Transformer Sizing Calculator

HP/kW → kVA · starting kVA · standard rating · FLC

Supply

System

Supply Voltage (V)

Motor

Motor Rating

Unit

Efficiency η (%)

Power Factor cosφ

η and cosφ are auto-suggested from the motor size when you calculate — always editable. Real nameplate data should take priority when you have it.

Starting Method

Starting Multiplier (×FLC)

Safety / Future-load Margin (%)

Add each motor with its rating, efficiency, power factor and starting method. The worst case models the largest motor starting while the others already run.

Demand / Diversity Factor (%)

Group Margin (%)

Existing Transformer

Transformer Rating (kVA)

Assumed Efficiency η (%)

Assumed Power Factor

Reserve Margin (%)

Result

Enter your motor rating, efficiency and starting method to get running kVA, momentary starting kVA, the recommended transformer size rounded to a standard rating, and full-load current — or use multi-motor and reverse modes

Transformers are rated in kVA (apparent power), not kW, because the current a transformer must carry depends on power factor — a lower cosφ means more current for the same real power. This tool sizes for the running load with margin and separately reports the momentary starting kVA; a transformer must tolerate that brief surge without excessive voltage dip, and a unit with higher percentage impedance will dip more for the same starting kVA. The efficiency, power factor and starting multipliers are typical values for guidance — always use actual nameplate and manufacturer data, and have the final selection reviewed by a qualified electrical engineer against local standards. This tool is for preliminary sizing and study, not a substitute for a full design.

Transformer Sizing Calculator: HP or kW to the Right Transformer kVA

A transformer nameplate is rated in kVA, apparent power, but a motor nameplate is rated in HP or kW, mechanical output. Converting between the two properly needs efficiency, power factor, the starting method and a safety margin, not just one multiplication. This tool takes your motor rating in HP, metric PS, kW, watts or direct kVA and works out the actual running kVA, the momentary starting kVA for whichever starting method you use, and then rounds the result up to the nearest standard transformer rating you can actually go and buy. It also handles a group of motors on one transformer with a realistic diversity factor and works in reverse to tell you the biggest motor an existing transformer can support.

How to Use

This tool has three tabs: Single Motor, Multiple Motors and Reverse (max motor).

Step 1: Setting up your supply (shared across all three tabs)

Select your System, Three-phase or Single-phase and enter the Supply Voltage (V). This is used later to work out full-load current.

Step 2: Using Single Motor mode

Enter your Motor Rating and select its Unit, HP, BHP, metric HP (PS), kW, Watts or Direct kVA if you already know the apparent power directly.
Check the Efficiency η (%) and Power Factor cosφ fields. These come pre-filled with sensible size-based defaults the moment you calculate, but you should replace them with your actual motor nameplate values whenever you have them, since real data is always more accurate than a size-based guess.
Select your Starting Method, DOL (Direct-On-Line), Star-Delta, Soft Starter or VFD/Inverter. Each one automatically fills in a typical starting current multiplier (6.5× for DOL, 2.5× for Star-Delta, 3.0× for Soft Starter, 1.5× for VFD), though you can edit the Starting Multiplier field directly if your motor’s actual locked-rotor data is different.
Enter your Safety / Future-load Margin (%), the extra capacity you want to build in for future expansion or a safety cushion. 25% is a common starting point.
Tap Size Transformer. The result shows your motor’s input power, running kVA, momentary starting kVA, which one actually governs the sizing, the recommended transformer size rounded up to a standard rating, and the full-load current for cross-checking cable and protection sizing.

Step 3: Using Multiple Motors mode

Switch to the Multiple Motors tab. Tap Add Motor for each motor on this transformer, and enter its rating, unit, efficiency, power factor, and starting method individually, since different motors on the same transformer often have very different characteristics.
Enter your Demand / Diversity Factor (%), since not every motor on a shared transformer typically runs at full load at exactly the same time. 80% is a reasonable starting assumption for a mixed group.
Enter your Group Margin (%) for the overall safety cushion.
Tap Size for Group. The tool doesn’t just add up every motor’s running kVA. It works out the genuine worst case, the single largest motor starting from cold while every other motor is already running at its demand-adjusted load, since that’s the actual moment a shared transformer is under the most strain.

Step 4: Using Reverse mode

Switch to the Reverse (max motor) tab if you already have a transformer installed and want to know what size motor it can safely support.
Enter the Transformer Rating (kVA), your Assumed Efficiency and Assumed Power Factor for the motor you’re considering, and your Reserve Margin (%), how much of the transformer’s capacity you want to keep free.
Tap Find Max Motor to see the largest motor output, in kW, HP and PS, that this transformer can comfortably support.

