Estimates only. These results may be wrong for your specific installation, and they are not engineering or code-compliance advice. Check every result against the code that applies to you and with your local inspector (AHJ) before you install anything. Terms of Service
Control and power transformer protection and DC power supply protection and loading. Results update as you type.
Transformer
Power supply
How the transformer and power supply calculator works
Enter a transformer's VA rating and voltages and the calculator works out primary and secondary current and the largest fuse or breaker the rule allows. A second tab does the same style of check for a DC power supply from its input, efficiency and output.
Steps
- On the Transformer tab choose control or power type, phase, VA rating, and primary and secondary voltage.
- Choose whether the transformer has primary protection only or primary and secondary protection.
- Read the primary and secondary current and the protection size, with the rule used shown beneath it.
- On the Power supply tab choose a common supply from the list or enter your own input voltage, efficiency, output voltage and current.
- Enter the connected load to see how heavily the supply is loaded.
Example
A 1,000 VA single-phase control transformer, 480 V primary and 120 V secondary: primary current is 1,000 ÷ 480 = 2.08 A and secondary current is 8.33 A. With primary-only protection the tool applies 167% of 2.08 A = 3.48 A and rounds down to a 3 A fuse or breaker.
Questions
- Why round down to 3 A?
- For this case the rule gives a maximum, and the maximum is rounded down to a standard size rather than up.
- Does it cover three-phase transformers?
- Yes. Choose three-phase and enter the rating and voltages. Control and power transformers use different tables, and the tool shows which one it applied.
- Does it pick a power supply for me?
- It lists some common supplies to save typing, but you can enter any supply's values. Always follow the supply manufacturer's datasheet.
The code references on this page are pointers, not a substitute for the code. NEC® and National Electrical Code® are registered trademarks of the National Fire Protection Association (NFPA). WireAndWidgets is not affiliated with or endorsed by NFPA. Rules differ by edition and by local amendment, so check the code your inspector (AHJ) enforces before you build or install anything.