**Q.** What is the minimum circuit ampacity and minimum size conductor required for the feeder conductors supplying the following group of electric arc welders?

- one each—80A rated, non-motor generator, 100% duty cycle.
- one each—70A rated, motor generator, 90% duty cycle.
- one each—60A rated, non-motor generator, 70 % duty cycle.
- one each—50A rated, non-motor generator, 80% duty cycle.
- four each—40A rated, motor generator, 60% duty cycle.
- four each—25A rated, non-motor generator, 40% duty cycle.

Terminals at the feeder overcurrent device and in the welder disconnects are rated 75°C. The feeder consists of one conductor per phase, which is specified as THHN insulated, dry location.

**A)** 350 kcmil THHN**B)** 400 kcmil THHN**C)** 500 kcmil THHN**D)** 600 kcmil THHN

**Answer: C**

**Explanation:** As per a change to the 2005 NEC, section 630.11(B) was revised to note that the minimum conductor ampacity shall be based on the individual currents determined in 630.11(A) as the sum of: 100% of the two largest welders, plus 85% of the third largest welder, plus 70% of the fourth largest welder, plus 60% of all remaining welders.

Therefore, the answer is calculated as follows (individual calculations are in same order as listed above):

80 x 1.00 = 80A

70 x 0.96 = 67.2A

60 x 0.84 = 50.4A

50 x 0.89 = 44.5A

40 x 0.81 x 4 = 129.6A

25 x 0.63 x 4 = 63A

80 + 67.2 + (0.85 x 50.4) + (0.70 x 44.5) + [0.60 x (129.6 + 63)] =

80 + 67.2 + 42.84 + 31.15 + (0.60 x 192.6) =

80 + 67.2 + 42.84 + 31.15 + 115.56 = 336.75, round up to 337A

From Table 310.16, 75°C column, we can see that a 500kcmil conductor is rated at 380A. This is the minimum size conductor that will work for this particular application.

*Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states. He can be reached at [email protected].*