Read your question please!

Your 20 amp breaker is opening when it sees a sustained 100 amps. Thank you, to the breaker company for protecting you.

Is your welder supposed to be connected via a 20 amp breaker? If it is, are you sure that you have the proper type breaker for use with the welder.

FYI - Additional information:

(I do realize that you are talking about the 100 Amp Output of the welder. You need to put an ammeter on the conductors feeding the welder to make sure that the breaker is seeing the correct amperage. Per the inputs (highlighted in red below, I would have chosen to use a 25 or 30 (see NEC 240.6 - you can link to it above.) amp breaker. What did the installation instructions call for?)


Specifications
Primary Input Voltage/Phase: 115/208/230 vac, 1 ph, 50/60 Hz
Voltage Tolerance: +/- 10%

115V Operating Data
- Output Range (Stick): 5A/20V to 85A/23.5V
- Stick Duty Cycle & Rated Output: 100% - 85A @ 23.4V
- Output Range (TIG): 5A/20V to 100A/14V
- TIG Duty Cycle & Rated Output: 100% - 100A @ 14.0V
- TIG Duty Cycle & Rated Output: 60% - 130A @ 15.0V
- Primary Input Current @ 100% Duty Cycle: 19A
- Primary Input Current @ 60% Duty Cycle: 26A
- Open Circuit Voltage (max): 83 V

230V Operating Data:
- Output Range: 5A/20V to 150A/26V
- Stick Duty Cycle: 110A/24.5V @ 100%, 140A/25.6V @ 60%
- TIG Duty Cycle: 110A/14.5V @ 100%, 150A/16V @ 60%
- Primary Input Current @ 100% Duty Cycle: 17A
- Primary Input Current @ 60% Duty Cycle: 23A
- Open Circuit Voltage (max): 83V

Net Weight: 18 lbs
W 5.75 in x L 15 in x H 11 in

Comes Complete With:
- Power Source
- 10 ft. Work cable & Clamp
- 10 ft. welding cable w/electrode holder
- 1# 6013LV electrodes
- Plastic carrying case