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DC Power Distribution is CHANGING

November 25, 2022

Implementing direct current (DC) power distribution in a home or commercial building is becoming increasingly more practical; about 74% of total electrical loads in homes need DC power to operate (if powering electric vehicles and HVAC equipment with a DC motor). And that number is on the rise, as all digital devices require DC power, and our homes and buildings are becoming increasingly “smart”.

Buildings receive alternating current (AC) power from the grid, but with an increasing number of our electrical and electronic devices needing DC power, many people are choosing to implement DC power distribution. Implementing a DC power distribution system can involve DC microgrids, Class 2 low voltage systems such as Power over Ethernet (PoE) or something new called a Class 4 power (CL4) system. Class 4 power systems will be present in Article 726 of the 2023 version of the National Electrical Code (NEC), and we'll discuss the implications of this later in the article. For now, all you need to know is that they provide higher voltages than PoE, but still only require low voltage wiring practices. 

In this article: 

  • We’ll break down the types of AC and DC power distribution systems
  • Electrical class ratings in America’s electrical code book (the National Electrical Code or NEC for short)
  • Discuss the exciting new addition to the 2023 version of the NEC, and what makes it so revolutionary for commercial buildings

AC vs. DC Power Distribution

When considering the distribution of electricity to communities, and throughout buildings, there are really just a couple things you need to know. Firstly, there are two types of electricity: alternating current (AC) and direct current (DC) power. Secondly, DC power is the most efficient to transmit across long distances, and distribute throughout buildings, but AC power is what flows through your walls. Why is that? AC power won the current wars in the late 19th century because the infrastructure for distributing AC is cheaper (due to its compatibility with electrical transformers).

Despite the fact that AC power won the war of the currents, the world has gone through many changes in the last hundred years, and now most of our everyday systems and digital devices require DC power. For example, HVAC systems with variable speed motors (also known as variable frequency drives or VFD’s), LED lighting, and batteries (including electric vehicle batteries) all require DC power. In fact, DC consumption currently makes up about 74% of total electrical loads in homes that use electric vehicles and HVAC equipment with DC motors. Because these devices are getting AC power, they need to convert this AC power into the DC power they need. These conversions are often executed by inefficient power converters in a load’s driver, and can waste upwards of 20% of the energy consumed by a DC powered load. 

Maybe now you’re wondering what makes DC different from AC power. Most of the differences between AC and DC power are a result of the fact that AC power has a frequency, and DC power does not.

One benefit of having a frequency is that it makes AC power compatible with transformers. Electrical transformers are used to step voltages up and down depending on the distribution stage. Transmission of electricity to towns and cities, for example, would need very high voltages, and these voltages must be stepped down for distribution throughout communities, which can be done easily with a transformer. Transformers also isolate voltages for safety to reduce the risk of electric shock and fires.

ac power sine wave graph
Graph of voltage and time for AC power

dc power sine wave graph
Graph of voltage and time for DC power

On the other hand, DC power is not compatible with transformers, so its infrastructure is generally more expensive because distributing it at high voltages additionally requires rectifier stations. Still, there are hundreds of DC power transmission systems because DC power incurs less line losses during transmission, and is therefore more efficient to transmit. DC power’s lack of frequency is what prevents it from suffering as many line losses along cables as AC power. Line losses like the skin effect, capacitive, or inductive losses occur in AC transmission systems, but DC transmission is not affected by them. We made a video all about line losses, and you can check it out here:

Aside from these differences, AC and DC power also differ in how they are distributed, and we’ll cover different types of distribution systems for both AC and DC power next. 

Types of AC Power Distribution Systems

AC power systems are generally categorized by how many phases they have, and the more phases they have, the more efficiently they can deliver higher voltages (to a limit).

These are two of the most common form factors for an AC power distribution system: 

Single (Split) Phase - 3 Wire:

  • In North America the most common electrical form factor in residential applications is what’s known as the “Edison System”, or the single (split)-phase four wire system. This AC electrical system powers either loads that require 120 V, or bigger loads (such as dryers, stoves, and level 2 EV chargers) that require 240 V.
  • This system has 3 wires with an additional wire for ground/earth: 2 live wires (one at either end of the secondary winding of the electrical transformer, 120V each), one neutral wire (0V tapped at the center of the secondary winding of the transformer), and one ground or earth wire (that’s bonded to the neutral once, for safety). 

3 wire electrical system
Single (Split) Phase 3 and 4 Wire

Three Phase - 4 Wire Wye:

  • This AC system is the most common in commercial building applications, and is used to power 120/208 Volt loads, lighting, and smaller HVAC systems.
  • It is also used in larger buildings for loads (like lighting and larger HVAC systems) of either 277/480 Volts (in the U.S.) and 347/600 in Canada. 
  • This is a 4 wire system with an additional wire for ground/earth. 

4 wire electrical system
Three Phase 4 Wire Wye

So now you know the form factor of typical AC power distribution systems with respect to wires, but what does it mean for an electrical system to be either single phase or three phase? And why aren’t there more than three phases? Why not 6 or 12? We won’t dive too deeply into this, but Anaa Lavaa from Linquip explained the difference well. In an article, she wrote:

“The flow of electricity in a single-phase connection is through a single conductor. A three-phase connection, on the other hand, is made up of three independent conductors that are required for electrical transmission.” - Linquip Technews

Essentially, 3-phase AC motors can more efficiently send higher voltages because different/new phases of power are sent in such quick successions that there is never a lull in power supply to a load. See the image below for a better understanding. 

single phase vs three phase
Image Source

Types of DC Power Distribution Systems

While AC power systems are generally divided up by how many electrical phases they have,  DC power distribution systems do not have phases because they don’t have a frequency. Instead, DC power distribution systems are typically categorized by whether they have 2 wires (unipolar system) or 3 wires (bipolar system).