Step 5: Exporting your result

Use Print / PDF for a clean printable copy or Copy to paste the figures into a report.

Key Features

Accepts motor rating in HP, BHP, metric PS, kW, Watts or direct kVA, converting everything to a consistent base automatically
Size-based efficiency and power factor defaults, auto-suggested the moment you calculate, but always editable in favour of real nameplate data
Four starting methods, DOL, Star-Delta, Soft Starter and VFD, each with its own typical starting current multiplier
Separately reports running kVA and momentary starting kVA and clearly shows which one actually governs your recommended size
Automatic round-up to a standard transformer rating from a real catalogue list (10 kVA up to 3150 kVA), not just a raw calculated number
Multiple Motors mode with a genuine worst-case model, the largest motor starting while the rest of the group already runs at a demand-adjusted load, not a naive sum of every nameplate
Reverse mode to find the maximum motor size an existing transformer can support
Full-load current calculated for direct cross-reference with cable and protection sizing
Export as PDF or copy the result.

Formula / Logic Used

Motor Rating to Output Power

$P_{output} (kW) = HP \times 0.746, \quad \text{or} \quad PS \times 0.7355, \quad \text{or} \quad kW \ \text{directly}$

Running kVA

P_{input} = \frac{P_{output}}{\eta}, \quad kVA_{running} = \frac{P_{input}}{\cos\phi}

Starting kVA

kVA_{starting} = kVA_{running} \times Starting\ Multiplier

Where the multiplier is roughly 6.5 for DOL, 2.5 for Star-Delta, 3.0 for a Soft Starter and 1.5 for a VFD.

Recommended Transformer Size

$kVA_{recommended} = kVA_{running} \times (1 + Margin), \ \text{rounded up to the nearest standard rating}$

Multi-Motor Worst Case

kVA_{worst} = kVA_{starting,\ largest} + (kVA_{running,\ others} \times Demand)

This models the moment the single largest motor starts cold while every other motor on the transformer continues running at its demand-adjusted load, which is the real peak stress point for a shared transformer, not just the sum of every motor’s own individual peak.

Full-Load Current

FLC_{3\phi} = \frac{kVA \times 1000}{\sqrt{3} \times V}, \quad FLC_{1\phi} = \frac{kVA \times 1000}{V}

Reverse Mode: Maximum Motor Size

P_{output,\ max} = \frac{kVA_{transformer}}{(1 + Margin)} \times \cos\phi \times \eta

Who Should Use This Tool

Electrical site engineers and contractors selecting the correct transformer for a new motor installation, a pump house or a small factory load. Also useful for diploma and B.Tech electrical engineering students learning the relationship between motor output power, apparent power and transformer selection.

Frequently Asked Questions (FAQs)

1. Why is a transformer rated in kVA while a motor is rated in HP or kW?

A motor’s HP or kW rating describes its mechanical shaft output, while a transformer must actually supply the electrical apparent power, which depends on power factor as well as real power. A lower power factor means more current is needed to deliver the same real power, so transformers are rated in kVA to capture that current-carrying requirement directly.

2. How do I convert a motor’s HP rating into the transformer kVA it needs?

Convert HP to kW using 0.746, divide by efficiency to get input power, then divide by power factor to get running kVA. This tool does the full chain automatically and then rounds the result up to a real standard transformer rating.

3. Why does the starting method matter for transformer sizing?

A Direct-On-Line start can momentarily draw over six times the motor’s running current, which briefly demands a much larger kVA from the transformer than its steady running load ever will. This tool calculates that momentary starting kVA separately and tells you whether your running load or your starting surge actually governs the recommended transformer size.

4. How do I size a transformer for several motors running from the same source?

Simply adding up every motor’s full running kVA overestimates the real load, since motors rarely all run at full output simultaneously, and it also misses the true worst moment, which is the largest motor starting while the others are already running. This tool applies a demand factor to the running total and separately checks that specific worst-case starting scenario.

5. Can I check if my existing transformer can handle a new, bigger motor?

Yes, switch to Reverse mode, enter your transformer’s kVA rating along with the motor’s expected efficiency, power factor, and how much reserve margin you want to keep free and the tool tells you the maximum motor output, in kW, HP and PS, that transformer can support.

Related Tools

Transformer Calculator – for full transformer analysis, FLC, regulation, and parallel operation once you’ve selected a size
Cable Size Calculator – size the feeder cable using the full-load current this tool calculates
Ohm’s Law Calculator – for basic voltage, current, resistance and power relationships elsewhere in the circuit