Electrical relationships help check a heater RFQ, but the formula must match the circuit. A single resistive element, several parallel heaters and a three-phase bank can have the same total power while using different current and resistance values.
Use this guide to prepare a design review or RFQ. Final ratings depend on the complete product, installation, controls and operating environment.
Selection inputs that change the design
| Decision | Information to provide | Why it matters |
|---|---|---|
| Single resistive load | P = V × I and R = V² / P | Checks rated current and hot resistance basis. |
| Parallel elements | Each branch sees supply voltage | Branch currents add at the supply. |
| Series elements | Supply voltage divides by impedance | One open element interrupts the path. |
| Three-phase bank | Connection and line voltage matter | Delta and wye use different element voltage. |
Name the voltage at the element
State supply voltage, phase and whether the quoted voltage is line-to-line, line-to-neutral or across each element. Draw the intended connection for multi-element banks.
Do not calculate element resistance from total three-phase power without first identifying delta or wye connection and element count.
Separate rated values from measured values
Resistance wire changes with temperature, and meter accuracy plus lead resistance affects low-ohm measurements. The quotation should state whether resistance tolerance is referenced cold.
Current measured in service also reflects actual supply voltage and control modulation. Compare values under a defined stable condition.
Coordinate circuit protection and control
Provide total current, branch current, switching method and number of control stages to the electrical designer. Account for conductor, terminal and enclosure temperature.
Protective devices and grounding must follow the finished equipment design and applicable electrical requirements; heater formulas alone do not size the whole system.
Common specification mistakes
- Using line voltage as element voltage without checking connection
- Confusing total bank resistance with one element
- Expecting cold and hot resistance to be identical
- Sizing controls from nominal power only
RFQ checklist
- Supply voltage and frequency
- Single- or three-phase connection diagram
- Total and per-element power
- Expected current and resistance tolerance
- Control stages, switching and protection interface
Review the matching Custom Heating Elements product range, then send this checklist with your drawing for an engineer-reviewed quotation.
Frequently asked questions
How do I calculate current for one heater?
For a primarily resistive single load at its rated voltage, divide power by voltage.
How do delta and wye differ?
In delta each element is connected across line voltage; in wye each balanced element sees line voltage divided by the square root of three.
Why does measured resistance differ from the label?
Temperature coefficient, measurement leads, tolerance and the distinction between one element and the assembled circuit can all matter.