Genset Weather Resistant Specs

Pritchard Brown Weather Resistant Genset Enclosure Specification No. 2120

(Replaces P-B Specification No.9920)

Design Criteria:


Weather resistant enclosure. (See Definitions below)

Rated to a wind load of 80mph.
Roof load equal to 40 lbs/ft2
Distributed floor load to 200 lbs/ft2
Basic structure meets all seismic requirements of Zone 4 or equivalent.


The enclosure will consist of a roof, two sidewalls, two end walls, and an optional floor/underframe incorporating prepainted aluminum stressed-skin semi-monocoque construction.


The roof shall incorporate a positive camber and be comprised of a mill- finish 6063-T6 extruded aluminum perimeter channel or “roof rail” with 16 gauge (minimum) roll-formed galvanized crossmembers or “roof bows” mechanically fastened to the roof rails. In a drop-over (no floor/underframe/tank) enclosure, two (2) roof lift rings per side (4 total) shall be located within the roofrail perimeter for the purpose of handling the enclosure.

The roof skin shall be a nominal 0.040-inch 3003-H16 or 3105-H14mill-finish aluminum sheet and be securely fastened to the roof bows and roof rails. The top skin shall be rolled over the perimeter of the roof rails to provide additional weather protection. A weatherproof mastic/sealant shall be used at the perimeter, as well as any joints required in the roof skin. The top skin shall be comprised of separate sheets of roof skin joined with lock-seam construction. This joint(s) shall include a high performance mastic tape or sealant for both joint structural integrity and weather protection.


The walls shall be manufactured utilizing mill-prepainted 0.040 inch (nominal) 3004-H36 aluminum panels hard-riveted to fabricated aluminized steel “Z” section wall posts located on 24 inch (maximum) centers. The enclosure walls shall incorporate an extruded structural “panel-cap” of mill finish extruded 6063-T6 aluminum.

The panel-cap will interlock into the adjoining roofrail for a weatherproof structural connection between the roof and sidewalls. The bottom exterior of the sidewalls will incorporate a mill finish extruded 6063-T6 aluminum “rubrail” for a structural connection of the sidewalls to the base perimeter or enclosure tie-down frame (drop-over enclosure, only).

Corner Posts:

Corner posts shall be pairs of mating/interlocking mill finish 6063-T6 aluminum extrusions with one-half of each pair attached to the end of the wall. The halves will interlock upon assembly forming a structural, weatherproof corner.

Removable End Walls (optional):

End walls shall be of a removable type retained with 0.250-inch stainless steel thread-forming hardware to facilitate equipment installation and maintenance. Available on all enclosure end walls without adjacent intake/exhaust plenums.

Insulation and Lining (Optional):

Thermo-acoustic insulation or a thermo-acoustic composite material shall be installed on the interior roof and wall panels of a weight and thickness consistent with the specified level of noise reduction. The insulation shall be covered with mill finish 0.032-inch thick (nominal) perforated aluminum interior lining for the purpose of protecting the insulating medium as well as allowing noise to permeate the absorptive material.

Floor/Underframe/Rupture Basin (if applicable):

The floor structure shall be rated for a minimum distributed load of 200 lbs/ft2 and reinforced as required to support prevailing point-loading. The floor and underframe assembly shall consist of rectangular steel tubing or structural I-beams welded to form the outer perimeter. This perimeter shall be combined with formed or structural steel crossmembers (nominal 16-inch centers) to create a welded steel support structure for the installed power generation equipment. Steel channels shall be incorporated into the floor structure for adequate structural support and attachment of the generator set and vibration isolators.

The crossmembers shall be overlaid with a composite of 0.72-inch (nominal) oriented strand board (OSB) covered by 14 gauge (minimum) diamond plate steel for the purpose of load distribution, vibration isolation and sound attenuation. The diamond plate sheet shall be coated with a wear-resistant, high quality anti-corrosive material. Truss head screws shall be inserted in optimal locations through the diamond plate sheets to establish a ground connection to the underframe crossmembers.

Fuel Tanks:

The fuel tank shall be installed beneath the floor and shall be listed as a “primary containment aboveground tank for flammable and combustible liquids” in accordance with UL Standard No. 142 and mounted within a combined rupture basin/floor/underframe. The interstitial space between the tank and basin shall be monitored (through electronic means) to indicate a rupture condition (see below, Tank Monitoring Systems). Fuel tanks shall be available as standard manual-fill tanks and remote location-fill tanks (day tanks). All fuel tanks will include drainage plumbing, supply/return lines, and supply valve control.

