Page last updated Oct 20, 2021 @ 10:12pm

Locating Generators


onan-largegen-encl.jpg (14000 bytes)   Outdoor Locations

Gensets that are housed in weather-protective enclosures are designed for installation outdoors. The following summary is from an Onan 12kW Installation Manual but most of the guidance is applicable to other sizes. Since this is only a summary, be sure to follow all the instructions contained in the Installation Manual for your specific model.

Choose a site close to the electric service and fuel supply lines (natural gas, propane, or diesel).  The image below shows a typical natural gas installation.  The main distribution panel, transfer switch and sub-panels are inside the building in this example.

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NFPA 37 2010, section 4.1.4 Engines Located Outdoors states, "Engines, and their weatherproof housings if provided, that are installed outdoors shall be located at least 5 ft. from openings in walls and at least 5 ft. from structures having combustible walls."  Leave at least 5 ft (or more if the housing and instructions for your particular unit require it) all around the genset enclosure for access to the inside (NEC Art. 110-26a, Art. 110-26b).  The genset must be at least 5 ft from any opening (window, door, vent, etc.) in a wall, and the exhaust must not be able to accumulate in any occupied area.  See figure below.  Note that some jurisdictions require larger clearances such as 10 feet.  Verify requirements with your local authorities.

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onan-largedg-open.jpg (21362 bytes)   Indoor Locations

We do not recommend locating generator sets indoors in residential applications and small commercial and industrial applications.  The reasons are primarily related to safety.  In addition to safety, it costs more to install a genset indoors than for the same genset provided with the factory weatherproof housing.

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When a genset is installed indoors, the building must be designed carefully to handle ventilation to remove heat and any fumes due to fuel, exhaust, lubrication and starting batteries.  The radiator must be provided with a duct adapter that interfaces properly with louvers on an outside wall of the building.  Adequate air intake flow must be available not only for the radiator fan but also for cooling the alternator.  The engine exhaust piping and muffler must be tight to prevent any leaks that would allow dangerous carbon monoxide to accumulate inside the building.   Automatic fire suppression systems may also be required.  Check with your local fire codes.  You should also contact your fire insurance provider to determine if an indoor genset is even permitted.

Even installations perfectly designed to meet all the above requirements and all safety codes can become a hazard later.  For an installation to remain safe, it has to be regularly inspected and maintained to ensure that leaks or other dangerous conditions do not develop with age or use.  Sites that do not have a knowledgeable maintenance staff trained to support an indoor generator set should not install a unit inside a building.

An additional factor is the initial cost.  It is impossible to construct a building to house a genset at a cost as low as the factory housing that can be ordered with the genset.  And even if the building already exists, the design expense and costs to adapt it for a generator set installation usually will exceed the enclosure cost available from the genset manufacturer.  For a small genset the cost of an open unit with duct adaptor and exhaust pipe kit is as little as $400 less than the same genset with the factory weather housing.  The additional costs for just the exhaust thimble and louvers exceed that savings.

If you decide to install a genset inside a building here are some technical details that are typical for most designs.
* The radiator side of the genset needs to be ducted to the outside.  Typically the radiator duct area needs
   to be at least 1.5X the area of the radiator.
* Intake louvers need to be mounted on the wall (usually the wall opposite the radiator duct).  The area of the
   intake louvers needs to be at least 2X the area of the radiator.
* The genset will usually come with the muffler and pipe package shipped loose.  The muffler is usually anchored
   to the wall or ceiling in the genset room.  The engine is then connected to the muffler using a piece of flexible
   pipe.  This flex will absorb most of the engine vibration and prevent damage to the muffler mounting.
* The installer will need to purchase a wall thimble.  Because the exhaust pipe gets very hot it must be isolated
   where it passes through the building wall or ceiling.  The thimble performs this function.  To purchase a thimble
   you will need to spacify the wall thickness and what it is constructed of.  Note that propane and natural gas
   fueled gensets burn hotter than their diesel counterparts.
* The genset building should NOT be constructed of wood.  It should be a masonry building.
* Per the National Electric Code you will need at least 3 feet clearance on all sides of the genset.
* Vibration isolators are used between the genset skid and the floor foundation to reduce sound and vibration
   transmission to the building.

As stated in the LLC Terms & Conditions engine driven generators sold by LLC are not intended for indoor operation at residential or small commercial and industrial sites because of safety risks.   The buyer and user assume all risk and liability when located and operated in such applications.


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