In short. A generator foundation has three jobs. It carries the set’s weight without settling. It is heavy and stiff enough that the set’s vibration does not travel into the building. It holds the skid level so that the engine and alternator stay aligned. For most canopy sets on firm ground this means a reinforced concrete block of M20 grade or better, weighing 1.5 to 2.5 times the set’s wet weight, extending 150–300 mm beyond the skid on every side, isolated from the surrounding floor and level to within about 1 mm per metre. Pair it with correctly loaded anti-vibration mounts and give the concrete at least seven days to cure before the set goes on.
Who this page is for: civil and electrical contractors, consultants, facility engineers and owners preparing a plinth for a new or relocated set, from a 7.5 kVA canopy set on a terrace to a 2,000 kVA set in a plant room.
Key facts
- A 1,500 rpm set has its main disturbing frequency at 25 Hz (shaft speed). A six-cylinder four-stroke engine also has a strong firing frequency of 75 Hz.
- Anti-vibration mounts isolate well only when the mounted system’s natural frequency is well below the disturbing frequency. A ratio of 3 or more gives roughly 85–90% isolation.
- Reinforced concrete weighs about 2,400 kg/m³. Foundation volume is found from the mass required.
- M20 concrete gains about 65% of its strength in 7 days and most of its design strength in 28 days when properly cured.
- A foundation for a canopy set typically stands 150–200 mm above the finished floor or ground to keep the base frame out of standing water and dust.
What the foundation must achieve
Carry the load without settlement
The total load is the set’s wet weight (with oil, coolant and a full base tank) plus the foundation’s own weight. On natural ground this is checked against the soil’s safe bearing capacity. Most firm soils in the Deccan plateau, including the murram and weathered rock common around Hyderabad, carry far more than a generator foundation imposes. Black cotton soil, filled ground and made-up plots need a structural engineer’s advice, because they swell, shrink and settle unevenly.
Resist and absorb vibration
A mass of concrete larger than the set acts as an inertia block. It lowers the amplitude of any vibration that passes through the mounts and stops the slab around it from ringing. A foundation that is too light, or that is cast as part of the building floor, becomes a loudspeaker for the engine.
Hold the set level and true
The engine and alternator are aligned at the factory on a common base frame. A foundation that is out of level, or that twists the frame when bolted down, puts stress on the coupling and alternator bearings. Uneven support also loads some anti-vibration mounts more than others, which spoils their isolation.
Sizing the foundation
Work through four quantities in order: plan size, required mass, depth, and bearing pressure.
- Plan size. Take the skid (base frame) length and width from the maker’s general-arrangement drawing and add 150–300 mm on every side. Use the larger margin for bigger sets and where service access is tight.
- Required mass. Foundation mass = 1.5 to 2.5 × set wet weight. Use the upper end for sets without integral mounts and for buildings sensitive to vibration. On upper floors the slab’s capacity governs instead; see the terrace section below.
- Depth. Depth = required mass ÷ (2,400 kg/m³ × plan area). Round up to a practical figure, usually in 50 mm steps.
- Bearing pressure. (Set weight + foundation weight) × 9.81 ÷ plan area, in kN/m². Compare this with the soil’s safe bearing capacity.
Worked example (illustrative): 250 kVA canopy set on natural ground
Data from the maker’s drawing (illustrative): skid 3.6 m × 1.3 m; wet weight with canopy and full base tank 4,000 kg.
- Plan size: add 300 mm on each side. Length 3.6 + 0.6 = 4.2 m; width 1.3 + 0.6 = 1.9 m. Area 4.2 × 1.9 = 7.98 m².
- Required mass: 2.0 × 4,000 = 8,000 kg.
- Volume: 8,000 ÷ 2,400 = 3.33 m³.
- Depth: 3.33 ÷ 7.98 = 0.418 m. Round up to 450 mm: 200 mm above finished ground and 250 mm below, on a compacted base with 75 mm lean concrete.
- Actual foundation weight: 7.98 × 0.45 × 2,400 = 8,618 kg.
- Bearing pressure: (4,000 + 8,618) × 9.81 ÷ 7.98 = 123,783 N ÷ 7.98 ≈ 15.5 kN/m². Firm natural ground typically carries 100–200 kN/m² or more, so this is a large margin.
- Reinforcement (typical): 12 mm bars at 200 mm centres both ways, top and bottom, 50 mm cover. Final detailing by the structural engineer.
