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Understanding Design Considerations for Electrical Systems in Residential Buildings

Blog Post created by dvigstol Employee on Apr 30, 2019

What goes into the design of the electrical system in a residential building? The answer to this varies with the complexity of the installation. A single family home has less to consider than a large multi-family apartment complex with common areas and amenities for the residents. In a single family dwelling unit, the major considerations are usually just the size of the service, receptacle outlet layout, and whether or not the appliances are electric or gas. There might be some variations on this - maybe there are outbuildings or a pool, or maybe a PV system - but those are not the norm for the majority of new construction. However, that’s not to say they won’t be in the future.

So what are the design considerations for the service? Well, again, this is determined based on the answers to many questions. What is the square footage of the house? Are the appliances gas or electric? Is there going to be air conditioning? Is the furnace/boiler electric? Do we know what other special electrical needs are being requested by the homeowner? For the most part, most single family homes fall into the 100, 150, or 200 ampere service range and in some cases a 200A might even be a request even though a 100A service would suffice per Article 220 of the National Electrical Code® (NEC®). For instance, my house was probably adequately supplied by a 60A service back in 1938 when it was built, long before the 100A minimum requirement. However, when I bought the house and changed out the service, I was more than happy to pay the extra money for the 200A service since now I know that I will be covered in my electrical needs down the road. This attitude, though, is something above and beyond what the NEC® requires.

The next big consideration that comes up is the layout of receptacle outlets. For the most part, the NEC allows you to place receptacles as you wish with a couple of conditions to keep in mind. Remembering that the purpose of the NEC is the practical safeguarding of persons and property from the hazards arising from the use of electricity, it helps to give some context to the minimum receptacle outlet requirements. When building a home, you often have no idea what types of equipment the homeowner is going to use, what the furniture placement will be, or how the electrical system is going to be used. This is theelectrical reason for certain requirements in Article 210 for maximum spacing of receptacle outlets and for requiring that receptacle outlets be within a certain distance of a feature of the home. For example, there must be a bathroom receptacle outlet supplied by a 20A branch circuit dedicated as a bathroom branch circuit, and located within 3’ of the outside edge of each basin or sink. Why? Because it can be reasonably assumed that the occupants are going to be using some form of hair care or beauty appliance like a hair dryer or curling iron while in that location. This eliminates the need for an extension cord to be used to supply power to this location. Not that the use of extension cords is inherently dangerous, but when used as the normal way to supply power to an area, they are being used in a way that they were not intended to be used and can be exposed to damage if they are left in place.

In addition to the requirement for placement of bathroom receptacle outlets, there are also maximum spacing requirements for receptacle outlets serving counter-tops or other work surfaces, maximum spacing requirements for receptacle outlets in habitable rooms, and minimum requirements for providing receptacle outlets in other areas of a residence where electricity is likely needed such as in the garage or outside. Keep in mind however that these requirements are just a minimum and that what you are asked to consider in the design of the building might be different. However, the final lay out must still meet the minimum. An example that always seemed to come up for me was installing receptacle outlets in a bedroom. Like I mentioned earlier, you don’t always know what the furniture layout will be but sometimes we can take an educated guess. It is often evident in floor plan of a master bedroom as to where it makes sense to put the bed. As a homeowner there is nothing worse than finding that perfect spot for the bed and when you finally get your new room put together, BAM! The only receptacle outlet that is even close to your nightstand is right in the middle behind the headboard. For this reason, I would always lay out the receptacle outlets based on whether a room has a logical spot for a bed. This might have caused me over the years to have put in an extra receptacle outlet or two, but it saved me from being the guy getting cursed out by the new homeowner because they don’t have power where they need it. But again, it’s important that I point out here that the NEC does not require that this approach be taken. The requirement in the NEC is that no point be farther than six feet from a receptacle outlet measured along the floor line. And while the receptacle outlet in the middle of the bed space might be annoying, it is probably also code compliant.

When it comes to design considerations for a residential installation I am always reminded of the fact that the NEC is not a design specification or an instruction manual for untrained persons. This point means that those who are installing systems covered by the NEC are intended to know what they are doing. It is also important to remember that the requirements that are found in the NEC are there to safeguard people and property from electrical hazards and that nothing in the NEC prevents a system from having convenience and functionality baked into the design or providing for peace of mind in knowing that the future was considered in the initial installation, even though it is 100 percent not a code requirement to account for future additions. As long as the minimums are met and the maximums aren’t exceeded, taking more than code into account during the design of residential occupancies can often be the difference between good reviews and rave reviews.

To the electrical contractor, it is a job or a project, but to the homeowner it is their world.

For more about this topic, subscribe to NFPA's NEC Connect Newsletter and visit our NEC webpage for additional information including related codes and standards, helpful products, training, certfications, news and resources, and more.

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