Transformer Sizing Calculator
Work out the transformer kVA rating you need from load, power factor, growth allowance and target loading, rounded up to the next standard size.
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Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
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About this calculator
Use this to turn a connected running load into a transformer rating. It adds a growth allowance, divides by the loading you want the transformer to run at, and rounds up to the next standard size so you can also see the secondary full-load current of that size.
Limits of this calculator
Harmonic-rich loads, motor starting and a high ambient temperature can call for a larger or K-rated transformer; none of these is modelled. Confirm the result against the manufacturer data before ordering.
Cross-check: 67 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Indicative
Indicative. Estimation only; further engineering checks are required. Basis: Selection from a standard-size list; harmonics, starting and ambient not modelled.
What this calculator checks
- Smallest standard kVA at or above the load, growth and loading target.
What remains unverified
- Harmonics (K-rating), motor starting and ambient temperature.
- Impedance, losses and cooling class: take them from the transformer data sheet.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | IEC 60076-1; IEC 60076-7, IEC 60076-1:2011 Ed 3.0; IEC 60076-7:2018 Ed 2.0 |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | Power-transformer ratings and, for mineral-oil-immersed units, loading/thermal ageing are relevant. The calculator sizes from kW, power factor, growth and a selected loading fraction; it does not check harmonics, cooling, ambient or cyclic load. |
| Cross-check test cases | 67 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Indicative |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; licensed clause-by-clause standard review; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| Connected running load | 450 kW |
| Power factor | 0.9 |
| Future growth (%) | 10 |
| Target loading | 0.8 |
| Secondary voltage (L-L) | 415 V |
- S = 450 / 0.9 × 1.1 / 0.8 = 687.5 kVA
- Next standard size = 800 kVA
- I = 800 kVA / (√3 × 415) = 1,113 A
Assumptions
- Balanced three-phase load
- Standard sizes: 25, 50, 63, 100, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3150 kVA
Harmonic-rich loads, motor starting and ambient temperature can call for a larger or K-rated unit.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Connected running load | kW | greater than 0 kW | |
| Power factor | (number) | greater than 0, up to 1 | Rough starting point only. Resistive heating and incandescent loads are 1.0; a fully loaded induction motor is typically 0.8 to 0.88 and falls sharply when lightly loaded; discharge lighting depends on its ballast or driver. For real work use the nameplate or a measured value. |
| Future growth (%) | (number) | 0 or more, up to 200 | |
| Target loading | (number) | 0.3 or more, up to 1 | Fraction of rating at design load; 0.7 to 0.8 leaves headroom |
| Secondary voltage (L-L) | V | greater than 0 V |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Required rating | kVA | Shown to 1 decimal place | |
| Next standard size | kVA | Shown to 0 decimal places | |
| Secondary full-load current | A | Shown to 0 decimal places |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
Typical values
| Connected running load (kW) | Required rating (kVA) | Next standard size (kVA) | Secondary full-load current (A) |
|---|---|---|---|
| 50 | 76.4 | 100 | 139 |
| 100 | 152.8 | 160 | 223 |
| 200 | 305.6 | 315 | 438 |
| 450 | 687.5 | 800 | 1,113 |
| 800 | 1,222.2 | 1,250 | 1,739 |
| 1,500 | 2,291.7 | 2,500 | 3,478 |
Other inputs are the same as in the worked example above.
Standards and further reading
- IEC 60076 series (power transformers)
Impedance, losses and cooling class are manufacturer values; enter the figures from the transformer data sheet. These are named as background reading. Elecmetric does not embed clause text or rating tables, and the reviewer record for this calculator is separate from this list.
Frequently asked questions
What do I need to enter?
Enter connected running load, power factor, future growth (%), target loading and secondary voltage (L-L). Change the unit of a value with its selector and the result stays the same.
Why not choose the transformer that exactly matches the load?
Transformers are sold in standard sizes, loads grow, and running a transformer near its full rating all day leaves no margin. The calculator adds a growth allowance and a target loading, then rounds up to the next standard size.
Which standard sizes does it use?
The standard kVA series listed in the assumptions below, from 25 kVA up to 3,150 kVA. If your utility or supplier uses a different series, take the required rating and select from their list.
What target loading should I enter?
It is a design choice. A lower figure leaves more headroom and lower losses at the cost of a larger unit. Enter the loading your specification or utility asks for.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 67 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- Transformer Formulas: Current and Fault Level: Transformer formulas for turns ratio, full-load current, impedance and fault current
- Parallel Transformer Load Sharing Formulas: Parallel transformer formulas: how a load divides by rating and impedance
- Transformer Cooling Class Rating Formulas: Transformer cooling formulas: the ONAN and ONAF ratings
- Transformer Inrush Current Formulas: Transformer inrush formulas: rated current, peak inrush as a multiple of rated current
- Transformer Vector Group Formulas: Transformer vector group formulas: the turns ratio of a delta-star unit
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