The same steps for a 62.5 kVA set weighing about 1,400 kg on a 2.2 m × 0.95 m skid give a foundation about 2.8 m × 1.55 m and 250–300 mm deep. Small sets often end up with a practical minimum depth rather than one set by mass.
Typical figures by set size
The table below gives illustrative planning ranges for canopy sets. Weights vary widely between makes and enclosure types, so always take the wet weight and skid dimensions from the general-arrangement drawing of the exact set.
| Set rating | Typical wet weight with canopy | Foundation mass at 2× | Typical depth range |
|---|---|---|---|
| 15–25 kVA | 600–900 kg | 1,200–1,800 kg | 200–250 mm |
| 62.5 kVA | 1,200–1,600 kg | 2,400–3,200 kg | 250–300 mm |
| 125 kVA | 2,000–2,600 kg | 4,000–5,200 kg | 300–350 mm |
| 250 kVA | 3,200–4,200 kg | 6,400–8,400 kg | 400–500 mm |
| 500 kVA | 5,500–7,000 kg | 11,000–14,000 kg | 400–550 mm |
| 1,000 kVA | 12,000–16,000 kg | 24,000–32,000 kg | 550–750 mm |
Concrete, reinforcement and isolation
- Concrete grade: M20 as a minimum; M25 for larger sets and upper-floor plinths. Use a designed mix where possible, rather than a site-batched nominal mix.
- Reinforcement: a top and bottom mesh controls cracking from shrinkage and vibration. Large blocks also need side bars and stirrups.
- Isolation joint: leave a gap of about 25 mm between the foundation and the surrounding floor, filled with a compressible joint filler and sealed at the top. This stops vibration passing sideways into the slab.
- Foundation bolts or anchors: follow the maker’s drawing. Pockets for grouted bolts are formed when casting; chemical or expansion anchors are fixed after curing. Keep at least 150 mm from any edge.
- Finish and level: trowel the top flat and level to within about 1 mm per metre, or provide steel sole plates and level with non-shrink grout.
- Drainage and spill control: slope the surrounding ground away from the plinth. Where the base tank or bulk tank has no integral bund, provide a bund or kerb to contain spills.
- Curing: keep the concrete wet for at least seven days. Place the set after 7–14 days and bring it to full load only after the concrete is well cured.
Anti-vibration mounting
Most canopy sets have anti-vibration mounts between the engine-alternator and the base frame, inside the enclosure. In that case the base frame is bolted directly to the foundation and no further mounts are needed. Adding a second layer of soft mounts under the base frame can make the set rock on starting and stopping, and can strain flexible connections. Open sets, large sets and sets on upper floors may need mounts or spring isolators between the base frame and the foundation; their selection is an engineering decision.
How isolation works
A mount behaves as a spring. Under load it compresses by a static deflection δ, and the set on its mounts then has a natural frequency of about fn ≈ 15.76 ÷ √δ, with fn in Hz and δ in millimetres. Isolation becomes effective once the disturbing frequency is more than √2, about 1.4 times, the natural frequency, and improves as the ratio rises. The transmitted share of vibration for an undamped mount is roughly T = 1 ÷ (r² − 1), where r is the frequency ratio.
Worked example (illustrative): checking mount isolation at 1,500 rpm
- Disturbing frequency at 1,500 rpm: 1,500 ÷ 60 = 25 Hz.
- Target a mounted natural frequency of 8 Hz. Required static deflection: δ = (15.76 ÷ 8)² = 1.97² ≈ 3.9 mm.
- Frequency ratio: r = 25 ÷ 8 = 3.125; r² = 9.77.
- Transmissibility: T = 1 ÷ (9.77 − 1) = 1 ÷ 8.77 ≈ 0.114, so about 89% isolation at shaft frequency.
- At the 75 Hz firing frequency of a six-cylinder engine, r = 75 ÷ 8 ≈ 9.4 and isolation is better still.
- Mount loading: share the set’s weight according to its centre of gravity, not equally. A mount that is overloaded bottoms out; one that is underloaded barely deflects. Either way the isolation in steps 2–4 is lost.
