Code Quiz

Dec 22, 2005 5:31 PM, By Steven Owen

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 necexpert@aol.com.