Calculating Arc Welders and Resistance Welders

Arc Welders

The procedure for determining conductor size for arc welders is found in 630.11. To determine the current the conductors, multiply the primary current rating listed on the nameplate of the welder by the duty cycle factor that is found in Table 630.11(A).

Once the current of the conductors is found in amps, use Table 310.16 to size the conductors. Keep in mind the requirements of 110.14(C)(1)(a) and (C)(1)(b) depending on the terminations as that will effect which ampacity column, 60°C or 75°C utilized. The procedure for determining overcurrent protection for arc welders is found in 630.12. The maximum permitted overcurrent protection for a single arc welders is determined by multiplying the primary current rating of the arc welder by 200%. If the OCPD does not correspond to a standard size, the next lower standard sized OCPD should be used.

Important note: You DO NOT have to utilize the maximum overcurrent protection, which is at 200% of the arc-welders primary current ratings, where the “I eff” is missing from the nameplate. The installer can choose to use any value less than 200% if desired and in those cases 630.12 permits the use of the NEXT SIZE UP rule for standard overcurrent devices as long as you do not exceed the 200% maximum expressed in 630.12(A).

Resistance Welders

The procedure for determining conductor size for resistance welders, utilized for a specific operation that remains consistent and remain unchanged are found in 630.31(A)(2). To determine the current of the conductors, multiply the primary current rating listed on the nameplate of the welder by the duty cycle factor that is found in Table 630.31(A). Once the current of the conductors is found in amps, use Table 310.16 to size the conductors.

However, what happens if you have a resistance welder where its not always consistent, in other words it has a variety of primary current ratings and/or duty settings. In those cases, if the welding is for seam and automatic fed welders, the conductors shall be sized at not less than 70% of the resistance welders primary current rating. If the resistance welder is used for non-automatic operation, in other words manually operated, then the conductor would be sized no less than 50% of the resistance welders primary current rating.

The procedure for determining overcurrent protection for resistance welders is found in 630.32. Overcurrent protection for resistance welders is determined by multiplying the resistance welders primary current ratings by 300%. Now, the 300% is the max, but like we did with arc welders, you can have a value less than 300% and if that is the application being utilized and the OCPD does not correspond to a standard size, the next higher standard size OCPD should be used.

Lastly, the conductors sized in 630.31(A) for a resistance welder also have to ensure they are protected against overcurrent protection as well. So, per 630.32(B) as long as the overcurrent protection is not more than 300% of the conductors ampacity, it is still considered protected. If by chance the overcurrent protection for the resistance welder is higher than 300% of the conductors ampacity you must decide to increase the conductors size or reduce the overcurrent protective devices to some value below 300% to a point where the overcurrent protection is NOT more than 300% of the conductors ampacity.

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