Resistor elements, materials and tolerances

All the stainless steels used to make NER elements increase in resistance as they get hotter, typically between 1% and 15% per 100¡C rise. There is a cost penalty when low-coefficient materials are specified; to what extent this is worth paying is not always clear.
Many NER's are over-specified in this respect, as much because engineers are intuitively uncomfortable with the idea of a resistor with a very wide current range than as a result of a performance analysis. There is certainly scope for cost saving by accepting the use of higher temperature coefficient, lower cost steels.
We make extensive use of grade 430 stainless steel; this is a low-cost 18% chrome alloy with little or no nickel content but with good corrosion and high-temperature performance.
It has a temperature coefficient of resistance of about 14% per 100¡C, which makes it unsuitable for continuous applications, but it is widely used in grid resistors for dynamic braking, motor starting and neutral earthing for which the key property is high heat capacity rather than close resistance tolerance.