Understanding Load Calculations and the 2020 NEC
If you haven’t seen or heard yet, the 2020 edition of NFPA 70®, National Electrical Code® (NEC®), is set to hit the shelves this fall. As we at the National Fire Protection Association (NFPA) are winding down our efforts on the 2020 revision cycle, electricians, contractors, and designers all over the country will be springing into action to learn what is new or revised. Many will spend nights and weekends combing through the book, attending update classes, and coming to terms with how the new version of the NEC will impact their world.
This article looks at one change that will have an impact on most non-residential installations going forward. A revision in Art. 220 affects many long-held values that have been used to calculate the electrical load in a building for many years.
When was the last time you sat down and looked at how the NEC calculates loads in Art. 220? If you are like some folks in the electrical construction industry, it probably isn’t something that you spend your nights and weekends obsessing about. However, that doesn’t mean that revisions made won’t influence your day-to-day work. It’s always important to understand the reason for a Code rule change and the background that led to the Code-Making Panel’s decision.
Over the course of the last couple revision cycles, questions have arisen around whether or not the load calculation values are still applicable in this day and age of energy efficiency and expanding technology. Back in the day of incandescent and T12 fluorescent lamps, 3.5VA/ft2 might have made sense for general lighting load calculations. Does that value still apply to today’s streamlined, energy-efficient office space full of LED luminaires and Power over Ethernet (PoE) technology? People started asking these questions a few years back, which spawned a Fire Protection Research Foundation project to look at the need to adjust the way feeder and branch circuit loads are calculated. The report, “Evaluation of Electrical Feeder and Branch Circuit Loading: Phase 1,” can be found on the Electrical News & Research section of the NFPA website.
In the interim, the NEC gave some wiggle room for installations being installed to meet local energy codes; however, there were additional conditions that needed to be put in place like a monitoring system to ensure that the power consumed doesn’t exceed the set values. Also, no demand factors were permitted to be used if the load was calculated based on lower values specified in an energy code. So, in the 2017 NEC, another exception was added to Sec. 220.12 that allowed the general lighting load to be reduced by 1VA/ft2 in office or bank areas of a building when the overall lighting density in the building was less than 1.2VA/ft2. These revisions set the tone for what was coming during the 2020 NEC revision cycle.
There are many revisions to talk about in Sec. 220.12, but let’s start with a few that were more reorganizational in nature. The first thing you might notice when you look at Table 220.12 is that “Dwelling Units” have been completely removed from the table. Since the general lighting load and the general receptacle load for dwellings have been included in the same number calculated at 3VA/ft2, dwelling unit load calculations for these values was combined in a single section, 220.14(J). While ANSI/ASHRAE/IES Standard 90.1 – Energy Standard for Buildings Except Low-Rise Residential Buildings does list the lighting loads for dwelling units at around 1W/ft2, this value remained the same in the NEC (3VA/ft2) due to the receptacle load being included in this value.
Another hard-to-miss change to Table 220.12 is that the list of occupancies has been largely redone. The reason for doing this is to bring the NEC in alignment with ASHRAE 90.1 and the International Energy Conservation Code. This will probably take some adjustment to the new list for those of us who have been using Table 220.12 for some time now. The good news is that also included in the 2020 NEC Table 220.12 are footnotes aimed at helping users make the transition. Here is the list of footnotes explaining what some of the old occupancy types have become in the revised table:
- Armories and auditoriums are considered gymnasium-type occupancies.
- Lodge rooms are similar to hotels and motels.
- Industrial commercial loft buildings are considered manufacturing-type occupancies.
- Banks are office-type occupancies.
- Garages – commercial (storage) are considered parking garage occupancies.
- Clubs are considered restaurant occupancies.
- Barber shops and beauty parlors are considered retail occupancies.
- Stores are considered retail occupancies.
The next big change that you are going to notice when looking at Table 220.12 is that a lot of the values have changed. Some have been greatly reduced, some have stayed the same, and others have even been increased. Unfortunately, there are no footnotes that state what the old values have become in the revision.
One example of a major change in the general lighting load values occurred with office building type occupancies. The calculation values changed from 3.5VA/ft2 to 1.3VA/ft2. This is a significant reduction in the standard lighting load calculation, since there are no demand factors allowed to be applied to office building lighting load calculations. These were — and still are — required to be added to the service calculation at 100% of the total calculated value. This is going to lead to a significant reduction in service equipment ratings in offices. However, per the energy conservation documents, this is more in line with what the actual demand is. Since the general receptacle load is factored in a separate calculation, either by 180VA per receptacle outlet or 1VA/ft2, this reduction is only applying to the lighting load. Let’s look at an example on how this might play out in an actual installation.
Take, for instance, a general use office building that is 15 stories high and has outside dimensions of 200 ft × 100 ft. This gives us 20,000 ft2 of office space per floor and a total of 300,000 ft2 for the building. In the 2017 NEC, the lighting load would be calculated at 300,000 ft2 × 3.5VA/ft2 for a total load of 1,050,000VA on the service. If the service supplying the building is 480Y/277V, 3-phase, this adds a total of about 1,260A of capacity to the required electrical supply for the building. All things remaining equal in the 2020 NEC, that 1,260A drops to around 470A. A reduction of nearly 800A. This is a considerable reduction in the size of the service and a significant reduction in the capacity of supply equipment that is required by the serving electric utility as well.
One of the questions that is currently being tossed around is whether oversized supply equipment might lead to safety concerns as there are larger transformers, bigger wire, and higher instantaneous trip settings on overcurrent protection devices; factors in what level of incident energy is available at the point where service personnel might be exposed to arc flash hazards. There are varying opinions on this matter, and it has been the topic of many a presentation in the electrical safety world over the past few years.
Vigstol is a senior electrical content specialist with the National Fire Protection Association in Quincy, Mass. He can be reached at firstname.lastname@example.org.
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