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Jul 26
2017

What Are the Top Considerations When Designing a Generator Pad?

By: Patti Stewkesbury

 

When purchasing a generator, there are many details to consider. In just installation alone, there are a wide variety of aspects to consider like airflow considerations, fuel availability, exhaust flow, sound, etc. The list goes on. Included on this list is the pad that the generator sits on. Before the details of the installation are concrete, there are a few considerations that require attention to ensure the mounting pad is designed appropriately:

  • Size – How large will the footprint of the pad be?
  • Composition – What type of concrete mixture is suitable for the application?
  • Thickness – How thick must the pad be to support the weight of the generator set?
  • Conduits/Stub-up Areas – Where will all relevant wires need to come up through the pad?

Size: The pad footprint is likely the quickest and most simple to determine. Using the generator footprint, simply add the desired clearance to all sides to determine the overall footprint of the pad. Kohler’s manufacturer recommendation is a minimum of 6 inches on all sides. Kinsley has seen many installations through the years, and recommends a minimum of 12 inches on all sides.  Our added 6 inch buffer will help to minimize site location issues, while allowing technicians to stand on the pad while servicing or repairing the generator sets, rather than being forced to service the generator at an awkward position.

Composition: The recommended concrete mixture for pads is described in Kohler’s TP-5700 Generator Set Installation Manual Section 3.2.4 Mounting Pad Specifications. To quote this section, “The recommended concrete mixture by volume is 1:2:3 parts of cement, sand, and aggregate, respectively.”

Thickness: The thickness of the pad is a function of the weight it must support. For the pad to be structurally sound, it must weigh equal to or more than the load it is intended to support. Based on the above mixture, the density of the pad should be approximately 150 lbs/ft3. Using the required weight, and the known density, calculate the total volume required by dividing the overall weight by the density. Knowing the footprint (surface area) of the pad, and the total volume required, calculate the thickness by dividing the volume by the surface area. See equations below:

 

Conduits/Stub-up Areas: This consideration is perhaps the most important. The conduit cannot change location once the concrete is poured, so it needs to be positioned correctly the first time. Depending on the generator, this task can be a difficult one. Ideally, the conduit is brought up directly under the junction box where the load leads will connect to the breakers or the alternator bus. For generators without a subbase tank, the area between the skids on the alternator end is likely to be mostly open, not restricting where the conduit can be brought up. Even if the conduit is not directly under the junction box, the leads are not restricted and can be connected regardless.

 

Things get tricky when a subbase tank gets involved. The area open to the bottom for a tank supersedes the location of the junction box, because if the tank is in the way, wires cannot be pulled through it. This area of the tank varies, and it does not always align with the location of the junction box. Furthermore, the area of the tank open to the bottom and the location of the junction box are typically on separate drawings making it difficult to determine actual location. Lastly, the tank potentially extends past the skid of the generator, so these drawings are difficult to match up. Improper placement for the conduit location can end up costing a lot of time and money; it’s best to consult with a generator professional beforehand.

              Situation A: Ideally, this is how all the generator sets would line up. The location of the junction box is inside the area the tank is open to the bottom. There should be no issue when pulling the conduit/stub-up. In the example pad drawing below, the hatched rectangle is the junction box location and the immediate blue box around it is the tank area open to the bottom:

              Situation B: In some generator set configurations, the tank area open to the bottom and the junction box location do not line up nicely. In these circumstances, looking at only the generator drawing can lead to problems in the field. As depicted below, the junction box location (hatched area) can overlap where the tank sits, and not rest solely in the area open to the bottom (blue outline). When a conduit is brought up within the junction box area, the tank may cover it up and the conduit is rendered useless. This scenario is represented in red in the below image. When the conduit is misplaced in that manner, the concrete pad would need to be demolished and poured again. This is a costly mistake not just for the added labor, but added rigging expenses and delays to project completion.

Obviously, there is a lot to consider when designing a generator pad. The pad must be the perfect fit and makeup for the generator itself and the space it is placed in. Because such precision is required, it’s best to leave the design process to the experts. Having a proven track record of success and over 50 years of experience, the engineering department at Kinsley has the ability to design project-specific generator pads for guidance to avoid costly mistakes in the field.


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