If you need kva with spare capacity, minimum kva required and single suggested size and you already have load current (i), load voltage (v), spare capacity, load current (i), load voltage (v) and spare capacity, this calculator does the substitution for you. It suits a quick check, a homework answer you want to confirm, or any figure you need to sanity test.
What Is the Transformer Sizing Calculator?
This transformer sizing calculator is for the kind of problem where you know the physical quantities but not the number you need. It turns load current (i), load voltage (v), spare capacity, load current (i), load voltage (v) and spare capacity into kva with spare capacity, minimum kva required and single suggested size.
Physics calculations go wrong in predictable ways, usually through mixing units or dropping a digit. Work through the result one step at a time and check that the figures are the size you would expect at each stage.
How the Transformer Sizing Physics Works
The relationship is applied step by step, and each step is a standard piece of physics rather than a single opaque formula. That means you can check the intermediate values against what the theory predicts, which is usually faster than checking only the final answer.
Keep every value in the same unit system throughout the calculation. Mixing metres and feet part way through is the single most common reason a correct formula gives a wrong answer.
Units, Assumptions and the Limits of the Model
The calculation assumes ideal conditions: no air resistance, no heat loss, and dimensions that do not change while you work. Those assumptions are reasonable for most classroom and engineering estimates, and unreasonable for anything more precise.
Where a value is a rate, it has to be quoted for the same period as everything else. A per-hour figure cannot be combined with a per-day figure without converting one of them first.
A Worked Example
Working through an example, with load current (i) set to 10, load voltage (v) set to 5, spare capacity set to 2, the calculator returns minimum kva required of 0.05, and minimum kva required (t rating) of 1.73205, and kva with spare capacity of 0.051. Substitute your own figures and the same steps produce the same kind of answer.
Frequently Asked Questions
What do I need to work out kva with spare capacity?
Enter load current (i), load voltage (v), spare capacity, load current (i), load voltage (v) and spare capacity. The calculator then works through the relationship and reports kva with spare capacity, minimum kva required and single suggested size.
What does the transformer sizing actually calculate?
It takes load current (i), load voltage (v), spare capacity, load current (i), load voltage (v) and spare capacity and converts them into kva with spare capacity, minimum kva required and single suggested size using the standard relationship for this problem. Each step is applied in turn, so every output is traceable back to the numbers you typed in.
How should I enter load current (i)?
Use the value you actually have, in the unit shown next to the field. Enter every value in the same unit system. The calculator assumes the figures you supply are consistent, so it cannot correct a mistake that happens before the calculation starts.
Do I have to fill in every field?
Only the fields a particular result depends on. If you leave one blank, the results that need it simply show a note instead of a number, and everything else still works.
Can I use this for a quick estimate as well as a precise answer?
Yes. The calculator is built for quick checks, so round your inputs to a sensible number of significant figures and the result will be just as useful. If you need more precision, keep the same method and add digits to the inputs rather than adjusting the method itself.