The Resistance Welding Manufacturing Alliance “RWMA” has defined duty cycle in the Resistance Welding Manual, 4th Edition, Section 19-3. It reads “Since resistance welding equipment is used intermittently, the rating is based on a 50 percent duty cycle over a one minute (60 sec) integrating period”. This value was determined long ago and has served the industry well. ISO 5826 also specifies the same 50% duty cycle one minute integration period.
Transformer and machine designers design to a 50% duty cycle. Since most resistance welders operate in the 20% range, equipment has proven to be very robust. Duty cycle is actually a heat rating to protect the insulation in the transformer.
In other industries and specific applications transformers are manufactured at various other duty cycles ranging from 20 – 100%.
Reference: RWMA – Resistance Welding Manual Section 19.3
ISO - ISO 5826