“Concepts of the NEC”: James Khalil, PE

James Image Final 

Slides are available on YouTube at Concepts of the NEC


Thank you for coming back for this afternoon session. Today we will discuss the NEC. Most importantly, we’ll discuss how PowerCalc takes the 1500 - 2000 pages of the NEC, codes its requirements, and puts all of this information into a single software program so the power distribution system is designed easily, faster, with much better quality, and at a much lower cost. When we mention costs, it is the cost (1) for you and your company and also (2) for your customer to build the project. So, let’s see how we managed to put this 150+ years of confusion into a single software program minimizing the complexity of the electrical design process.

In 1897, various stakeholders including insurance companies and electrical and construction firms, wrote the first electrical safety rules. Since publication, no court has ever faulted a party for meeting the requirements of the NEC. The NEC also has the distinction of being the least amended model codes.

The NEC addresses the requirements for the electrical service entrance, conductors, as well as branch circuits including resistive or inductive circuits. The NEC sets forth minimum requirements for the electrical design. For residential projects, the NEC is specific with requirements for the number of circuits, the size of the minimum service, and even the placement of outlets and receptacles. In contrast, for other buildings such as industrial, commercial, and government buildings, the NEC puts the burden on the electrical professional to design for the project’s specific requirements.

NEC Structure: Article 90

The NEC’s structure is set out in Article 90. Article 90 makes it clear that the NEC is not intended as a design specification, instruction manual, or construction manual. The NEC’s main purpose is to safeguard persons and property from hazard arising from the use of electricity. We all know how much energy comes into the house or facility in the form of electricity and how powerful this electrical energy is. The NEC sets forth the requirements for how this electrical power can be used safely.

Start at Branch Circuit

How does PowerCalc implement these requirements? The NEC always starts from the branch circuit or from the electrical load. Our software works the same way. It provides a “Branch Circuit Wizard” where the electrical professional inputs three values: load kVA, load type, and the number of poles. Then, the software calculates all the specifications for the entire power distribution system from the overcurrent protection device to the branch circuit conductor. This calculation includes the ground, the conduit, and the voltage drop. It also accumulates all loads to then identify the panel size that feed these branch circuits.

Our software also includes a “Panel Board Wizard”. With this Wizard, the electrical professional sets up the parameters for the panel. Looking at this Wizard, you'll see three or four items…these result in populating 16 cells set on “Auto” with another 26 for selections from drop down menus.

As an example, let’s look at the conductor insulation. There's a drop down menu to select the type of conductor based on the temperature required for the project. Next to it, you also select whether the conductor is rated for 60, 75, or 90 degrees. On the wire size, you can then select whether it is rated for 75 or 90 degrees.

Same thing happens for the branch circuit. It has the same selections, so you can select your wire rated for 75 or 90.

Looking at the ambient temperature, especially for those that work overseas, the default value is again based on the NEC’s requirement that all conductors be rated for ampacity at 30 degrees Celsius. Then, there's a de-rating of values or factors associated with other degrees: 40, 50, 60, or 80 degrees Celsius. Once you set the conductor’s ampacity on 30, the software automatically updates the conductor size. And then, any further changes will result in automatic updates to the value of the conductor


Another purpose of the NEC is adequacy. Besides complying with the NEC’s requirements, the electrical professional’s design should be cost effective to build, operate, and maintain. For instance, the electrical professional should design a power distribution system that is adequate for future expansion and increases in loads. The NEC sets the minimum requirements, but recognizes that “adequacy” requires additional considerations.

PowerCalc provides space and spares for future expansion. We also added statistical calculations: the panels show how much load it can handle, what the actual demand load is, and what the connected load is. If you look at the statistical calculations for a particular panel, you have room for future load expansion if the panel’s load, demand factor load as a percent of the connected load, and the true utilization factor are all under 100 percent. If these values are over 100 percent there is no room for future load capacity. This information either confirms that your design provides for future expansion or alerts you to add more loads or increase the size of the panel to allow for future expansion.

International Standards – Both US and Metric Units

The third issue considered by the NEC is its relation to other international standards. There are a lot of similarities between the NEC and the IEC in terms of the requirements for the size of the circuit, the gauge of the wire, the overcurrent protection. But, there are always some minor differences between codes.. An example that comes to mind is the size of the transformer in the Canadian code. Even though the Canadian code addresses the same capacity sizes for transformers of 45, 75, 112 and so forth, it does not require that an overcurrent protection device be sized for the total load. In contrast, the NEC requires an overcurrent protection device, whether it's on the primary or the secondary breaker, to satisfy the full load of the transformer regardless of the actual load that's on it. I disagree with the Canadian approach as in a few years, an additional load could easily be added to the transformer without checking as to whether the overcurrent protection for the transformer is adequate.

To address this concern, PowerCalc is available in both US and Metric (SI) units. The calculations will just change back and forth. At my MEP firm, we’ve done projects around the world . We do the calculations in the US because the numbers make more sense as we are familiar with them. Then we click on the metric button to have these numbers converted.


