Standard FAQs

The EMerge Alliance DC & Hybrid AC/DC Microgrid Standard creates an integrated, open platform for power, infrastructure, peripheral device and control applications to facilitate the hybrid use of AC and DC power within single power system domains.

The Alliance has prepared a list of Frequently Asked Questions about the EMerge Alliance DC & Hybrid AC/DC Microgrid Standard. If you have questions on topics not discussed in this section, please contact us. Please note that some information about the DC & Hybrid AC/DC Microgrid Standard may be confidential and for member use only.

The EMerge Alliance DC & Hybrid AC/DC Microgrid Standard is available to all Governing, Participating and General members of the Alliance. If you would like to receive a full version of the standard, please join the Alliance.

For questions about the Alliance and its mission, please visit our Alliance FAQs section.

1. Are you distributing DC power everywhere and trying to replace a building’s AC wiring?
   No. The relatively straightforward change in power distribution architecture involves making a single conversion of the system’s incoming line voltage of AC to 350700 VDC, and then combining it on a common buss with other ac and/or dc converted local sources, then distributing it directly to storage and distribution equipment. This approach simplifies the otherwise unnecessarily complex power management provisions generally used in today’s AC-power systems. DC power collection and distribution systems simplify the use of locally generated power including PV, wind, fuel cells and other site-based generation and storage devices and/or combinations thereof. See System Graphics.
2. Why is the EMerge Alliance DC & Hybrid AC/DC Microgrid Standard set for 350/700 VDC?
   350/700 Vdc is the global sweet spot for standardized components with the best balance of economics and safety. These standardized components are commonly used in power supply systems for today’s computers, electric vehicles, solar power, etc. Power distribution at 350/700 Vdc has inherent advantages over lower voltages. 350/700 Vdc is the next generation for many electric power applications, providing enhanced distribution capabilities, lower equipment costs, and improved sustainability with smaller bus and wire sizes and reduced copper. With today’s personnel and circuit protection technology, DC and AC are on par with each other with regard to safety.
3. Does the EMerge Alliance DC & Hybrid AC/DC Microgrid Standard help increase reliability?
   Yes. Since DC power conversion and distribution equipment have fewer components than AC equipment, they suffer fewer heat-related device failures caused by excessive AC to DC conversions. DC can be between 200% and 1000% more reliable than AC, improving system reliability significantly.
4. Can this system help simplify the integration of on-site energy generation and storage?
   Yes. By directly utilizing native dc power from on-site sources, such as solar panels, wind turbines, and fuel cells, a dc power collection and distribution system provides a simple and direct delivery of controlled power.
5. Isn’t DC less efficient than AC power? Don’t you need bigger wires?
   This would only be the case when distributing extremely low voltage DC (or AC) over long distances, but EMerge Alliance standards are not designed or implemented that way. Moreover, 350/700 Vdc actually has the advantage over AC in terms of wire loss. In any case, in most building level power distribution systems it is the conversions of AC to DC and DC to AC that waste the most energy. And although voltage conversions waste some energy, the EMerge Alliance DC & Hybrid AC/DC Microgrid Standard reduces the number of these conversions in a system. Simply put, EMerge standardized DC power distribution reduces net energy loss and thus significantly contributes to improved energy efficiency in most power systems.
6. Can solar photovoltaic panels be used in conjunction with the EMerge Alliance DC & Hybrid AC/DC Microgrid Standard?
   Solar photovoltaic (PV), like most renewable energy sources, produces native DC power. Properly designed, a PV system can deliver its natively DC power directly to the supply bus for the microgrid system with minimal voltage conversion loss. So, in an EMerge Alliance DC & Hybrid AC/DC Microgrid Standard compliant system, the solar PV system can contribute more net energy to the DC bus then to an equivalent AC system which requires an inverter. The Electrical Power Research Institute (EPRI) estimates the efficiency gain for direct use of on-site renewable energy without conversion to AC at 10-15 percent.
7. What about cost?
   Using DC power distribution reduces the total cost of ownership when equipping, installing and operating a power system. By reducing the investment in infrastructure, maintenance and operating costs, DC coupled power distribution allows a greater percentage of investment to be leveraged with more productive information technology assets.
   • • Lower infrastructure (including space) and equipment costs (CAPEX):  DC Power supplies and distribution gear are simpler and require less space than their AC counterparts. This results in generally lower capital expenditure and installation cost.
   • • Lower maintenance costs:  By significantly reducing the number of power conversions occurring in a data microgrid, DC coupled power distribution reduces overall component content while decreasing the amount of heat generated by the power conversion equipment, which in turn lowers maintenance costs by reducing heat related equipment failures.
   • • Lower operating costs (OPEX):  AC to DC conversions, DC to AC conversions and unnecessary voltage conversions consume energy and produce heat in a power system. By reducing the number of these conversions, DC power distribution reduces energy and cooling operating costs.

