Conductor's base or derated ampacity

[designer82]

240.4(C) requires OCPD to be sized so the conductor's ampacity is equal to or greater than the OCPD rating.

Does this mean the conductor's base ampacity or derated ampacity?

I can't find where this is stated in the code.

 

[don_resqcapt19]

If by "base ampacity" you mean the ampacity shown in Table 310.16, that ampacity only applies to the conditions stated.

310.16 Ampacities of Insulated Conductors in Raceway, Cable, or Earth (Directly Buried).
The ampacities shall be as specified in Table 310.16 where all of the following conditions apply:
(1)Conductors are rated 0 volts through 2000 volts.
(2)Conductors are rated 60°C (140°F), 75°C (167°F), or 90°C (194°F).
(3)Wiring is installed in a 30°C (86°F) ambient temperature.
(4)There are not more than three current-carrying conductors.

If the conditions are not as shown above you must adjust or correct the ampacities.

310.15 Ampacity Tables.
(A) General.
Ampacities for conductors rated 0 volts to 2000 volts shall be as specified in the Ampacity Table 310.16 through Table 310.21, as modified by 310.15(A) through (F) and 310.12. Under engineering supervision, ampacities of sizes not shown in ampacity tables for conductors meeting the general wiring requirements shall be permitted to be determined by interpolation of the adjacent conductors based on the conductor’s area.

The temperature correction and adjustment factors shall be permitted to be applied to the ampacity for the temperature rating of the conductor, if the corrected and adjusted ampacity does not exceed the ampacity for the temperature rating of the termination in accordance with the provisions of 110.14(C).

That section requires the use of the ampacities that have been modified for temperature and/or number of current carrying conductors and also permits the use of the reduced size conductors for dwelling unit services. The rules in 310.15(B) are the ambient correction factor rules, and the ones in 310.15(C) are the rules for adjusting the ampacity for more than 3 current carrying conductors.
The ampacity of the conductor is always under the conditions of use as stated in the definition of ampacity.

Ampacity.
The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. (CMP-6)

 

[Dennis Alwon]

The de-rated ampacity of a conductor cannot be less than the overcurrent protective device except in a few cases such as motors.

 

[wwhitney]

Wherever the bare word "ampacity" is used, it means what you might call derated ampacity. That's what the definition in Article 100 means.

Unfortunately, some CMPs misuse the word to mean "ampacity value from the table, without adjustment or correction", what you might call base ampacity. E.g. 215.2(A)(1)(a) wrongly uses "ampacity" when what it means is base ampacity. In this case we are left to infer the correct meaning because of the contrast with 215.2(A)(1)(b), which clearly does mean "ampacity".

Cheers, Wayne

 

[Tulsa Electrician]

I get into this all the time at work. They say 14 #12 thhn current carrying conductors are ok in a 3/4. I said yes you put that many in there.
However there no longer good for a 20 amp branch circuit after derating. They always say I'm wrong because the inspector did not turn it down. They think of it it fits your good.

 

[kwired]

I get into this all the time at work. They say 14 #12 thhn current carrying conductors are ok in a 3/4. I said yes you put that many in there.
However there no longer good for a 20 amp branch circuit after derating. They always say I'm wrong because the inspector did not turn it down. They think of it it fits your good.

I think that also depends on what loads are.

general use multi receptacle outlets, you have to assume load can be up to 20 amps on each circuit if you have a 20 amp OCPD.

If all the conductors are supplying fixed loads, but none put you over adjusted ampacity you maybe could use the 20 amp OCPD, though there also sort of no reason to use anything but 15 amp OCPD other than for motor starting reasons - but I do know you can do this with motor circuits - the motor overload protection will still limit current and ultimately still protect the conductors as well.

Been several times I've run multiple motor circuits in raceways and gotten into the 50 or even 45% deration factors. With 480 volts especially you still can run a lot of 5 hp and less motors on all 14 awg, 7.5 and 10 hp adjustments start making you run larger conductors though depending on exactly what the adjustment factor might be.

 

[ggunn]

The de-rated ampacity of a conductor cannot be less than the overcurrent protective device except in a few cases such as motors.

Only if the current is greater than 800A. For current of 800A or less the next standard size OCPD up from the conductor ampacity derated for conditions of use is acceptable. 240.4(B) compliant with (1), (2), and (3).

 

[Dennis Alwon]

Only if the current is greater than 800A. For current of 800A or less the next standard size OCPD up from the conductor ampacity derated for conditions of use is acceptable. 240.4(B) compliant with (1), (2), and (3).

I said except in a few cases....The general rule is what , I believe, he/she was looking for

 

[ggunn]

I said except in a few cases....The general rule is what , I believe, he/she was looking for

Maybe I am misreading what you wrote. I don't deal with OCP rated greater than 800A all that often; for OCP rated 800A or less 240.4(B) applies which allows the next standard size up OCPD, so the derated conductor ampacity can usually (for me, anyway) be less than the rating of the OCPD protecting it. Am I missing something?

 

[Fred B]

I get into this all the time at work. They say 14 #12 thhn current carrying conductors are ok in a 3/4. I said yes you put that many in there.
However there no longer good for a 20 amp branch circuit after derating. They always say I'm wrong because the inspector did not turn it down. They think of it it fits your good.

Your correct. If you look at table 310.15(B)(16) thhn #12 starts at 30Amp then you derate from there one based on the table 310.15(B)(3)(a). up to 9 conductors you might still be ok if no other derating is needed but more than that definitely you will be under 20Amp allowed. Conduit fill is different than allowable ampacity.

Just because the inspector missed it doesn't make it right. No more than if an inspector called it a violation of a totally code compliant installation.

 

[don_resqcapt19]

Maybe I am misreading what you wrote. I don't deal with OCP rated greater than 800A all that often; for OCP rated 800A or less 240.4(B) applies which allows the next standard size up OCPD, so the derated conductor ampacity can usually (for me, anyway) be less than the rating of the OCPD protecting it. Am I missing something?

it does as long as you are not supplying more than one receptacle outlet and as long as the ampacity of the conductor is at least equal to the load.

 

[ggunn]

it does as long as you are not supplying more than one receptacle outlet and as long as the ampacity of the conductor is at least equal to the load.

Which I stipulated in post #7.

 

[augie47]

240.4 seemingly confuses a lot of people.
The paragraph "(1) The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord-and plug-
connected portable loads." is often overlooked.