Transformer Sizing Calculator

Inputs

• Present coincident demand in kW — the measured maximum demand (a year of bill maximum-demand readings, or a load-logger survey), not the sum of nameplates. 0 for a site with no load yet.
• Future coincident demand in kW — what the planned additions add to that peak when they run together, from their duty cycles. 0 if nothing is planned.
• True power factor at the transformer (more than 0, at most 1) — kW ÷ kVA including harmonic distortion, not only the displacement power factor an APFC relay shows.
• Planning utilisation — the share of the rating you are prepared to load the transformer to, as a percentage (more than 0, at most 100). You choose it; the tool recommends none.
• The ratings you can buy, in kVA — prefilled with the preferred three-phase ratings of IS 1180 (Part 1):2014 for systems up to 11 kV, and editable.

Method

1. Total coincident demand (kW) = present + future.
2. Apparent power at that demand (kVA) = total ÷ power factor.
3. Planning kVA = apparent power ÷ planning utilisation — the same as total ÷ (power factor × utilisation).
4. Rating = the smallest one in your list at or above the planning kVA. If the planning kVA is above the largest rating in the list, the tool says so and selects nothing; it does not invent a size or suggest a configuration.
5. For the selected rating and the listed ratings either side of it: the loading at the entered demand, and the coincident kW still in hand at your utilisation and power factor (rating × utilisation × PF − total demand; negative means over).
6. Sensitivity: the planning kVA and the rating it rounds to, recomputed at the power factor ±0.05 and the utilisation ±10 points.

Assumptions

• Both demands are coincident. The present figure is a measured peak that already includes the plant's diversity; the future figure is what the additions add to that peak. Nameplate kW, or a future load that will not run at the peak, overstates the rating.
• One power factor applies to the whole load at the peak, and it is the true power factor. On a drive-heavy plant a displacement-PF reading overstates it.
• Planning utilisation is your allowance for ambient temperature, loading cycle, redundancy policy and growth beyond the stated expansion. No value is recommended, and the tool applies no derating of its own.
• The prefilled list is the preferred three-phase ratings of IS 1180 (Part 1):2014 for nominal system voltages up to and including 11 kV — the standard marks 6.3, 10, 20 and 40 kVA non-preferred, so they are left out. For a 22 kV or 33 kV supply, a dry-type unit or anything above 2,500 kVA, confirm the ratings with your supplier and edit the list.

Limitations

• No harmonic (K-factor) derating, no ambient or altitude derating and no manufacturer's derating — the planning utilisation is not a substitute for any of them.
• No fault level, impedance, vector group, voltage ratio, cooling type or HT switchgear selection.
• No redundancy decision: whether the site needs two transformers, N+1 or a separate transformer for essential load is design work.
• Does not check the DISCOM sanctioned load or contract demand, which caps what can be connected — an expansion can fit the transformer and still need a load-enhancement application.
• Steady state only: motor starting, inrush and other transients are not modelled.