Performing Calculations: A Necessity to Apply the Requirements of the NEC

# Performing Calculations: A Necessity to Apply the Requirements of the NEC

NFPA 70, National Electrical Code® (NEC®), is the industry standard for providing our world with safe electrical systems that, if installed and used correctly, will provide the power we need and keep us safe from the dangers that inherently accompany electricity. However, in order for the NEC to keep us and our property safe, those installing our systems must at least meet or exceed the requirements within the NEC. Inevitably, this means that it is likely that at least once in the process, some form of calculation must be performed. Whether we are calculating what size service a building will need, determining the ampacity of our conductors for delivering the power to where we need it, or simply trying to figure out how many receptacle outlets are needed on our kitchen countertop island, chances are you will have to perform some sort of calculation in order to apply the requirements of the NEC and provide that installation that safeguards people and property from the hazards that arise from the use of electricity.

Let’s start by taking a look at the process of load calculations since this is a step that must be taken prior to a single raceway, conductor, or box being installed. After all, until you know what equipment is needed, how are you supposed to know what to install? This is where Article 220 of the NEC comes into play. The values determined by the requirements in Article 220 are what sets the stage for the entire electrical system in the building. It would be safe to say that a firm understanding of how Article 220 works is important for the system designer to have. At the very basic level, Article 220 can be broken down into two approaches: branch circuits and non-branch circuits.

Starting with branch circuits, let’s break these down into branch circuits that we know the load value for and for those we don’t. For the most part, when calculating the load for equipment that we know will be installed, we can simply use the value provided from the equipment manufacturer as the load we will use. For example, if we will be installing a 5000W heater in the garage bay of a major service center that works on automobiles, we pretty much know that we will need to have a circuit that is capable of supplying the equipment. Other requirements, such as 210.19, may require the branch circuit to have a rating larger than the 5000W, but we know that we must at least supply what the heater is rated for.

The other option for sizing branch circuits applies to when we don’t know the exact load to be connected. General lighting and convenience-type receptacle outlets tend to fall into this category. This is a question that comes up in conversation quite often. How can we know what size circuit we need if we don’t know what will get connected down the line? Article 220 often handles these cases by assigning a unit load either based on the number of devices/equipment or based on the building’s square footage. Dwelling units are a prime example of this practice since it is very common for the installer/designer to have no idea what will get connected in the long run. Therefore, the general lighting and receptacle load is calculated at 3 VA per square foot. This allows the service to be configured so that there is enough capacity to handle most of what will be connected within the dwelling.