Unipolar DC Distribution System (2-Wire DC System)

This system uses two wires running in parallel to each other, and it’s used to power most electrical loads such as LED lighting, mobile phones, laptops, and small off-grid appliances. This system is also what's used for high voltage DC (HVDC) power transmission. The two wires in this system are simply positive (+) and negative (-), just like the two poles of any battery.

2 wire system
Circuit Diagram - Image Source
2 wire electrical system
Diagram of a 2-wire electrical system with context - Image Source

Bipolar DC Distribution System (3-Wire DC System)  

These systems are less commonly used, but they can double the power provided to a load while distributing the same voltage with reference to earth or ground. They also make two different voltages available at the same time, from one electrical distribution system.

The anatomy of this system is 3 wires: two outer wires (one positive, and one negative), and a ground wire or 0 volt conductor (similar to a neutral in an AC system). 

3 wire electrical system
Circuit Diagram - Image Source
3 wire electrical system
Diagram of a 3-wire electrical system with context - Image Source

Class Ratings for Electrical Systems

Electrical codes throughout the world have also categorized AC and DC power distribution systems in their own way. For example, the electrical code book in the US, the National Electrical Code (NEC), categorizes traditional electrical systems (these provide AC power) under the section “wiring methods”. Then, specialized electrical systems with power limitations are given a class rating of either 1, 2, or 3. These class rated electrical systems distribute either AC or DC power depending on the circuit, and they are typically organized based on fire safety and safety from electrical shock. Power over Ethernet (PoE), for example, is a class 2 electrical system, and it’s power limited because it cannot exceed 60 V DC or 100 W. 

Revisions to the NEC are made every three years, but there’s some especially big news coming for the 2023 version: it will include the addition of a fourth electrical class rating in Article 726. This rating is not power limited (like the other 3 ratings), it will actually be applied to “fault-managed power systems” that safely distribute higher voltages of DC electricity. It will probably even be able to replace traditional electrical systems in some applications because it can safely provide up to 450 Volts DC over cables that closely resemble Ethernet cables (like CAT 5e).

NEC front cover for the 2023 version with class 4 power in article 726

Big news for DC power distribution, and for electrical engineers in general, is the upcoming addition of Article 726, and class 4 power systems, to the 2023 version of the NEC. With systems rated as class 4 under this article, engineers, for example, will be able to save buildings energy and copper. Simply distributing DC power throughout a building makes it more energy efficient because it eliminates the need for our DC powered devices to make an inefficient AC to DC conversion at the load level, as I mentioned above. 

DC powered devices waste energy when connected to an AC source
AC to DC conversions can waste about 20% of the energy consumed by a building

What is a Fault-Managed Power (FMP) System?

No classes have been added to the NEC for decades. However, those in charge of the code decided that fault-managed power systems are important enough to change that. Along with the addition of Article 726 in the NEC, comes a new rating for class 4 power systems, which are fault-managed power systems. They distribute up to 450 Volts DC, with no wattage limit, and do so safely because of their fault management. Let’s talk about what being a fault-managed power (FMP) system means. Essentially, in a fault-managed power system, power cables are monitored continuously by an onboard computer for defined faults, and power is stopped immediately when a fault is detected. There is an additional safety measure that’s always checking to make sure that the fault monitoring is working properly. So, this is a double layered, intelligent, monitoring system, and it’s used in class 4 power systems. 

A class 4 power system monitors for the following fault conditions: 

  • An abnormal condition such as abnormal voltage, current, waveform, or load condition is identified in the system
  • A short circuit
  • Human skin contact on energized parts
  • A ground-fault condition
  • An overcurrent condition 
  • Intentional shorting of the line at the receiving or transmitting end to force de-energization for purposes of maintenance or repair

One example of a Class 4 and fault managed power system is Cence Power's Digital Current™ (Cence HVDC).  

The Proliferation of DC Power Distribution Systems

Digital devices (that require semiconductors and batteries), as well as LED light fixtures, variable speed HVAC systems, electric vehicle (EV) charging, brushless DC (BLDC) motors, and more, all require DC power. But not only do these devices and building systems need DC power, they are also being implemented more often in commercial buildings because of their advantages. For example, variable speed HVAC systems are about 50% more efficient than their AC-powered counterparts, which run continuously. As systems, like HVAC, are replaced with those that require DC power, they will act as tailwinds for the proliferation of DC power distribution throughout commercial buildings, and it will become more and more crucial for building managers and designers to implement DC power distribution for their buildings (whether they be old buildings that require retrofits, or new builds). PoE does not provide enough power to be implemented cost effectively everywhere in a building, so even though it’s a DC power distribution system, now is the time for class 4 (CL4) power systems to enter the spotlight.

Implementing DC power distribution can future proof a building, and boost its value by aligning the electricity distributed to devices, with the electricity they need. This basic revolution in electricity is becoming less like an option, and more like a necessity, as it provides buildings with the electrical foundation to support more digital devices and increasingly become more intelligent.

If you’d like to learn about how to implement a class 4, DC power distribution system, based on Article 726, check out our website for more information.

You can additionally follow Cence on LinkedIn or YouTube to stay up to date on new technologies in the commercial building industry, and the Cence products

blog author image

Erin Law

Erin is the Creative Director at Cence Power. She has a New Media degree from the University of Toronto and 5 years of experience in the communications field. She has also done digital content creation for dozens of clients through her own business called Story Unlocked. Erin loves technology, especially when it makes the world a better place.

Cence Brings Buildings Into The Future

We improve the value of commercial and multifamily buildings with an intelligent DC power distribution system that's pain-free to install. It combines the benefits of low-voltage wiring practices with voltage capabilities of up to 450 Volts DC.

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