Transformer Size Calculator

Because the two things that destroy a transformer are both driven by current and voltage rather than by useful work. Copper loss is I²R in the windings and depends only on how many amperes are flowing, regardless of whether those amperes are in phase with the voltage; core loss depends on the applied voltage and the frequency. A 100 kVA transformer at a load power factor of 0.5 delivers 50 kW and is fully loaded; the same unit at unity delivers 100 kW and is equally fully loaded. Rating it in kilowatts would mean the plate changed meaning every time the load did, which is also why a load stated in kW has to be divided by its power factor before it reaches this page.

No, and that is the point of rounding into the manufactured series rather than reporting the arithmetic. Three-phase dry-type distribution transformers are built at 15, 30, 45, 75, 112.5, 150, 225 and 300 kVA and the single-phase series runs 15, 25, 37.5, 50, 75, 100 — so 43 kVA becomes a 45 kVA three-phase unit and there is nothing in between to specify. Above 300 kVA three-phase and below 15 kVA the series thins out and diverges between manufacturers, and control and buck-boost transformers are a different product family with a much finer series of their own; confirm against a catalog rather than against this page.

The low-voltage winding carries the higher current and gets the bigger conductor, by exactly the turns ratio — a 480 to 208 volt transformer puts 2.3 times as many amperes on the secondary as on the primary. Each winding's conductors are sized on its own current, and this is where a 450.3(B) answer and a conductor answer part company: the primary device is permitted to be 125 percent of primary full-load current, or 250 percent where a secondary device is also fitted, and those percentages protect the transformer, not the conductors on either side of it. Secondary conductors need their own protection, and the exceptions that let them run unprotected for a distance are separate rules with their own length and termination conditions. A device chosen from the transformer table alone has not been checked against any wire.

The nameplate kVA is a continuous rating at a stated ambient temperature and a stated winding temperature rise, so 100 percent loading is legal, sustainable and not the same as being at a limit you should approach with confidence. What it costs is insulation life: the published loading guidance is written in terms of hot-spot temperature and the rate at which insulation ages, not as a percentage of nameplate, and a transformer run hot in a closed electrical room with poor ventilation is being aged faster than the same unit at the same load in open air. The growth allowance on this page defaults to 25 percent because replacing a transformer is a shutdown, but it is a design choice with no code figure behind it — set it from growth you can name, and set it to zero for a fixed load that will never change.

It is measured on the individual transformer at the factory and stamped on its nameplate, and no code or standard prescribes a value — which is why this site has no impedance table and will not pretend to one. It matters twice. Available fault current at the secondary terminals is inversely proportional to it, so a 5 percent unit lets through roughly twenty times its own full-load current and a 3 percent unit closer to thirty-three times; guess it low and you overstate the interrupting rating the downstream gear needs. It also sets voltage regulation: a higher-impedance transformer sags more between no load and full load, which is why the impedance somebody chose to limit fault current is the same impedance that shows up as a lighting flicker when a large motor starts.

Yes, and not by a percentage you can guess from the kVA. Harmonic currents from switch-mode power supplies, LED drivers and six-pulse drives raise eddy-current losses in the windings much faster than they raise the RMS current, so a transformer at 80 percent of nameplate feeding a heavily nonlinear load can be running hotter than the same unit at full load feeding motors. The answer the industry settled on is the K-rated transformer, built with larger conductors and a heavier neutral to tolerate the extra loss, with the K number describing how much harmonic content it is designed for. What this page returns is the volt-ampere requirement; whether that quantity has to be bought as a K-rated unit is a question about the load spectrum, and the harmonic content is measured rather than calculated.