Most voltage stabilizer sizing goes wrong on one number: the nominal voltage. A unit quoted as 5kVA is rated at its nominal input, usually 230V. When the mains sits at 170V — which it does every evening on a long rural feeder, and often in summer in tier-2 and tier-3 industrial areas — that same 5kVA stabilizer cannot deliver 5kVA to the load. It can deliver roughly 3.7kVA. Buyers who size on the load and ignore the input voltage end up with a unit that trips, hunts, or burns out its relay board inside a year.
This guide covers the three stabilizer types sold into India, Pakistan, Bangladesh and Sri Lanka, how to size one against a real input range rather than a nominal figure, and the installation details that decide whether it lasts five years or fifteen months. For the product families, see the voltage stabilizer range.
Nominal is 230V single-phase and 415V three-phase at 50Hz. Measured, the supply moves through a daily and seasonal cycle that is predictable enough to design for:
| What you measure | Where it happens | What the load sees |
|---|---|---|
| 245-260V at night, 205-215V at peak | Urban and semi-urban feeders near a distribution transformer | Lamps brighten and fail early; electronics run hot; compressors work harder at night |
| 160-190V for hours at a time | Tail end of long rural feeders, agricultural pump feeders | Motors stall or overheat, UPS units transfer to battery constantly, refrigeration fails |
| Frequent short dips of 30-60V | Any feeder with welding sets, pump starters or a shared LT line | Contactor chatter, control boards resetting mid-process, PLC faults |
| Neutral shifted or floating | Older installations, shared neutrals, poor earthing | One phase reads high and another reads low at the same time — a stabilizer cannot correct a phase-to-neutral fault |
| 50Hz ± 0.5Hz, occasionally worse | Weak grids during high demand | Motors run slightly fast or slow; clocks drift; usually tolerable |
The last two rows are the ones that waste money. A stabilizer corrects voltage magnitude. It does not fix a wiring fault, and it does not change frequency. If the neutral is floating, spend the money on the earthing and the panel first.
| Relay type (SVR) | Servo type (SVC / TND) | Static / IGBT (AVR) | |
|---|---|---|---|
| Output accuracy | ±8-10% typical | ±1-2% | ±1% |
| Correction | Stepped, by tap relays | Continuous, servo motor drives a brush over the winding | Electronic, no moving parts |
| Input range | 150-270V (model dependent) | 140-280V wide range | 170-270V typical |
| Correction speed | Fast per step, but steps are visible | Fast and smooth, 30-50V per second | Very fast, per cycle |
| Cost per kVA | Lowest | Mid | Highest |
| Best for | Lighting, refrigeration, IT, general single-phase loads | Medical equipment, CNC, laboratory instruments, sensitive process control | Critical IT, data, precision test benches |
| Weak point | Relay contact life under frequent correction | Brush and servo motor wear; needs periodic service | Price; limited surge capacity |
For a shop, a house or a small workshop the SVR relay series is almost always the right commercial answer. The step correction is visible as a small flicker in incandescent lamps, which is the main customer complaint — LED lighting and switch-mode supplies do not care. Move to servo where the load is a measuring instrument, a CNC control or a medical device, because a ±8% output window is outside the tolerance of those devices.
A stabilizer’s output capacity is limited by its input current rating. Inside the unit, the transformer and the tap changer are built for a maximum input current at nominal voltage. When the input falls, the current needed to deliver the same output rises in proportion — so usable output kVA falls.
Usable output kVA ≈ rated kVA × (actual input voltage ÷ nominal voltage)
| Rated capacity | At 230V input | At 200V input | At 170V input | At 150V input |
|---|---|---|---|---|
| 3kVA | 3.0kVA | 2.6kVA | 2.2kVA | 2.0kVA |
| 5kVA | 5.0kVA | 4.3kVA | 3.7kVA | 3.3kVA |
| 10kVA | 10.0kVA | 8.7kVA | 7.4kVA | 6.5kVA |
| 15kVA | 15.0kVA | 13.0kVA | 11.1kVA | 9.8kVA |
Two practical consequences. First, if the site regularly drops below 180V, size for the load at that voltage, not at 230V — a 5kVA unit holding up a 4.5kVA load will be overloaded every evening. Second, motors add inrush: a compressor that runs at 3kVA draws 9kVA for a fraction of a second, and the stabilizer must ride that without tripping. That is why the working rule in the field is 1.3× for motor loads and 1.5× where several motors start together.
| Laden Sie | Running load | Recommended stabilizer | Grund |
|---|---|---|---|
| House: lights, fans, TV, fridge, 1 AC | 3.5kW mixed | 5kVA relay | Fridge inrush plus diversity |
| Retail shop: POS, displays, 2 AC, freezer | 5kW | 7.5kVA relay | Two compressors, possible simultaneous start |
| Clinic: analyser, centrifuge, AC | 4kW | 5kVA servo | ±2% needed for the analyser |
| Workshop: 2 machines, compressor, lighting | 8kW | 15kVA relay or three-phase servo | Motor inrush plus wide input swing |
| CNC cell | 6kW | 10kVA servo | Control tolerance, no step flicker |
Most field failures we see are installation failures, not product failures. The sequence that works:
Three-phase loads need a three-phase decision: a three-phase servo stabilizer on the LT panel, or three single-phase units on separate phases. Three single-phase units are cheaper and give per-phase correction where the phases are unbalanced, which is common on rural feeders. A single three-phase unit is the better answer where the load is a three-phase motor that needs balanced volts on all three phases. For the high-capacity end of that, the SBW series from 20kVA to 1000kVA covers panel-level compensation.
Four lines get you a priced recommendation instead of a catalogue: the measured input voltage range at the site (high and low, not nominal), the total load in kW with motors flagged, whether the load is single-phase or three-phase, and the first order quantity. If you have a load list with a power factor on it, send that too.
Die SVR series datasheet (PDF) covers the relay range from 0.5kVA to 10kVA with sizing notes, and the stabilizer model selection guide walks through the same decision for a single installation.
Only if the mains stays near 230V and the load is resistive. At 180V a 5kVA unit can deliver about 3.9kVA, and a 5kW motor load needs roughly 1.3× margin on top. For a 5kW mixed load on a feeder that drops to 180V, specify 7.5kVA or 10kVA.
Relay. The load is lighting, fans, refrigeration and electronics, all of which tolerate ±8% comfortably, and the relay unit costs much less per kVA. The only reason to go servo at home is if someone in the house runs medical equipment that specifies a tighter supply.
It is stepping between taps because the input is hovering right at a tap boundary. Two causes: the input is genuinely unstable within a narrow band, or the unit is undersized and the load is dragging the input down each time it corrects. Check the measured input first, then the load against the derating table above.
No. A floating or shifted neutral produces a phase-to-neutral fault, not a voltage sag, and no stabilizer can correct it. The output will read something plausible while the load still fails. Have the earthing and neutral checked before buying equipment.
Annually in a clean plant room, more often in a dusty industrial area. The service is a brush inspection, a winding clean and a check of the servo motor drive belt or coupling. Relay units need no scheduled service but their contactors have a finite life — roughly 100,000 operations, which a badly sited unit can consume in two years.
Not to any useful degree. It corrects slow voltage variation. Surge protection is a separate device at the incoming supply, and in lightning-prone areas it is the more important purchase.
Not sure which frame fits your measured input range? Send the site readings to our engineers and we will confirm the model and the capacity.
