How should resistance tolerance be specified for industrial heating elements?

Specify resistance tolerance from the rated voltage and power, then correct for sheath diameter, material, watt density and operating temperature, since cold and hot values differ through the element's temperature coefficient, lead welds and oxide layer. At acceptance, measure each branch cold against the manufacturer's declared tolerance; on three-phase units also check branch spread to limit load imbalance. Record measured power, insulation resistance and dielectric results, and confirm tolerance together with installation geometry, control and protection settings.