240.4(B) and Adjustable LT Settings

Xptpcrewx

Senior Member
Does anyone know if the next-size rule applies nonstandard overcurrent device ratings (other than listed in Table 240.6(A))? For example, nonstandard ampere ratings for Adjustable-Trip Circuit Breakers... Thanks in advance.
 

Carultch

Senior Member
Why would you need to use a "next size up" rule if the trip is adjustable? Just set the trip to protect the conductors.
One explanation: the trip setting may not be continuously adjustable. You may be able to set it to every 8th of the full capacity, and some of those fractions don't align with a standard rating. Consider a breaker with 600A full capacity, with a trip adjustment that goes to the eights. The 7/8 setting corresponds to 525A, which is non-standard. Using 2 copper sets in parallel, it is a question between 2x250 (510A "rounding up" to 525A) and 2x300 (570A fully exceeding 525A).

I've had this question before, with non-standard fuse ratings. The consensus was that due to it being non-standard,240.4(B) doesn't apply, because 240.4(B) requires standard ratings. It makes for a non-intuitive conclusion, that 350 kcmil can be protected by a 350A fuse, but not a 330A fuse. Why use the non-standard rated fuses? Because those were the options for that particular piece of equipment.
 
Consider a breaker with 600A full capacity, with a trip adjustment that goes to the eights. The 7/8 setting corresponds to 525A, which is non-standard. Using 2 copper sets in parallel, it is a question between 2x250 (510A "rounding up" to 525A) and 2x300 (570A fully exceeding 525A).
But couldn't you round up to the full 600A setting in this scenario? And if so, setting it to down to 525A seems like it should be permissible as well.

My take on this is considering that code is the minimum level of safety. It allows you to round up to the next standard size. If you set your adjustable setting to be below that next standard size that it would have allowed you to round up to, and you're not violating other code sections by doing so, then this should be fine. You're actually exceeding the code at that point.
 

Carultch

Senior Member
But couldn't you round up to the full 600A setting in this scenario? And if so, setting it to down to 525A seems like it should be permissible as well.
Ok, maybe not the best example, because I didn't realize that possibility. Suppose there did exist an adjustable setting on this 600A breaker between 450A and 500A, and neither 450A nor 500A are available options. Suppose it were 480A. It is a question between 460A worth of wire "rounding up" to 480A as if 240.4(B) applied, and a conservative solution of just using 510A worth of wire.
 
Ok, maybe not the best example, because I didn't realize that possibility. Suppose there did exist an adjustable setting on this 600A breaker between 450A and 500A, and neither 450A nor 500A are available options. Suppose it were 480A. It is a question between 460A worth of wire "rounding up" to 480A as if 240.4(B) applied, and a conservative solution of just using 510A worth of wire.
Interesting scenario, but I still think my second paragraph applies since it was a general statement. Basically if your wire is 460A, and you can round up to 500A, then a 480A setting is exceeding the requirement IMO and is therefor permissible.

If you don't think so, then consider the same scenario but let's say your breaker setting only goes down to 500A, and you would say this is OK. So you can set it to 500A but not 480A, if that were the lowest setting? Unless I'm completely ignoring some other fact or piece of code, that seems to go against the intent of it.

IIRC there may be some other things required for this scenario such as making sure that the breaker setting is only accessible by qualified persons, but that's probably another discussion.
 

don_resqcapt19

Moderator
Staff member
As long as the adjustable setting is equal to or less than the next larger standard rating and the wire has an ampacity greater than the next lower standard rating, I would not have an issue with it.
 

Carultch

Senior Member
Unless I'm completely ignoring some other fact or piece of code, that seems to go against the intent of it.

IIRC there may be some other things required for this scenario such as making sure that the breaker setting is only accessible by qualified persons, but that's probably another discussion.
240.4(B) only specifies standard sizes in it, so to meet the exact wording of it, you could only apply it if you are using a standard rating, and it wouldn't apply when you are using a non-standard size. The existence of non-standard ratings was probably not anticipated when it was first written. I agree that the physical reality of the situation is no different rounding up 460A to a 480A device, than it is rounding it up to a 500A device. If anything, it is more safe. You may meet the intent, but you wouldn't meet the exact words.
 
I think it’s debatable that the adjustable setting is against what’s actually written. You’re correct that it doesn’t meet 240.4(B) exactly, but you possibly could get there with logic and noting that it says “shall be permitted to be used” rather than “must be used”. It’s allowing, not forcing the jump to the next size. So if you’re allowed to go all the way to the next standard size, I think it is implied that you are allowed to go less than that (as long as something else isn’t being violated).

But to your point, this isn’t abundantly clear. If 240.4(B) started with the words “Up to” it would probably be explicit like we’d want. Would you agree?
 
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