230.79

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charlie b

charlie b

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This is not as simple a question as it may, at first, appear to be. What does 230.79 mean when it says the "service disconnecting means shall have a RATING (my emphasis) not less than . . . "? To be specific, if a main service switchboard has a rating of 5000 amps, and if its main breaker has a frame size of 5000 amps but is set to trip at 4500 amps, what is the "RATING" of that breaker, in the intended context of the article?
 
charlie b said:
This is not as simple a question as it may, at first, appear to be. What does 230.79 mean when it says the "service disconnecting means shall have a RATING (my emphasis) not less than . . . "? To be specific, if a main service switchboard has a rating of 5000 amps, and if its main breaker has a frame size of 5000 amps but is set to trip at 4500 amps, what is the "RATING" of that breaker, in the intended context of the article?
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Interesting - never gave it much thought until you brought it up. Huummmm:
1. Looking at 230.79.A-D makes it sound like it is the number on the CB handle or the fuse size.

2. However, if one looks at 240.6.B and 240.6.C which is likely more applicable , it would depend on if the trip adjustment was restricted.

3. And then there is 230.95, which for picking a GFE, is based on the largest fuse or CB adjustment available.

For the "intended context of the article" you picked, I like door number 2. Restricted access - 4500A. Un-restricted access - 5000A

ice
 
I am no longer in close proximity to a code book, but I think I know the answer to my follow up question. Here it is:
  • The service load calculation gives me a result of 4800 amps. I have accounted for the continuous load, the largest motor, and all applicable demand factors.
  • We all know that the actual load will almost never exceed 4000 amps, and a max of 3500 is more likely. Nevertheless we follow the code.
  • So I select a service switchboard at 5000 amps.
  • I select a main breaker with a frame size of 5000 amps.
  • I provide for service conductors rated at least 4800 amps.
  • And for reasons I will give you later, I choose (with owner's agreement) to set the trip at 4500 amps.

Would I have broken any code rule?
 
I cannot find a violation.

Dialing down the breaker to 4500A instead of 4800A, your calculated load and conductor size, only changes the trip limit to a lower setting than required.

The only violation I see would be setting it over 4800A.

Am I missing something?
 
jumper said:
I cannot find a violation.

Dialing down the breaker to 4500A instead of 4800A, your calculated load and conductor size, only changes the trip limit to a lower setting than required.

The only violation I see would be setting it over 4800A.

Am I missing something?
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Yes, you are.
The quoted text says "not less than", so setting the trip too *low* compared to what the later part of the sentence references would be the problem.
 
230.79 mentions the rating of the disconnecting means, it does not care what its overcurrent setting is: therefore 'rating' must be the maximum continuous amperes (e.g. the value per the devices listing).
230.90(A) deals with the rating, or setting, of the overcurrent protection. Clearly, Article 230 considers rating and setting as two different items.
 
jim dungar said:
230.79 mentions the rating of the disconnecting means, it does not care what its overcurrent setting is: therefore 'rating' must be the maximum continuous amperes (e.g. the value per the devices listing).
230.90(A) deals with the rating, or setting, of the overcurrent protection. Clearly, Article 230 considers rating and setting as two different items.
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:thumbsup:
 
My take is that 230.79 is satisfied by way of the 5000 amp frame breaker. I am concerned (and will check the NEC in the morning) about something in 240 requiring the OCPD to be sufficient for the load. It will be sufficient for the ampacity of the conductors, but the load is a different question.

This is a real world situation. The problem we have is that the utility is telling us their CTs cannot handle more than 4500 amps. At present, the only "easy" solution under discussion is the 4500 amp trip rating. The "much harder" solution would be to ask for two services. Given that the design was issued for construction already, that becomes a very hard solution indeed. We are considering changes now because a tenant was selected and that tenant has new ideas (i.e., new and larger loads than the owner expected, and that we designed for).
 
The way I see it, you have met the requirement of 230.79 as the rating of your equipment and device are not less than the calculated load and you have met the requirement of 230.90 by setting the device so as to protect the conductors.
Strange to me that you would be requested to set it below the load but not a violation, IMO.

You would need to comply with 240.82
 
This morning I learned from the switchboard manufacturer that the breaker can set to 4500 amps. But it would have to be locked at that setting (using a password), and the switchboard would have to be labeled as having a 4500 rating. If that turns out to be true, this solution would no longer be viable, as it would fail on 230.79. More updates as they become available.
 
charlie b said:
This morning I learned from the switchboard manufacturer that the breaker can set to 4500 amps. But it would have to be locked at that setting (using a password), and the switchboard would have to be labeled as having a 4500 rating. If that turns out to be true, this solution would no longer be viable, as it would fail on 230.79. More updates as they become available.
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Rember that most manufacturers do not know NEC requirements and interpretations. They are great with manufacturing standards, but they usually have limited experience with installation codes.

I see no NEC provisions that would prevent your 5000 rating (bussing and breaker frame) along with a 4500A overcurrent rating.
However, if the utility CTs are being installed internal to the switchboard, their 4500A rating becomes the limiting factor for the equipment labeling.
 
charlie b said:
My take is that 230.79 is satisfied by way of the 5000 amp frame breaker. I am concerned (and will check the NEC in the morning) about something in 240 requiring the OCPD to be sufficient for the load.
Click to expand...

Without knowing the section off the top of my head, I do agree with you about the article 240 issue. Doesnt the OCPD have to meet or exceed the load calc? It seems pretty basic, but again I dont know where that is stated. Are the rules ever different for an adjustable trip setting breaker vs a fixed breaker?
 
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