   While most DC load equipment is just entering the market at relatively low volumes, It has already been reported that a well designed DC system can save at least 15% in capital cost, and 20 % in installation cost and reduce the facility footprint by at least 25% as compared to an equivilent ac coupled system.

8. What are safe, class 1 power levels for DC as defined by the National Electrical Code?
   Class 1 circuits are voltage and current limited according to local code requirements.  The NEC limits Class 1 voltage to under 600V while the IEC extends a similar requirement to 1500V.   350/700 Vdc circuits operate well within these long established safety standards.  The connectors, wiring and protection requirements specified in EMerge Standards require all EMerge registered products to be certified and/or listed to the appropriate authorities having jurisdiction.  EMerge Standards are application standards and as such are completely subordinate to government and industry recognized safety standards.
9. What is the difference between AC and DC power?
   AC – or alternating current – is the type of electrical power that comes from standard wall outlets and has been historically used to move electrical power produced by rotating generators at centralized power stations to buildings where it is consumed.  Technically, when we say “277 VAC” for example, a voltage commonly used in commercial building branch wiring, we actually mean a regularly oscillating voltage that ranges from  a peak of plus 392 volts to peak of minus 392 volts.  In North America, this oscillation or cycling takes place at a frequency of 60 times every second.  While the average (RMS) voltage in this example is 277 volts, the maximum voltage is 392 volts. This is important to understand since insulation and safety equipment must protect against the highest voltage not the average.
   DC  – or direct current – is the type of electrical power that typically comes from batteries, wall chargers, fuel cells and solar panels and has been increasingly used in electronic devices from cell phones, computers, LED lights and fluorescent ballasts to whole systems in cars, ships, trains and planes.  For DC systems, maximum and average voltages are the same, because DC voltage doesn’t oscillate.  A 350/700 Vdc circuit is just that, 350/700 volts, no more, no less.  In addition to making insulation, safety and circuit protection a straight forward proposition, it also makes DC easier to control and simpler to use when integrating multiple sources of power.
10. Can DC power be changed into AC power?
    Yes, but with some efficiency loss and additional equipment complexity.  A DC to AC power converter (often called an inverter) is used to allow AC-powered things (like today’s buildings) to work from DC power sources (like a photovoltaic Solar Panel). While this is relatively straight forward if the DC is the only source of power, additional consideration must be given when the DC power is being combined with AC power from a generator or utility feed.  If the devices powered by the distribution system are natively DC (i.e. servers and other data/telecom equipment), the power must be converted back to DC at the device level.
11. Can AC power be changed into DC power?
    Yes, but with some efficiency loss. An AC to DC power converter (often called a rectifier) is used to accomplish this task.  Think of the “brick” you use to run and/or charge your digital devices, like laptops or cell phones. This is an external converter for changing AC to DC power. However, most AC to DC conversions occurring in buildings today are invisible, occurring internally within devices, like electronic lighting ballasts or video displays. This causes them to be relatively inefficient due to voltage level, scale and cost considerations of the individual converters. It’s generally better to make AC to DC conversions at higher voltages with more efficient components in more economic, larger scale converters.
12. Can you co-locate AC and DC equipment in an installation?
    Yes, the two systems can operate independently so traditional AC devices can be used in the same space. Typically the DC system will have one or more AC feed connections to it. In some cases, where excess DC power is available, the AC system will have a feed connection to it from an inverter.
13. Can you connect multiple sources of both AC and DC power to feed the same distribution system?
    Yes, and this in increasing required due to the need for higher levels of site based back-up supply and the growing desire to use site based renewable sources of power.  To connect multiple sources of AC power (utility feed, AC generators, etc.) together, the oscillating voltages (frequencies) must be matched or synchronized and voltage adjusted (regulated) to the “system” voltage.  Typically this synchronization is accomplished by taking one (or both) of the AC sources and converting them to DC so the frequencies and voltages can be “adjusted” electronically to match. To connect DC sources (i.e. solar, wind, batteries, fuel cells, DC generators, etc.) with AC sources, the DC must first be converted (using an inverter) to AC and the voltage adjusted to the system voltage. This AC source must then be electronically synchronized with the first one.  This process is repeated for each new AC or DC source that is connected. Each additional connection significantly raises the risk of dynamic and transient voltage instability and propagated disturbances between the sources and the loads, particularly if the added sources are intermittent (i.e. solar, wind, etc.).  