'next size up'-ing ocpd for branch circuits...

[ToolHound]

Subject: ocpd 'next size up' rule(s), 240.4 etc. as applied to branch circuits.


I aim to shape my understanding of 'next size up' v's branch circuits.

What general statement(s) can be made about 'next size up'-ing OCPD's for branch circuits ?

In light of

240.4(D) (ocpd ratings for small conductors), and in light of

210.3 'Rating' (Branch Circuits, including max ocpd ratings for branch circuit ratings set forth therein), and in light of

Table 210.24 (summary of branch circuit requirements, including OCPD protection), and in
light of

240.4(B)(1), a pre-condition of 'next size up'-ing saying "The conductors being protected are not part of a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads." (similar requirement in 210.19A(2) ),

instances would seem rare and slim that a 'next size up' decision could be made for a branch circuit, except for applications under 240.4(G) Overcurrent Protection for Specific Conductor Applications as compiled in Table 240.4(G).


Would you find any credibility in the above statement?

How could it be changed to be more accurate or in accord with the code ?

I am trying to solidify or gain perspecitve on this subject of 240.4(B) provision, quote,

"The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met...."

Any comments, observations, suggestions, etc, appreciated.

--Toolhound

 

[Dennis Alwon]

My understanding is that once all the conditions of the NEC were applied to arrive at the overcurrent protective device then one could use the next size up rule if applicable. There are situations were the next size will not apply

 

[infinity]

Branch circuits that have nonstandard values after calculation such as derating can use the next standard size up rule. This could apply to many circuits just not those with more than one receptacle.

 

[kwired]

I don't quite understand what you are asking. But I think you are asking why are we allowed to use next size up ocpd.

Bottom line I think is that a properly sized conductor is not going to be overloaded to a point that creates significant heat that may have destructive results. If we have a conductor with an ampacity of 57 amps, only apply 52 amps of load to it, it is considered properly protected by a 60 amp device. True it can be overloaded by 3 amps, but resistive loads are fixed and this wouldn't happen, and motors have overload protection that would still protect the conductor according to full load rating of the motor. (minimum size of a motor conductor is 125% but so is max overload setting in most cases)

You don't design a circuit by selecting overcurrent device before selecting conductor size.

When designing a circuit you first must know the load and whether it is continuous or non continuous. You next must have a conductor with a minimum amount of ampacity to carry the load. Depending on conditions you may need to adjust ampacity of the conductor(s) but you are still determining minimum ampacity of conductor required. After you have selected minimum ampacity (not the actual size, but the calculated minimum ampacity) of the conductor you then have to provide overcurrent protection for that conductor. If the overcurrent device is going to be over 800 amps then the minimum ampacity of the conductor must be equal or more than the overcurrent protection value. If the device is 800 amps or less you are permitted to use next standard size higher.

 

[hurk27]

I hope the OP is not talking about 14,12, 10, or #8 AWG as these wire sizes all correspond to a breaker rating that would not allow next size up, not until you get to #6 NM at 60? rating do we see a rating of 55 amps where one could use a 60 amp breaker for the next size up and #6 THHN/THWN at 65 amps where one could use a 70 amp breaker, this is because after 45 amps the breaker sizes in 240.6 starts on the tens as 50,60,70 etc....

Also the smaller conductors have a set rating in 240.3(D) 1-7 unless E or G apply

Derating could get you to a place that you might be able to use the next size up on smaller conductors where only one receptacle outlet is used or the load is hard wired, but that would be a rare case.

 

[kwired]

I hope the OP is not talking about 14,12, 10, or #8 AWG as these wire sizes all correspond to a breaker rating that would not allow next size up, not until you get to #6 NM at 60? rating do we see a rating of 55 amps where one could use a 60 amp breaker for the next size up and #6 THHN/THWN at 65 amps where one could use a 70 amp breaker, this is because after 45 amps the breaker sizes in 240.6 starts on the tens as 50,60,70 etc....

Also the smaller conductors have a set rating in 240.3(D) 1-7 unless E or G apply

Derating could get you to a place that you might be able to use the next size up on smaller conductors where only one receptacle outlet is used or the load is hard wired, but that would be a rare case.

True the values in table 310.15(B)(16) are all in increments of 5 and do correspond to a standard size until we reach 55 amps. But start making ampacity adjustments and even the smaller conductors will end up being in situations where you have to decide if you can go to the next size up with overcurrent protection.

Example - take a 8AWG 75C conductor rated 50A and apply a .82 adjustment to it for ambient temp and you end up with conductor ampacity of 41. This conductor can not be used if the load is greater than 41. It can be protected with a 45 amp overcurrent device though.

 

[ToolHound]

Edit post button ???

Edit post button ???

​

is there a button/capability that will allow me to return to my own post and edit it?

I have looked but cannot see any button to click for going back into my post so I may edit/correct what I originally wrote. Is it possible to edit my own post ?

I am still learning how to use the various thread commands here in the Holt forums. Is there a button to edit my own post ?

Thanks,
ToolHound

 

[GoldDigger]

​

is there a button/capability that will allow me to return to my own post and edit it?
I have looked but cannot see any button to click for going back into my post so I may edit/correct what I originally wrote. Is it possible to edit my own post ?
I am still learning how to use the various thread commands here in the Holt forums. Is there a button to edit my own post ?

Thanks,
ToolHound

When you first make a post, there is indeed an edit button (but it does not include the option to completely delete the post.)
However, to maintain accountability and keep the flow of comments readable, there is a timeout after which the OP can no longer edit the post. Moderators can always edit and much more.

PS: This is an edit.