NEC Interpretation: Overcurrent Protection for Electric Welders

[ProjectDelta]

I am a 4th year apprentice and we are currently covering electric welders in school. My instructor gave us an example of the information on a welder name plate and told us to calculate the conductor size and overcurrent protection required. He and I came up with different answers and I believe that he may be incorrect based on the way he is interpreting the code.

As stated in 630.32 "....where the values as determined by this section do not correspond with the standard ampere rating ratings provided in 240.6 or where the rating or setting specified results in the unnecessary opening of the overcurrent device, a higher rating or setting that does not exceed the next higher standard ampere rating shall be permitted."

In the example given, the calculation for overcurrent protection did not correspond to a standard breaker size however, the next higher breaker size exceeded 300 percent of the conductor ampacity. So based on the beginning of 630.32, I interpret the verbiage to allow the use of the next higher breaker size even though 630.32(B) states that overcurrent protection cannot exceed 300 percent of conductor ampacity. He stated that you cannot round up in this scenario. His reasoning was that the opening paragraph to 630.32 only applies to 630.32(A) and not (B). This confuses me because based on the excerpt above, specifically "....where the values determined by this section....." would encompass the entire section and not just half of the section. He's been doing this a lot longer than I have but it just doesn't seem right. Perhaps someone can offer a different explanation/interpretation?

 

[Never Stop Learning]

630.32 states the calculations from (A) and (B) shall be standard per 240.6 - ultimately Table 240.6(A), up to the nearest standard amperage rating.

630.32 has two situations, (A) and (B).
(A) For Welders
(B) For Conductors

(A) States the overcurrent protection cannot exceed 300% of the welder primary current (welder is not plural). If the supply conductors are protected by an overcurrent device not more than 200%, a separate overcurrent device is not required.
(B) States the conductors shall not be protected by an overcurrent device rated more than 300% of the conductor ampacity.

If your calculations put your overcurrent protection @ 299A for 300%, then selecting a 250A breaker would be allowed. Selecting a 300A breaker would not be allowed because it exceeds the 300% limitation.

Think of the 300% as your upper limit. That does not mean you have to hit that limit. If a standard breaker size puts you at 275% and the next larger breaker would put you @ 325%, you would be limited to the 275% option. At least this is how I interpret this section.