Ampacity and OCPD size

[wdingman]

The definition of ampacity is as follows: “The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating” (2020 NEC).

I know that conditions of use include factors such as ambient temperature, number of conductors in the raceway, temperature limitations of terminals and splices, etc.

In a recent class with my 2nd year apprentices, we worked on a conductor/OCPD sizing question. This question came up: Does the rating of an OCPD count as part of the conditions of use? For example, if a 4 AWG copper conductor is terminated to a 20 A OCPD because a larger wire size is needed in order to reduce voltage drop, is the ampacity of the 4 AWG conductor now not more than 20 A because it cannot carry more than 20 A?

Thoughts?

 

[wwhitney]

In the 2017 NEC (the latest version for which I have a searchable PDF), the term "conditions of use" occurs 20 times. The first use is in the definition of ampacity, and the last two are in Example D3(a) in the Annex D. The rest of the uses are in contexts other than ampacity.

So there is no definition of "conditions of use" and we left to infer from those 3 examples what is meant. The first use in Example D3(a) is in the sentence (regarding ampacity): "The conductors must independently meet requirements for (1) terminations, and (2) conditions of use throughout the raceway run."

As such, I infer that "conditions of use" refers only to the conditions "throughout the raceway run," and does not include termination limitations or OCPD sizing. I.e. only those factors that show up in (2020) 310.14/310.15 in calculating the conductor ampacity.

Cheers, Wayne

 

[kwired]

The 4 AWG on a 20 amp OCPD for voltage drop reasons isn't normally subject to exceeding it's temperature rating since the OCPD will essentially be a limiting factor on how much current it will carry. It can still carry ~85 amps (less than 3 current carrying conductor in raceway or cable anyway) before temperature becomes an issue.

A 4AWG on a 20 amp OCPD because of ampacity adjustments being required however is now a 20 amp conductor in that set of circumstances that required the adjustments

 

[don_resqcapt19]

The OCPD never reduces the ampacity of a conductor, but limits the amount of current that can be supplied to that conductor.

 

[wdingman]

The OCPD never reduces the ampacity of a conductor, but limits the amount of current that can be supplied to that conductor.

Do you have a code reference for this?

 

[infinity]

The OCPD never reduces the ampacity of a conductor, but limits the amount of current that can be supplied to that conductor.

That's similar to other sections of the code that reduce the ampacity from the values in 310.16. It doesn't change the actual conductor ampacity. For example a #12 THHN conductor has a maximum ampacity of 30 amps even when it limited to 20 or 25 amps by Article 240 or some other code section it is still a 30 amp conductor.

 

[don_resqcapt19]

That's similar to other sections of the code that reduce the ampacity from the values in 310.16. It doesn't change the actual conductor ampacity. For example a #12 THHN conductor has a maximum ampacity of 30 amps even when it limited to 20 or 25 amps by Article 240 or some other code section it is still a 30 amp conductor.

And just like the provisions of 240.4(B) that permits the conductor to be protected with a OCPD rated higher than the conductor ampacity, but the conductor ampacity is not changed. The rule simply permits a higher rated OCPD on a lower rated conductor.

 

[don_resqcapt19]

Do you have a code reference for this?

There is no such rule, but there is also nothing in the code that says the OCPD has anything to do with the ampacity of the conductor.

 

[kwired]

There is no such rule, but there is also nothing in the code that says the OCPD has anything to do with the ampacity of the conductor.

correct, is actually sort of the other way around. the general rule before considering special allowed conditions is conductors need to be protected at their ampacity and to answer wdingman this is in the first part of 240.4.

 

[charlie b]

Putting more than 3 current-carrying conductors in a conduit requires applying an "adjustment factor." Installing a circuit in an ambient temperature over 30C requires applying a "correction factor." Both of these circumstances literally result in a change (i.e., reduction) of the conductor's ampacity. Both can also result in a need to change the OCPD rating. On the other hand, voltage drop and ampacity are completely unrelated concepts.

 

[jaggedben]

...

In a recent class with my 2nd year apprentices, we worked on a conductor/OCPD sizing question. This question came up: Does the rating of an OCPD count as part of the conditions of use? For example, if a 4 AWG copper conductor is terminated to a 20 A OCPD because a larger wire size is needed in order to reduce voltage drop, is the ampacity of the 4 AWG conductor now not more than 20 A because it cannot carry more than 20 A?

Thoughts?

Among other things that have been said, the answer should be 'No' to avoid confusion from circular logic. Ampacity is determined by various code sections that don't mention OCPDs. Then article 240 (or sometimes other articles) determines what rating of OCPD protects the conductors at their ampacity. If the rating of the OCPD changed the ampacity, that would make it difficult if not impossible to follow 240. It could be somewhat like a circular reference in a spreadsheet, where the OCPD changes the ampacity, which in turn changes the OCPD requirement, and so on ad naseum...

 

[kwired]

Among other things that have been said, the answer should be 'No' to avoid confusion from circular logic. Ampacity is determined by various code sections that don't mention OCPDs. Then article 240 (or sometimes other articles) determines what rating of OCPD protects the conductors at their ampacity. If the rating of the OCPD changed the ampacity, that would make it difficult if not impossible to follow 240. It could be somewhat like a circular reference in a spreadsheet, where the OCPD changes the ampacity, which in turn changes the OCPD requirement, and so on ad naseum...

Too many are in the habit of sizing the conductor to the OCPD, particularly with general use 15 and 20 amp circuits, and can get away with it most of the time as long as no adjustments are needed

The approach from code perspective is determining the load and then determining what minimum ampacity of the conductor should be including any adjustments or corrections. Then overcurrent protection is selected and adjusted/corrected conductor ampacity is a consideration of that selection.