Amada Miyachi Unitek STA-100A, Operation Manual, Page: 91 / 154

STA SERIES AC CONTROLS
990-351 C-1
APPENDIX C
SCR, WIRE GAUGE, AND CIRCUIT BREAKER
SELECTION
Basis for SCR, Wire Gauge, and Circuit Breaker Selection

SCR size is determined as the larger size needed to support
a) machine rated kVA
b) machine demand kVA

The maximum demand current an SCR can handle is conservatively assumed equal to 2.5 X rated
current (at the 50% duty cycle).

Maximum machine kVA ratings (50% duty cycle) for a given SCR allows for a 300% safety factor.

Input wire gauge must meet two criteria:
a) Less than or equal to 5% of the voltage drop with 100 feet of mains wiring at maximum
demand current, and
b) Ampacity must be larger than effective 100% RMS current based on NEC table.

Circuit breaker is determined by
a) Trip current being the lowest specified unit at least 1.25 times the maximum RMS current,
normalized to 100% of the duty cycle.
b) Trip characteristics are determined by maximum demand current and weld duration.

Wire gauge determination assumes 90°C, 600V insulation.
SCR Selection
SCR Current Rating 240V 480V
SCR 100%
DC
Current
Rating
SCR 50%
DC
30-Cycle
Weld
Current
Rating (A)
Max kVA
50%
Equivalent
Continuous
Current (A)
Max kVA
Demand
Max
Current
Demand
(A)
Max
kVA
50% Equivalent
Continuous
Current (A)
Max kVA
Demand
Max
Current
Demand (A)
300 470 35 103 85 354 70 103 170 354
600 1100 85 250 220 916 170 250 440 916
1200 1800 130 383 300 1250 250 383 600 1250
1800 2500 200 590 500 2083 400 590 1000 2083
2200 3200 250 736 583 2430 500 736 1166 2430