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Tuesday, September 27, 2016
Question:
Determining the ampacity of arc welder supply conductors and overcurrent protection in Article 630 seems a little confusing. What would be the minimum allowable conductor size and maximum OCPD rating for a single arc welder (non-motor generator) rated at 47.5 amps input with a 20% duty cycle? What does it mean in 630.12 when it states where the OCPD rating specified results in unnecessary opening of the overcurrent device, the next higher standard rating shall be permitted?
Thank You,
Steve Manak
A
Answer:
Hey Steve thanks for your question. If the welder nameplates are not marked with an I1eff value the rated primary current can be multiplied by the applicable factor in Table 630.11(A) as stated in 630.11(A) to determine the minimum conductor size. In your example a factor of 0.45 (20 % duty cycle) would be multiplied by 47.5 resulting in a minimum conductor size that has an ampacity of 21.375 (21) based on the conditions of use for your specific application.
The overcurrent protection for the welder is covered in 630.12(A) and the overcurrent protection for the welder supply conductors is covered in 630.12(B). Use the I1max value if given or 200 % times the rated primary current (200 X 47.5 = 95 amperes in your example. Because 95 is not a standard rating in 240.6 a 100 ampere overcurrent device is allowed as stated in 630.12.
If unnecessary opening of the overcurrent device is caused by frequent starting of the welding process the next higher standard rating or setting is also allowed.
Keep in mind that many welding tasks are not continuous, due to prep work, changing electrodes etc. so the conductor size is based on the heating effect on the conductor. Frequent starting can result in "inrush currents" to be present more often - which can cause the overcurrent device to operate. The duty cycle for an arc welder is the amount of time the welder can operate at full rating in a 10 minute period (20% means 2 minutes operation, 8 minutes rest).