2029 PI: 240.4(B)

[wwhitney]

Change:

240.4(B) Overcurrent Protective Devices Rated 800 Amperes or Less.
The next higher standard OCPD rating (above the ampacity of the conductors being protected), or any lower rating, shall be permitted to be used, provided all of the following conditions are met:

(1) The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord- and plug-connected portable loads.
(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).
(3) The next higher standard rating selected does not exceed 800 amperes.

If the OCPD is an adjustable trip device installed in accordance with 240.4(B)(1), 240.4(B)(2), and 240.4(B)(3), it shall be permitted to be set to a value that does not exceed the next higher standard value above the ampacity of the conductors being protected as shown in Table 240.6(A) where restricted access in accordance with 240.6(C) is provided.


Substantiation:

A rating lower than the maximum allowed should also be allowed, as it is obviously no less safe. An adjustable trip setting is the not the only way this can occur; there are listed OCPD available with fixed trip settings at values other than the standard ratings. So the permission to use values between the ampacity of the conductor and the next higher standard rating should be extended to include such OCPD.

And once that is done, there is no need for the extra language on OCPD with adjustable trip settings. 240.6(C) already adequately covers how to determine the rating of such a device, and that rating would be used for applying 240.4(B).

 

[Carultch]

Agreed with this change. I've been in non-intuitive situations where a 330A fuse governs a larger size than a 350A fuse, because 330A is non-standard. Not up to me that 330A fuses were what the equipment could come with. If 350A fusing can safely protect it, anything between 301A and 350A that's large enough for the load should be allowed in the same situation.

It's common that adjustible trip ratings yield trip settings that are non-standard. It's commonly either a decimal or binary fraction of the maximum, set by dipswitch or dial, that won't necessarily equal a standard rating. You won't find a standard 375A breaker, but it's a likely setting to expect when setting a 600A breaker to 1/8 increments via a dipswitch.