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Leader in promoting the greater use of DC and hybrid AC/DC microgrids & power systems

Occupied Space Standard FAQs

The EMerge Alliance Standard is a new power distribution platform for the use of safe, low voltage DC power in commercial interiors.

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

The EMerge Alliance 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.

  1. Are you distributing DC power everywhere and trying to replace a building’s AC wiring?
    No. EMerge is focused on creating a safe, low voltage DC power distribution layer at the interior level. Conversions from higher voltage AC to lower voltage DC power are made throughout a building at this level in a distributed network of available power. The building still maintains its basic AC branch wiring, which feeds the hybrid DC layer. This keeps low voltage wiring runs deliberately short (typically less than 15 feet). See System Graphics. Native AC “plug” load devices will still utilize traditional AC wall outlets at 120 Volts.
  2. Isn’t DC less efficient than AC power? Don’t you need bigger wires?
    This would be correct if you went a long distance at low voltage DC, but EMerge applications are not designed or implemented that way. In system configurations at 24 Volts with power distribution in NEC Class 2 limited circuits, DC wiring should be as efficient as high voltage AC for these lengths. The same size wire used in AC branch wiring is generally used for the low voltage DC connections. The difference is that only two wires are used for DC versus three wires for AC.
  3. Why is the standard set for 24 Volt DC?
    Power over 30 Volts is not considered “safe” when conductors are exposed (without insulation) and must be enclosed in metal jackets, conduits and enclosures. Power less than 24 Volts (i.e. 12 Volt DC systems) can create more significant power losses in the wiring. The selection of 24 Volt DC provides the dual benefits of flexible, modular wiring and safe, efficient power distribution. Also, many digital devices used within commercial interiors today that inherently use DC power – such as occupancy and day lighting sensors – are already based on this voltage.
  4. What are safe, class 2 power levels as defined by the National Electrical Code?
    Class 2 circuits are power limited to 100 Volt-Amperes (Watts) and can be installed using more relaxed Class 2 wiring methods. A Class 2, 24 Volt DC circuit can deliver up to 4.1 Amperes of current and provide acceptable protection from electrical shock and fire initiation.
  5. What about line losses?
    Again, the 24 Volt DC power distribution is not used for distances that materially affect efficiency. AC-DC power servers will be dispersed throughout a space in order to purposely keep their output cable lengths short. See System Graphics. This means that individual power supply components will likely be designed to handle from 100 Watts to 2 Kilowatts. Distribution wiring is maintained at 12 AWG, the same as used in traditional AC wiring circuits. However, because they carry “safe” low voltage DC, they do not generally require metal jacketing, metal junction boxes, ground wires and other expensive or cumbersome protection means. The length of low voltage cables that connect power supplies to distribution busses are also limited by the Standard.
  6. Can any lights be used in this type of system, or only LEDs?
    Many types of lights and other electrical devices can be used, including but not limited to solid state lighting. LEDs are particularly well suited for direct integration with 24 Volt DC distribution because their electronic drivers are inherently DC-based. However, electronic fluorescent lighting fixtures can also be easily made or modified for low voltage DC distribution since they too are driven internally on DC power. A building’s AC power typically gets converted from AC to DC at the point of use in many types of fixtures and devices.
  7. I don’t use low voltage lighting. How is lighting performance affected?
    Luminaires with DC input ballasts or LED drivers use the same lamping, have the same output lumens, have the same output wattage per fixture, and have the same lighting performance characteristics as AC-input devices. The key change is that the electrical input of the device goes from high voltage AC to low voltage DC. This simple change can have the important added benefit of improving the fixture’s efficiency and reliability. For LEDs, the efficiency gain can be significant and is estimated at 10-15 percent.
  8. How does the Standard relate to ASHRAE 90.1, Title 24, UL and related industry codes and guidelines?
    All components used in EMerge compliant systems need to meet their own product-specific or industry-specific codes and guidelines (electrical and/or structural). The Standard details for members what codes and guidelines are relevant for typical applications of the Standard. EMerge has been working closely with key bodies like ASHRAE and UL where additional guidance may be necessary for engineers and specifiers. More details in this area are expected later this year.
  9. What is the difference between AC and DC power?
    AC – or alternating current – is the power that comes from standard wall outlets and is well suited for traversing long distances (i.e. from a power station to a building). The power that naturally comes from batteries and solar panels is direct current, or DC. Some electrical devices use AC power and some use DC power. Generally big motors and older (buzzing) fluorescent lighting use AC power while modern digital devices (IT equipment, sensors, etc.) and electronic lighting use DC power.
  10. Can DC power be changed into AC power?
    Yes, but with some efficiency loss. An AC/DC power inverter is used to allow AC-powered things (like today’s buildings) to work from DC power sources (like a photovoltaic Solar Panel).
  11. Can AC power be changed into DC power?
