More DC? Yes, Please! Who really won the current war, part 2

Published On
Aug 13, 2021

Last month, I presented an overview on the possibility of DC wiring in the home and office. But how DC will be distributed in these buildings is a subject of much discussion and debate. Below are some of current—pun intended—considerations.

While researching this subject, I found there is wide acceptance for the idea of moving toward installing DC wiring in new buildings. However, I found no agreement regarding the standards for these installations. Let’s look into a few of these discussions.

Safety

One of the arguments for low-voltage DC wiring is that it is safer than AC at higher voltages. This was very difficult to verify. The consensus of information from scientific, engineering and collegiate articles was that DC is safer, but not for the reason you might think. The amperage that gets through your skin is responsible for injuries and deaths with AC and DC. The resistance of human skin is highly variable for a number of reasons, including the skin’s level of moisture, internally and on the surface. The consensus I found was that AC is as much as five times more dangerous than DC due to its frequency. It turns out 60 Hz interferes with your heart rhythm.

Voltage

Now that we know DC is safer, what voltage will the standard be? I have read arguments for voltages from 5 to 48. A popular proposal is the standard USB outlet at 5V. This is already in use for millions of cellphone and tablet chargers. Some point to the combination duplex receptacle/USB outlet as a way to upgrade existing installations. However, this defeats the effort to get rid of inefficient AC to DC converters, because there would be one in every receptacle. Also, USB outlets can only power low-amperage loads at this time. Of course, proposals for 12V are numerous. Proponents point to recreational vehicles as examples of how dual AC/DC wiring systems can be configured. Most of the wiring is DC, with a few receptacles for convenience power and larger appliances. Some argue that 48V is superior because it will have less voltage drop. As you can see, it may be a while before standards for voltage and outlets are developed.

Wiring systems

The proposal for dual-voltage wiring for recreational vehicles is popular because it offers the most convenience to users. However, some people argue that dual wiring is a waste of resources and is ecologically unsound. They quote a statistic that claims the average house contains 400 pounds of copper wire, so why add another wiring system? An all-DC house is a future possibility. Large appliances and HVAC units are already being developed to run on DC. The proponents of DC-only wiring are also calling for changes to the National Electrical Code to allow for fewer AC receptacles.

Some of the people I interviewed are concerned about the lack of guidance from the NEC regarding DC installations. I was told several times that local inspectors lack the knowledge to ensure DC installations are safe. I even heard a story about a fire caused by the poor installation of a solar panel array. The NEC has been and will continue to be updated to cover these new technologies. In the meantime, know what you are doing and use common-sense practices to provide safe installations.

The configuration of future DC wiring installations will depend on what standards are adopted. One proposal I read called for existing AC wiring to be reused for DC wiring. The existing circuitry could carry the same current loads no matter which DC voltage is used.

A future building today

A concept I just learned from one of my students is the nano grid. This type of installation is possible now with technologies such as photovoltaic panels and roof tiles, natural gas generators and battery storage systems. These technologies all generate, store and distribute DC power, with no AC/DC or DC/AC conversion losses. The generator is for backup purposes only and used when the solar battery is not charged enough to get a home or office through the night.

The impact on estimators

As estimators, it falls on us to keep up with all these changes. I am fortunate that several of my associates, customers and students are at the forefront of these new technologies. They, and the internet, have made it possible for me to learn how to estimate these new systems as they appear. Education is the key to continuing a successful career as an estimator.

About the Author

Stephen Carr

Estimating Columnist

Stephen Carr has been in the electrical construction business since 1971. He started Carr Consulting Services—which provides electrical estimating and educational services—in 1994. Contact him at 805.523.1575 or steve@electrical-estimating.com, and...

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