable efficiency of operation. See the informational notes following 210.19(A) and 215.2(A). Voltage drop requirements for fire pumps in the NEC are extracted from another NFPA standard. NFPA 20, “Standard for the Installation of Stationary Pumps for Fire Protection,” has purview over the performance of electrically driven fire pumps. The NEC contains prescriptive electrical installation requirements that mirror the rules in NFPA 20. Section 9.1.6 of that standard requires all power supplies to comply with the voltage drop requirements in 9.4. Those requirements are extracted into Section 695.7.

Two receptacle outlets?

If two duplex receptacles are installed in a 4-inch box in a wall, can I count that as two receptacle outlets to comply with 210.65 for meeting rooms? After all, I have four receptacles.

No. An outlet is defined in Article 100 as a point on the wiring system at which current is taken to supply utilization equipment. Compliance with the NEC relies heavily on the user’s understanding of definitions. In your question, there is a single box; it could be single gang, two gang or more, but it is a single point on the wiring system at which current is taken to supply utilization equipment.