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Generator Set Foundations: Concrete Pad Recommendation
Properly sizing the concrete pad is fundamental to a generator installation. There are many variations to how a concrete pad can be sized properly. The pad can be poured to accommodate only the skid frame or subbase tank of the generator set, thus having the enclosure intake and discharge hoods extend beyond the pad on both ends. The pad can also be poured utilizing the enclosure dimensions versus frame dimensions, thus centering the entire unit on the pad. Both methods will work and neither is functionally better than the other.
The following are critical elements to consider when sizing and pouring a pad:
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Look at the submittal for the stub up drawings for the skid frame or subbase tank, and compare these to the weather enclosure elevation drawing. The skid frame or subbase tank are the part of the generator set that will rest on the pad. The weather enclosure mounts above the skid/tank, and is your longest dimension, but does not rest on the pad. Now is the time to decide if you want the entire unit centered on a longer pad, or if a shorter pad for just the skid/tank is all you need.
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The choice made above will be pivotal to placing your conduit properly. Sketch up your equipment using start of enclosure, start of skid/tank, end of skid/tank and end of enclosure as your reference points. Now use your drawings to verify your conduit location is correct on the sketch.
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Plan for a housekeeping apron around your generator set of at least 6” minimum on all four sides. With your plan above and the decision on the size of the apron, you now can determine your overall pad length and width.
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A common specification for a concrete pad is 2500 psi compression strength concrete reinforced with #6 rebar on 12 inch centers. This pad should be seated on a solid subsurface. The pad should be flat and level to within ½ inch.
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The depth of the pad is calculated as 125% of the unit wet weight (fuel included). A 10,000 lb generator would thus require a 12,500 lb pad. Concrete weighs approximately 150 lbs/cu ft.
Use the following formula, marking sure to use feet instead of inches:
Pad Weight= (Pad Length x Pad Width x Pad Height) x 150
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