The fuel tank will be outfitted with an electronic fuel monitoring system. The system shall be comprised of a programmable, digital process meter, and an electronic fuel level sender. The system shall be powered by nominally 11 to 38 VDC. Output from the sender shall be 4-20 mA and shall be wired directly to the input of the process meter. Local digital LED display will read directly in U.S. gallons (or other units as specified). The standard configuration shall include local indication of “FUEL FULL” and Form A output contacts for “HIGH LEVEL” and “LOW LEVEL”. An independent float operated contact will be supplied for indication of “RUPTURE” (leak) condition. Optional monitoring systems shall be available which include 4-20 mA analog output, additional relay output, additional display features, and optional enclosures.


The door(s) shall be fabricated utilizing mill-prepainted 0.040-inch (nominal) 3004-H36 aluminum panels. The door(s) shall be incorporated into fabricated 14 gauge (minimum) aluminized steel opening frames. The door(s) shall include stainless steel butt-type or aluminum piano-type hinges with stainless steel pins. The entry hardware shall consist of a key-lockable D-ring style cadmium plated latch assembly. Door(s) shall include a positive restraint to prevent the door from opening more than 100 degrees in normal operation.

Lift Rings:

Lift rings shall be provided at the base or tank perimeter for the purpose of lifting the complete enclosure with installed genset and empty fuel tank (if applicable) into place. The lift rings shall be fabricated of 1.25-inch (nominal) steel plate and welded into the base or rupture basin (if applicable) perimeter at 4 locations.

Air Handling Devices:

Inlet and exhaust air handling equipment shall be designed so as to maintain a combined total maximum static pressure drop of 0.5 inches of water gauge through the enclosure, including all air handling devices.

Inlet air will be through a fixed or operable louver, weather hood, acoustigrid, or a combination thereof, as specified, and shall provide the necessary level of attenuation. Inlet air handling devices shall be sized and designed so as to minimize the entrance of debris, rain and snow. Inlet openings will be screened to prevent the entrance of rodents, miscellaneous debris, etc.

Dampers shall not be used as inlet air handling devices, except where incorporated within the primary air handling device for the purpose of providing a controlled temperature environment within the enclosure. Gravity dampers shall not be used in an inlet application.

Air discharged from the enclosure shall be through gravity or motor operated discharge dampers, weather hoods, acoustigrids, plenums, or combination thereof, and shall provide the necessary level of attenuation. Discharge openings in fixed devices shall be protected with screen.

Third party test data shall be available to support the devices used within the air handling system to insure that the maximum allowable 0.5 inches watergauge static pressure drop has not been exceeded.

Available Options (specify when required):

  • Interior Lining:
    • Perforated mill finish aluminum
    • Stucco-embossed white or mill finish aluminum
    • Smooth prepainted aluminum
    • Plywood
    • Others on request
  • NEC/NFPA Electrical Package.
  • Mechanical installation of customer-furnished equipmen
  • Electrical installation of customer-furnished equipment
  • Gas detection systems
  • Combination fixed louvers with motor operated dampers
  • Sand filters
    Sand louvers (highly restrictive, suitable only for small gensets or for remotely cooled gensets)
  • Enclosure removable from base or tank
  • Electronic Fuel Monitoring System Options (Additional relays, alarm conditions, etc.)
  • Load testing
    Sound attenuation _*_ dB(A) @ 1 meter

    *Choose 10 or 20 dB(A) reduction


Weather Proof– Allows moisture or the elements into the enclosure in amounts less than that which will ultimately contact “live” electrical equipment.

Weather Resistant or Drip-proof– Allows moisture or the elements into the enclosure in amounts less than that which will cause installed equipment to malfunction or fail. (See Specification No. 2120)

Sound Attenuated– Reduces source noise by a minimum of 3 dB(A). Levels of reduction through 40 dB (A) are routinely specified depending on site conditions and prevailing local codes. (See Specification No.’s 2120 or 2130)

Louver– A fixed or movable blade device used generally for inlet air handling where the blades, when fully open, are at an angle opposed to inlet airflow. Louvers are generally not used in an air discharge application due to their restrictive qualities. The movable blade or operable louver is either manually operated or motorized with associated controls. Louver blades may be of fabricated or extruded design, with extruded blades of the drainable type offering superior weather protection.

Dampers– Operable blade air handling devices generally used in a radiator or fan discharge opening. They may be used within an inlet air opening otherwise protected by an inlet air handling device such as a louver, inlet hood or acoustigrid for the purpose of controlling the internal enclosure environment when the genset is not operating. Since their blades, when fully open, are at 90 degrees and offer no weather protection, dampers should never be used as primary inlet air handling devices. Inlet dampers are generally motor operated, while exhaust dampers may be gravity or motor operated.

Hood- An inlet or discharge air handling device typically covering an inlet or discharge opening and used for weather protection and/or sound attenuation.

Acoustigrid– A sound attenuating inlet or discharge air handling device comprised of acoustical materials arranged in a matrix or grid. When used as an inlet device, an acoustigrid is set within the enclosure wall at an angle so as to minimize weather penetration and permit self-draining.