Even small mismatches in synchronization can have catastrophic impact on the feed circuits, the power distribution system and loads being connected.  This includes possible equipment damage and/or total power loss due to the high fault currents produced. To connect multiple sources of AC to a DC power distribution system, the oscillating voltage of each AC source must be converted (using a rectifier) to DC and its voltage adjusted to the system voltage.  Since the DC does not oscillate, no synchronization is necessary.  There is no practical limit to the number of sources that can be combined together as there is little risk of fault currents being produced. To connect multiple sources of DC power together no conversion or synchronization is necessary.  The voltages of the added sources must be simply adjusted to the “system” voltage.  There is no practical limit to the number of sources that can be combined together as there is little risk of fault currents being produced.
14. Can this DC architecture be scaled to full-scale facilities?
    Yes, it can be scaled from a single system application to modular additions and even to entire buildings and capuses utilizing a microgrid distributed network topology. It generally does not require the changing of a building’s basic AC feed or distribution systems while making the incremental or evolutionary scaling task as well as future integration of on-site renewable or other energy sources easier.
15. Is anyone else doing something like this with DC in buildings?
    Yes, there are several significant efforts underway to move many power systems and microgrids toward the use of DC coupled power. Being aided by the continuing development of additional EMerge Alliance standards, organizations like such as Intel, NTT, Ford Motor Company and others are working on practical examples of this.  In addition, most telephony and public transportation systems in the U.S and around the world already use DC power systems.
16. Isn’t DC power more dangerous than AC power?
    All electricity at high voltages (above 60 volts in the US) should be considered dangerous and thus safety agency recognized equipment and personnel safety practices should be strictly adhered to in its use.   The 350/700 Vdc level called for in the EMerge Alliance DC & Hybrid AC/DC Microgrid Standard is the global sweet spot for both safety and economics when it comes to already recognized components. These standardized components are commonly used in power supply systems for today’s computers, electric vehicles, solar power, etc.  Safety organizations like NFPA, UL, IEC, ETSI and others have already recognized the value of DC systems and are working on making their requirements for its safe use more explicit.
17. Are there any additional safeguards built into the Standard?
    Yes. With today’s personnel and circuit protection technology, DC and AC are on par with each other with regard to safety according to most internationally recognized safety organizations.  In addition, EMerge Alliance standards will not call for, or allow, the Registration of products that do not meet the requirements of any authority having jurisdiction in building codes or safety.
18. Won’t you end up having two electrical distribution systems in a building, one AC and the other DC?
    No, it’s still one system that can deliver both forms of power. The AC and DC portions of the system are interconnected at the distributed power supply level in the EMerge Alliance DC & Hybrid AC/DC Microgrid Standard. The DC portion of the power distribution system is generally not redundant with the core AC portion of a building’s electrical system. The more efficient conversion of AC power to DC — and the optional connection to native DC power from alternate energy sources — are done in bulk for all devices in a defined local area where a traditional AC system would be branched to circuits that feed individual electrical devices (like light fixtures, etc.). The Alliance is focused on eliminating many of the device-level conversions and aggregating them up at a higher level to improve efficiency and flexibility.
19. How do EMerge Alliance standards fit into Smart Grid efforts?
    To put it simply, Smart Grids need “smarter buildings.” Use of EMerge Alliance Standards provides the opportunity to optimize local power use within buildings while still being connected to and even supporting the macro grid. And because the EMerge Alliance is working to use the same wireless control technology platforms utilized in Smart Grid efforts, this will allow for future integration opportunities in combined control and/or power management.  This should also allow buildings to make better use of intelligent data from the Smart Grid to minimize energy use during high cost – low availability times.
20. Where can I see a demonstration of the EMerge Alliance DC & Hybrid AC/DC Microgrid Standard?
    The EMerge website (www.EMergeAlliance.org) has video and graphic resources that show applications. The Alliance also hosts demonstrations at RE+ events throughout the year. Some members also have demonstration sites at their facilities. The Alliance will share news of public demonstrations and commercial installations as these become available. To stay informed, please join our Interest Community.