240.6(c) and the accuracy and precision of adjustable-trip breakers

[kwired]

I'm running two of them in parallel :roll:

My question was about breaker trip settings, not about how much current 4/0 can handle.

Why dial it down at all? You've got over 400A of ampacity with the parallel 4/0's.

Exactly, conductors are considered protected by NEC with a 400 amp OCPD. Should the equipment being supplied require a different overload/overcurrent protection level that also needs considered.

If ampacity adjustments lowered those 4/0 to less than a combined 400 ampacity - then we possibly have more to discuss.

 

[electrofelon]

More specifics weren't given because she wasn't asking a question about conductors; only about trip settings. The discussion about conductor size, type and rating is inconsequential to the OP.

But we like to pick apart and know everything on here. What else does anyone of us have to do with our time ? :roll: :angel:

 

[david luchini]

Thanks to all who've replied! I've come to the same conclusion first mentioned by @electrofelon - set the adjustable trip to the next higher standard breaker size; there's no need to dial it exactly to the load current as I was initially thinking, since the NEC always considers "one-size-up" to be adequate protection.

Here's a more complete example:

• 150A continuous load with 16 current-carrying conductors in the raceway, fill derating = 50%.
• 125% of 150A is 187A so I need a 200A breaker, if 80%-rated.
• 300MCM is required to carry the effective 300A derated load.
• Termination heatsinking of 187A is fulfilled for 75C terminations by 300MCM, since its ampacity at 75C is 240A.
• The derated ampacity of 90C 300MCM is 160A. NEC requires that the conductor be protected by the next standard size up, which is 175A.

Therefore the adjustable trip needs to be set at 175A to protect the conductor. If not using an adjustable trip breaker, you'd have to increase the conductor size, because a 200A breaker is not the next size up from 160A.

The rating of the overcurrent device needs to be at least 187.5A. You can't set the adjustable trip to 175A.

 

[kwired]

Thanks to all who've replied! I've come to the same conclusion first mentioned by @electrofelon - set the adjustable trip to the next higher standard breaker size; there's no need to dial it exactly to the load current as I was initially thinking, since the NEC always considers "one-size-up" to be adequate protection.

Here's a more complete example:

• 150A continuous load with 16 current-carrying conductors in the raceway, fill derating = 50%.
• 125% of 150A is 187A so I need a 200A breaker, if 80%-rated.
• 300MCM is required to carry the effective 300A derated load.
• Termination heatsinking of 187A is fulfilled for 75C terminations by 300MCM, since its ampacity at 75C is 240A.
• The derated ampacity of 90C 300MCM is 160A. NEC requires that the conductor be protected by the next standard size up, which is 175A.

Therefore the adjustable trip needs to be set at 175A to protect the conductor. If not using an adjustable trip breaker, you'd have to increase the conductor size, because a 200A breaker is not the next size up from 160A.

I didn't check your math so assuming it is correct:

you need a 187 amp conductor for the 75C terminations taken from 75C ampacity column
you need a 150 amp conductor in the raceway for 90 C insulation rating. taken from 90C ampacity column

you need 187 amp or next size up standard rating overcurrent protection or 150 amp if a 100% rated device there is no 75C or 90C rating involved here

 

[electrofelon]

I'm a bit confused why the numbers changed. In the OP we were looking at 278 A. Is this a different situation now?

 

[david luchini]

I'm a bit confused why the numbers changed. In the OP we were looking at 278 A. Is this a different situation now?

The original question is a hypothetical.

 

[kwired]

I'm a bit confused why the numbers changed. In the OP we were looking at 278 A. Is this a different situation now?

Second set of figures was a different situation, don't know which if either was hypothetical.

 

[kingpb]

Here's a more complete example:

• 150A continuous load with 16 current-carrying conductors in the raceway, fill derating = 50%.
• 125% of 150A is 187A so I need a 200A breaker, if 80%-rated.
• 300MCM is required to carry the effective 300A derated load.
• Termination heatsinking of 187A is fulfilled for 75C terminations by 300MCM, since its ampacity at 75C is 240A.
• The derated ampacity of 90C 300MCM is 160A. NEC requires that the conductor be protected by the next standard size up, which is 175A.

Therefore the adjustable trip needs to be set at 175A to protect the conductor. If not using an adjustable trip breaker, you'd have to increase the conductor size, because a 200A breaker is not the next size up from 160A.