Diagnostic table: foundation and mounting faults
| What you see | Likely cause | Check |
|---|---|---|
| Vibration felt in nearby rooms or floors | Foundation tied into building slab; foundation too light | Isolation joint present and clear; foundation mass |
| Cracks across the plinth | No reinforcement, poor curing, or settlement | Crack pattern; levels at the four corners |
| Set rocks on starting and stopping | Double soft mounting; mounts underloaded | Mount type under base frame; deflection of each mount |
| One corner mount flat, others tall | Uneven loading or out-of-level foundation | Level of plinth; mount positions relative to centre of gravity |
| Coupling or alternator bearing failures | Base frame twisted when bolted down | Shim gaps before tightening; frame flatness |
| Water standing under the base frame | Plinth too low; drainage towards set | Height above ground; surrounding falls |
| Loose foundation bolts | Grout failure or undersized anchors | Torque check; anchor type against drawing |
For vibration symptoms that are not caused by the foundation, see generator vibration and abnormal noise.
Terraces, upper floors and plant rooms
On a roof or upper floor, the question changes from soil bearing to structural capacity. The set, its foundation and its fuel add a concentrated load that must be checked by a structural engineer against the slab and beams. A thinner inertia block on spring isolators is usual, with the springs selected for low natural frequency. Flexible connections on exhaust, fuel and cables become essential, and the crane lift itself needs planning. Plant rooms need the same isolation joint discipline and attention to airflow; see generator room ventilation.
Foundation design and handover sheet
| Item | Value / reference |
|---|---|
| Set rating and model; drawing reference | |
| Skid length × width (m) | |
| Wet weight with canopy and full tank (kg) | |
| Margin added each side (mm) | |
| Foundation length × width (m) and area (m²) | |
| Mass factor chosen (1.5–2.5) and required mass (kg) | |
| Calculated depth (m) and adopted depth (mm) | |
| Height above finished floor or ground (mm) | |
| Bearing pressure (kN/m²) and soil capacity (kN/m²) | |
| Concrete grade and reinforcement | |
| Isolation joint width and filler | |
| Anchor type, size and positions | |
| Level achieved (mm per metre, four corners) | |
| Casting date; set placed on date | |
| Mount type and measured deflection per mount | |
| Checked by (name, date) |
Exceptions and cautions
- The figures on this page are typical planning values. The maker’s installation manual and a structural engineer’s design take precedence, especially for upper floors, large sets and poor soils.
- Black cotton soil, filled ground and sites with a high water table need a soil investigation and possibly a raft or piled foundation.
- Sets in parallel plants, on trailers or in containers have their own mounting requirements.
- Never weld the base frame to embedded steel unless the maker allows it; it transmits vibration and can distort the frame.
Common mistakes
- Casting the plinth as part of the floor slab with no isolation joint.
- Sizing the foundation from the set’s dry weight, forgetting fuel, oil, coolant and canopy.
- Placing the set on concrete cast two days earlier.
- Adding rubber pads under a base frame that already has internal mounts.
- Leaving the plinth flush with ground level in a location that floods in the monsoon.
- Tightening foundation bolts without checking for gaps under the frame, twisting it.
What to send us
To check a foundation design or a vibration problem, send the set model and rating, the maker’s general-arrangement drawing if you have it, the proposed location (ground, terrace, plant room) and photographs of the existing plinth, mounts and surroundings. Use the service and AMC enquiry, or the generator sales enquiry for a new installation, or WhatsApp 98852 01608.
Questions engineers ask
How big should a generator foundation be?
Typically 150–300 mm larger than the skid on every side, with a mass of 1.5 to 2.5 times the set’s wet weight. Depth follows from that mass divided by the concrete density and plan area.
What grade of concrete is used for a DG set foundation?
M20 as a minimum, with M25 for larger sets and upper floors, reinforced top and bottom and cured for at least seven days.
Does a canopy generator need anti-vibration mounts under it?
Usually not, if it already has mounts between the engine-alternator and the base frame. The base frame is then bolted directly to a level foundation. Open and large sets may need external mounts.
How long should concrete cure before placing a generator?
Keep it wet for at least seven days and place the set after 7–14 days. Full design strength develops over about 28 days.
Why does my generator shake the building?
Most often the foundation is tied into the floor slab, is too light, or the mounts are wrongly loaded. An isolation joint, a heavier inertia block or correctly selected mounts solve most cases.
Can a generator be installed on a terrace?
Yes, if a structural engineer confirms the slab can carry the concentrated load. Use spring isolators, flexible connections and a planned crane lift.
Related pages
- Installation planning
- Generator site planning checklist
- Generator installation
- Generator vibration and abnormal noise
- Acoustic enclosures
- DG set earthing, step by step
- Commissioning
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer. Last updated 5 November 2026.