Transformer Sizing Calculator
Transformer Sizing Calculator — quick estimate
Estimate the distribution transformer rating (kVA) your building needs from its connected load, diversity and growth margin. Adjust the inputs, then get an engineered electrical design from our team.
Your load
Estimates maximum demand = connected load × diversity, then converts to kVA at your power factor and adds margin. Motor inrush and harmonics may need a larger unit.
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Our electrical engineers will firm up the load schedule, transformer rating and HT/LT scheme for your site. We'll attach the estimate above automatically.
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Indicative estimates only — not a binding design. Actual transformer rating depends on the detailed load schedule, diversity, harmonics and DISCOM sanction.
How this calculator works — and where it stops
What it calculates
The distribution-transformer rating in kVA for a site from its connected load, the loading it will run at, and whether one transformer or two is the sensible configuration.
Inputs
- Connected load in kW
- Diversity factor — the share of connected load expected to run together
- Power factor
- Design margin for growth
Method
- Maximum demand = connected load × diversity; kVA = (maximum demand ÷ power factor) × (1 + margin).
- Selects the next standard rating from 100 kVA to 3,150 kVA (or a 500 kVA step above), reports the loading percentage at maximum demand, and suggests two transformers for redundancy at 2,000 kVA and above.
Assumptions
- Diversity is the user's judgement; the tool does not derive it from the load schedule.
- Power factor is the corrected value at the transformer — an APFC panel is assumed where the load needs it.
- One transformer serves the whole load; no allowance for a separate essential-load transformer.
Limitations
- No harmonic derating (K-factor), no ambient or altitude derating, no fault-level or impedance selection.
- Voltage level, vector group, cooling type and the HT switchgear are outside the tool.
- The DISCOM's sanctioned load — which caps what can be connected — is not checked here; see the sanctioned-load calculator.
Worked example
Produced by running this calculator with the inputs below.
Inputs
- Connected load: 800 kW
- Diversity factor: 0.7
- Power factor: 0.9
- Design margin: 20%
Working
- Maximum demand = 800 × 0.7 = 560 kW
- kVA = (560 ÷ 0.9) × 1.20 = 746.7 kVA
- Next standard rating above 746.7 kVA is 800 kVA
- Loading at maximum demand = (560 ÷ 0.9) ÷ 800 = 78%
Result. 800 kVA transformer, 78% loaded at maximum demand — single transformer with an LT panel. Check the figure against the DISCOM sanctioned load before applying.
How engineers use the result
To size the substation footprint and the DISCOM application at concept stage, and to check a quoted rating against the load before the load list is final.
When a professional design must replace it
Before the CEIG drawing submission: the rating is fixed from the load list with diversity justified, the DISCOM sanction and the protection study — and the CEIG approves that drawing, not this estimate.
Got your number — what next?
A calculator gives you a first figure. These take it further.
The work itself
Read before you specify
The terms behind it
Delivered work with this scope
Need an exact BOQ, rate analysis or measurement sheet?
This is a quick engineering estimate. For tendering, billing or audit, our QS & estimation team prepares an item-wise BOQ, rate analysis and a verified measurement sheet to IS / CPWD norms — backed by 15+ years and ISO 9001:2015 quality processes.