Article 90 defines the scope of the NEC. As discussed Article 90.1 states the purpose of the NEC to safeguard the installation for the personnel and the facility. Next Article 90.2 defines the scope. What is the scope? The NEC governs all electrical design on the supply side. This includes all five buildings types: residential, commercial, healthcare, education, military, industrial, government, and ports. The only buildings and facilities not under the NEC’s umbrella are utility transformers and the power grid, including. generating equipment and buildings used for power generation. Other facilities associated with utility companies are governed by the NEC. Looking at a 1 Line Diagram, anything from the service point into the building is under the NEC.

Other Articles 90.3 and the Tables

Next is Article 90.3 that outlines the nine chapters of the NEC. Chapter 9 is the tables. Again, applicable as reference in all the chapters. All these tables specify the characteristics and properties of the conductors, the conduits you can use/cannot use, the cables you can/cannot put in the conduit, resistivity, Z-values, cross-sectional areas, and so on. At the end is the affirmative, it is stated that these requirements are not mandatory. But, I disagree as this is the authority detailing these requirements

Regardless of whether its mandatory, again PowerCalc’s approach is to do all the calculations and to use the values in the NEC’s look up tables. As relevant, PowerCalc uses these NEC values as the mandatory minimum or cap.

Then Article 90.4 is about enforcement. It gives the jurisdiction having authority some flexibility in enforcement and waivers.

Article 90.5 distinguishes mandatory rules, permissible rules, and explanatory material. These are as follows: mandatory rules state “shall or shall not”; permissive “shall be permitted or shall not be required”, and explanatory material is included as  informational notes that are not enforceable as an NEC requirement.

Let’s look at Chapter 2, wiring and protection, article 215, field, section 215.1, minimum rating and sizes. The mandatory rule says the feeder conductor shall have a capacity not less than required to supply the load as calculated in part three, four, and five of article 220. Then, load by load, circuit by circuit  you figure out how much power is required, etc. Again, the key point is “shall have.”

What is permissive is when a rule has exceptions. In this case, the exception is that when you combine loads together on the same circuit (such as when you have multiple motors on the same circuit), then  you can size that particular conductor or that particular feeder for 125% times the largest full load Amps then plus the remaining full load for that particular load. If you go by the rule, you will actually have to add all the full-loading apps for these motors and then provide the circuit to protect all of

them. But the NEC allows you to be more cost effective by taking the largest of them and multiplying it by 125, then just add the rest of them.

The explanatory material addresses voltage drop. Although not mandatory, many people over or under engineer when it comes to voltage drop. The NEC does not require that the voltage drop be addressed, but merely explains that it's an additional consideration and provides a guideline. This guideline suggests that the voltage from the inception of the circuit to the load not exceed 5%. Further, it recommends 2% on the feeder and then 3% on the branch circuit. While only a recommendation, for instance, in Florida this guideline is enforced for our energy requirements.

PowerCalc uses 2% and 3% as its default calculation. This calculation updates every time the user updates it, by increasing or decreasing the load. All of these calculations are happening in the background automatically for the electrical professional. The purpose of PowerCalc is not to do the work for you, but to support your design. The electrical professional needs to be aware of the NEC’s requirements and guidance.  Although PowerCalc 100% automates the calculation process, but the electrical professional can override calculations.


All electrical professionals, every hour, every day, work on calculations and things get easily mixed up.

So, what PowerCalc does is integrate is all these calculations into a one big integrated calculation. This is automation to simplify the way electrical professionals design. PowerCalc gets rid of the tedious work to simplify your development of the design…all those thousands of circuits. PowerCalc allows you to stop number crunching and really engineer the power distribution system.

Right now we are working on the renovation of a number of schools in the Broward County School District. The existing mistakes we see are amazing…all because the electrical professional was so busy calculating instead of looking at the design overall, i.e, does the far corner of the building have more power than the center?

And the issues and expense this causes is incredible. As an example, a rooftop unit is to be added, but where is the circuit? Instead of finding the circuit, the electrical professional has gone to the main, and dropped yet another panel, just to feed that roof top unit. There's a couple of schools, they had to add an air conditioning unit for the IT wing, and it's just a small air conditioning to keep the IT room from overheating. The unit itself is probably about three to four thousand dollars. The electrical would be fifteen to twenty thousand dollars just to add the 20 Amp or 30 Amp all because no one knew there was capacity. This design information is just not usually available the way we are designing today, but PowerCalc tracks everything. So, not only does PowerCalc automate the design calculations, but it also automatically maps the entire power distribution design.7

So PowerCalc takes all these calculations from the branch circuit values, the field values, the NEC for the panels, short circuit calculations, voltage calculations, and integrated them into one integrated calculation. Then we put together the one line diagram which summarizes the design and is required at project end.  So, now, if there is a change at the end, the modification of the one line diagram is automatic and no longer a show stopper. Whatever you add, delete or change, is immediately reflected in the one line diagram.

Actual Project

Several actual projects were then walked through using PowerCalc.

Thank you for your interest.