    Yes, but with some efficiency loss. Think of the “brick” you use to charge your digital devices, like laptops or cell phones. This is an external converter for changing AC to DC power. However, most conversions occurring in buildings today are “invisible,” occurring internally within power-using devices, like electronic lighting ballasts. This causes them to be relatively inefficient due to scale and cost considerations of the individual converters. It’s generally better to make AC to DC conversions with more efficient components in more economic, larger scale converters.
  12. Can you co-locate alternating-current (AC) and direct-current (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 using standard 20 Amp 115-277 Volt AC branch circuits from a local breaker panel.
  13. Can this DC architecture be scaled to full-scale facilities?
    Yes, it can be scaled from a single room to an entire building utilizing multiple converters organized in a distributed network with short DC wiring runs. It generally does not require the changing of a building’s basic AC distribution of power.
  14. Is anyone else doing something like this with DC in buildings?
    Yes, the telecommunications industry has been using DC power systems in their central office switching facilities for many, many years with great success and significant reliability. More recently, there are several significant efforts underway to move data centers toward the use of DC power. This is being led by organizations like the Electrical Power Research Institute (EPRI) and companies such as Intel, NTT and Sun Microsystems. In addition, most public transportation systems use DC power in the U.S and around the world.
  15. Does the standard address loads with induction motors?
    Most of the devices expected to be used in the 24 Volt DC portion of the power distribution will not involve large motor loads. However, there is a significant opportunity going forward as induction motors are more frequently being controlled with adjustable speed drives. These drives inherently convert the AC to DC before conditioning the output to control frequency. Therefore, DC can be applied directly to the electronic drive to power the motor. Some motors have already been converted to DC motors so they can be powered directly by DC.
  16. Isn’t DC power more dangerous than AC power?
    Either type of electricity at high voltages can be dangerous. That’s why the EMerge Standard only allows 24 Volt DC at the point of use. 24 Volt DC power is significantly safer than any line voltage AC system (such as 120 Volt AC wall outlets or 277 Volt AC branch distribution). This is why it is allowed for use without significant restriction/protection by the National Electrical Code and is recognized by UL and other regulatory entities.
  17. Are there any additional safeguards built into the Standard?
    Yes. The DC distribution protocols in the Standard have built-in current-limiting fault protected circuits that work much like ground fault devices sometimes used in AC systems. The difference is that every DC circuit is protected and when the fault is removed, the power in the 24 Volt DC circuit is automatically restored, so no reset button or switches must be toggled by the user to restore power.
  18. Solar Photovoltaic panels are designed for voltages higher than 24VDC. How does this work in an EMerge-compliant system?
    Most renewable energy sources produce native DC power. Depending on the size of an alternative energy installation, this can range in voltage from under 10 Volts to over 350 Volts. Converting power from one voltage to another (whether it is AC or DC) always generates some efficiency loss. The most efficient conversion is made from one DC voltage to another DC voltage. These conversions are typically made at 98 percent efficiency or higher. So, in an EMerge-type system, a higher voltage DC source input like solar is “stepped down” to lower voltage DC for use in this local distribution system. The overall energy loss will be much less than the typical conversion required for AC power distribution within a building. 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.
  19. What about the increased cost of wire?
    Because no ground wires are needed and the wiring is not required to be encased in metal jackets, the total cost of wiring is expected to be the same or less than conventional AC wiring.
  20. 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 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.). EMerge is focused on eliminating many of the device-level conversions and aggregating them up at a higher level to improve efficiency and flexibility.
  21. How do EMerge applications fit into Smart Grid efforts?
    To put it simply, Smart Grid needs “smarter buildings.” EMerge provides the opportunity for local power use within buildings to be connected more directly into these enhanced macro delivery efforts. Some of the same wireless control technology platforms utilized in Smart Grid efforts will also be deployed locally in EMerge applications at the building level. This should allow buildings to make better use of intelligent data from the Smart Grid to minimize energy use during high cost times.
  22. Where can I see a demonstration of the standard?
    The EMerge website ( has video and graphic resources that show applications. The Alliance also hosts demonstrations at events throughout the year. Some members also have demonstration sites at their facilities. EMerge will share news of public demonstrations and commercial installations as these become available.
  23. What changes were made in Version 1.1 of the EMerge Alliance Occupied Space Standard as compared to Version 1.0?
    The EMerge Alliance Occupied Space Standard Version 1.1 addresses important changes and clarifications, including a narrower allowable voltage range, new inrush limits imposed to reduce voltage fluctuations, stricter polarity protection language, and some nomenclature updates.
  24. If I already have an EMerge Alliance Occupied Space compliant system and equipment, how will I be affected by version 1.1?
    If you are using EMerge Alliance Registered products, this equipment is still compatible. The intention of this and future revisions is to be inclusive of existing system requirements.