The rule of thumb

Take your total running load in kVA, add a surge allowance for any motors or compressors, and add 25% headroom on top. Round up to the nearest standard AVR size. That's the whole calculation in one sentence, the rest of this article is about doing each step well enough that the AVR you install actually lasts.

Why sizing matters

An undersized AVR runs hot, trips frequently, and wears out its servo motor years early. An oversized AVR costs more up front, takes more floor space, and, on a lightly-loaded supply, can actually regulate less accurately than a correctly-matched unit. Getting within one size step of the right answer is the goal; getting further off wastes money on either side.

The cost of being wrong is asymmetric. Undersizing burns out the AVR and the equipment it's meant to protect. Oversizing only wastes capital. When in doubt, go one step larger, never smaller.

Step 1: List your loads

Walk the site and list every piece of equipment that will be connected downstream of the AVR. For each, note:

Nameplate rating, usually in kW or kVA
Quantity
Running load, what the equipment actually draws in normal operation (often less than nameplate)

If the nameplate is in kW and you don't know the power factor, assume 0.8. Multiply by 1.25 to get kVA.

kVA = kW ÷ power factor

A 75 kW compressor motor at 0.8 PF draws about 94 kVA running. Modern VFD-driven motors can run at 0.95+ PF; older direct-on-line motors can sit as low as 0.7.

Step 2: Add surge allowance for inductive loads

Motors, compressors, pumps and welding equipment draw 3-7× their running current during startup. An AVR sized only for running load will see its regulation collapse every time a big motor starts, and the equipment downstream will experience exactly the voltage dip the AVR was supposed to prevent.

The practical rule:

Motors started across-the-line (DOL), double the motor's running kVA in your total
Motors on soft-start or VFD, add 50% of the running kVA
Welding sets, double the running kVA (very surge-heavy)
Induction heaters, double the running kVA
Continuous loads (lighting, heating, electronics), no surge allowance

Step 3: Apply headroom

Once you have the sum of running loads plus surge allowances, add 20-25% headroom on top. This accounts for:

Equipment additions that turn up within the first couple of years
Operating at higher ambient temperatures than the rated 40 °C
The AVR running below its thermal limit, which extends servo life

A site with 200 kVA of summed running + surge load should be specified for a 250 kVA AVR, not a 200 kVA.

Step 4: Round up to standard sizes

The Vener 7 three-phase range comes in standard increments from 10 kVA up to 2000 kVA, typical sizes are 10, 15, 20, 30, 50, 75, 100, 150, 200, 300, 500, 750, 1000, 1500 and 2000 kVA. Single-phase units run from 3 kVA to 20 kVA. Round your answer up to the next standard size. Never down.

Worked example 1: Small workshop

A workshop in Industrial Area runs:

3 × 7.5 kW motors (DOL), 3 × 9.4 kVA = 28 kVA running
Lighting + compressor room fans, 3 kVA continuous
10 kW welder (intermittent), 12.5 kVA running

Step 1 running: 28 + 3 + 12.5 = 43.5 kVA Step 2 surges: 28 kVA motor surge + 12.5 kVA welder surge = 40.5 kVA Total demand: 43.5 + 40.5 = 84 kVA Step 3 headroom (25%): 84 × 1.25 = 105 kVA Step 4 snap up: next standard size is 150 kVA

This workshop gets a 150 kVA three-phase AVR. A 100 kVA unit would work on paper but would run hot when all three motors started simultaneously.

Worked example 2: Mid-size factory with welding

A steel fabrication plant in Athi River runs:

5 × 22 kW CNC machines on VFDs, 5 × 27.5 kVA = 138 kVA running
4 × 200 A welding sets, intermittent, ≈ 180 kVA running
50 kW compressor, DOL, 62.5 kVA running
Office + lighting, 15 kVA continuous

Step 1 running: 138 + 180 + 62.5 + 15 = 395.5 kVA Step 2 surges: 69 kVA (VFD 50%) + 180 kVA (welders 100%) + 62.5 kVA (compressor 100%) = 311.5 kVA Total demand: 395.5 + 311.5 = 707 kVA Step 3 headroom (25%): 707 × 1.25 = 884 kVA Step 4 snap up: next standard size is 1000 kVA

This plant gets a 1000 kVA oil-cooled three-phase AVR. Air-cooled at this size is possible but harder to house in Kenyan ambient conditions, oil-cooled is the usual choice above 300 kVA.

When rule-of-thumb sizing isn't enough

Some sites deserve a formal load study rather than a spreadsheet calculation:

Data centers and hospitals, harmonic distortion from non-linear loads changes the effective kVA significantly; a power quality analyser needs to run for a week before sizing.
Sites with heavy welding or induction heating, surge duty cycle matters as much as peak magnitude; the AVR needs to be sized for thermal RMS, not just peak.
Sites already experiencing failures, the first job is diagnosing why existing equipment is failing, not just sizing a bigger stabilizer.

For any of these, a free site visit within Nairobi gives us a much more accurate answer than a form can.

Try the sizing tool

The residential path of the sizing tool does this calculation automatically from an appliance list. The factory path collects enough information for an engineer to do a rough estimate and book a site visit.

Or talk to an engineer directly and we'll quote you from your load schedule.

How to Size an AVR for a Factory or Commercial Site