2017 NEC - 430.52(C)(3) Instantaneous Trip Circuit Breaker for motor branch circuit short-circuit and ground-fault protection

[ZachStonePE]

Looking to hopefully clear this up. This seems pretty straightforward to me however I've been unable to find a black-and-white example either on forums, google, or by discussing with peers.

When sizing an instantaneous trip circuit breaker for motor branch circuit short-circuit and ground-fault protection, does 430.52(C)(1) Exception No. 1 apply that allows rounding up to the next standard size when the calculated size does not correspond to a standard size?


NEC 430.52(C)(3) states that instantaneous trip circuit breakers may be set to no more than the table 430.52 value:

430.52(C)(3) Instantaneous Trip Circuit Breaker. An instantaneous trip circuit breaker shall be used only if adjustable and if part of a listed combination motor controller having coordinated motor overload and short-circuit and ground-fault protection in each conductor, and the setting is adjusted to no more than the value specified in Table 430.52.

However, 430.52(C)(1) Exception No. 1 allows rounding up to the next standard size when the applied table percentage does not correspond to a standard size:

430.52(C)(1) In Accordance with Table 430.52. Exception No. 1: Where the values for branch-circuit short-circuit and ground-fault protective devices determined by Table 430.52 do not correspond to the standard sizes or ratings of fuses, nonadjustable circuit breakers, thermal protective devices, or possible settings of adjustable circuit breakers, a higher size, rating, or possible setting that does not exceed the next higher standard ampere rating shall be permitted.

For reference, this is based on the 2017 NEC, and assumes that the breaker size is adequate for starting (i.e., do not need to consider 430.52(C)(3) Exception No. 1).

Thanks for hopefully a fruitful discussion .

 

[don_resqcapt19]

The exception to (C)(1) has nothing to do with the rule in (C)(3). Exceptions are required to follow the rule that they apply to per the NEC Style Manual.

 

[ZachStonePE]

Thanks for the response Don_resqcapt19! Hoping to discuss this further.

430.52(C)(1) is titled "In Accordance with Table 430.52" for the Rating or Setting (C) of branch-circuit short-circuit and ground fault protection of individual motor circuits.

Would exception no. 1 of this rule, 430.52(C)(1), not apply to all of 430.52(C)(1) including in accordance with Table 430.52, including the Table 430.52 sizing column for Instantaneous Trip Breakers?


A few rules down, in 430.52(C)(3) Instantaneous Trip Circuit Breaker, there are additional percentage values to not exceed specifically for Instantaneous Trip Circuit Breakers if the initial Table 430.52 sizing does not allow for motor starting. 430.52(C)(3) points to using Table 430.52 first, so I'm just having a hard time seeing how 430.52(C)(1) exception no. 1, which applies to using Table 430.52 does not apply to instantaneous trip breakers (MCPs).

If the instantaneous trip breaker sizing percentages did not appear in Table 430.52, and instead appeared only in a separate table all on their own under 430.52(C)(3) Instantaneous Trip Circuit Breaker it would be clear as day.

Otherwise to me, it would appear that the exceptions in 430.52(C)(1) would apply to all of the conditions in the table especially since exception no. 1 says for "possible settings of adjustable circuit breakers" would include an adjustable (instantaneous) circuit breaker.

 

[don_resqcapt19]

I don't believe there are standard size ratings for instantaneous trip breakers.

The "adjustable trip breakers" in the exception is referring to the breaker where there are adjustable long time trip settings and not just an adjustable instantaneous trip. The magnetic only breakers with the instantaneous trip adjustments are not considered "adjustable trip breakers'"

240.6(B) Adjustable-Trip Circuit Breakers.
The rating of adjustable-trip circuit breakers having external means for adjusting the current setting (long-time pickup setting), not meeting the requirements of 240.6(C), shall be the maximum setting possible.

Where the 800% trip setting in the table does not work, you apply the permissions in 430.52(C)(3).

 

[ZachStonePE]

Thanks for the follow-up, it helps and also matches most of what I've found this morning digging through the 2017 NEC, the newest 2023 NEC where this portion has been slightly re-written, and digging through several manufacturer spec sheets and tables for sizing motor circuit protectors (like this one from Eaton).

 

[don_resqcapt19]

Thanks for the follow-up, it helps and also matches most of what I've found this morning digging through the 2017 NEC, the newest 2023 NEC where this portion has been slightly re-written, and digging through several manufacturer spec sheets and tables for sizing motor circuit protectors (like this one from Eaton).

Just don't forget that the instantaneous trip breaker can only be used as a part of a listed combination motor starter. It is rare that an electrician would be sizing one, as when you order the combination starter you specify the motor horsepower and the starter manufacturer installs the correct size of instantaneous trip breaker.
Such a breaker could only be field installed as a replacement device in a listed combination motor starter.

 

[ZachStonePE]

Just don't forget that the instantaneous trip breaker can only be used as a part of a listed combination motor starter. It is rare that an electrician would be sizing one, as when you order the combination starter you specify the motor horsepower and the starter manufacturer installs the correct size of instantaneous trip breaker.
Such a breaker could only be field installed as a replacement device in a listed combination motor starter.

Absolutely, and great to note.

My experience with sizing these in the past in the industrial sector was always assisted by the manufacturer that involved not just sizing the MCP but the entire "bucket" with starter and OL.

There was another good thread on here that I found while trying to find more information on the above exception that was a good read-through and reminder about the UL requirements