150A continuous load x 1.25 = 188A, requires a 200A breaker when not 100% rated, and Adj breaker will probably still be set at 200A.
Cable derating due to 50% capacity is going to dictate size; 400KCMIL (380A @ 90 degC) is 190A

 

[david luchini]

400KCMIL (380A @ 90 degC) is 190A

:thumbsup:

 

[JuliaTruchsess]

you need 187 amp or next size up standard rating overcurrent protection or 150 amp if a 100% rated device there is no 75C or 90C rating involved here

The breaker and conductor have to be sized so that the conductor is protected. The derated ampacity of 90C 300MCM in this example is 160A, so the next size up for that conductor is 175A. A 200A breaker is mandated (if 80%-rated) for device protection (187A). Therefore in the example, if using a non-adjustable 200A breaker you'd have to move up to 400MCM to satisfy the one-step-up criterion. With a restricted-access adjustable trip you could use 300MCM with the long-time pickup set to 175A under 240.6(c). That's the point.

 

[david luchini]

With a restricted-access adjustable trip you could use 300MCM with the long-time pickup set to 175A under 240.6(c). That's the point.

That is incorrect. You cannot set the breaker to 175A. The breaker rating must be at least 187A. 400mcm is required for the example you have given.

 

[JuliaTruchsess]

That is incorrect. You cannot set the breaker to 175A. The breaker rating must be at least 187A. 400mcm is required for the example you have given.

That is not what 240.4 says. It says "Conductors... shall be protected against overcurrent in accordance with their ampacities specified in 310.15." The 125% allowance applies to breaker ratings, for device and termination protection. It does not apply to overcurrent protection.

Have a look here as well: https://iaeimagazine.org/magazine/2003/07/16/sizing-conductors-for-all-load-conditions/

 

[david luchini]

That is not what 240.4 says. It says "Conductors... shall be protected against overcurrent in accordance with their ampacities specified in 310.15." The 125% allowance applies to breaker ratings, for device and termination protection. It does not apply to overcurrent protection.

Have a look here as well: https://iaeimagazine.org/magazine/2003/07/16/sizing-conductors-for-all-load-conditions/

I suggest you read 215.3. Your OCPD cannot be less than 187A. 300mcm (and 350mcm) cannot be protected by a breaker of that size, the conductors would need to be 400mcm.

 

[kingpb]

That is not what 240.4 says. It says "Conductors... shall be protected against overcurrent in accordance with their ampacities specified in 310.15." The 125% allowance applies to breaker ratings, for device and termination protection. It does not apply to overcurrent protection.

Have a look here as well: https://iaeimagazine.org/magazine/2003/07/16/sizing-conductors-for-all-load-conditions/

Read the article, a little out of date; i.e. circa 2003, but for the most part, other than references it is still reasonably applicable. In the end, it supports the fact that the cable would need to be 400KCMIL for the example proposed. Personally, I have never worried about the differences of the cable between the terminations and the "middle". I have no desire to introduce additional J-boxes or splices to use a smaller cable in the middle, just not practical. Whatever the worst case is, then that's the cable size for the entire run. Switching cable sizes mid circuit is a disaster for any electrician trying to do maintenance or mods later on.

 

[JuliaTruchsess]

Read the article, a little out of date; i.e. circa 2003, but for the most part, other than references it is still reasonably applicable. In the end, it supports the fact that the cable would need to be 400KCMIL for the example proposed. Personally, I have never worried about the differences of the cable between the terminations and the "middle". I have no desire to introduce additional J-boxes or splices to use a smaller cable in the middle, just not practical. Whatever the worst case is, then that's the cable size for the entire run. Switching cable sizes mid circuit is a disaster for any electrician trying to do maintenance or mods later on.

That's not what he's saying in the article; he never suggests running different size cable in a run. He talks about "imaginary pull boxes" and "lock your calculation results in a drawer for now" to help you mentally separate the termination requirement from the wireway requirement. The point being that the calculations must be made independently, with the final conductor size being mandated by whichever requires a larger conductor. In some cases the size will be dictated by the termination temperature rating (for which calculation the 125% factor is used) and in other cases it will be determined by the wire ampacity as determined by its insulation temperature rating, for which calculation the 125% factor is not used (but fill and ambient temperature are).

 

[david luchini]

That's not what he's saying in the article; he never suggests running different size cable in a run. He talks about "imaginary pull boxes" and "lock your calculation results in a drawer for now" to help you mentally separate the termination requirement from the wireway requirement. The point being that the calculations must be made independently, with the final conductor size being mandated by whichever requires a larger conductor. In some cases the size will be dictated by the termination temperature rating (for which calculation the 125% factor is used) and in other cases it will be determined by the wire ampacity as determined by its insulation temperature rating, for which calculation the 125% factor is not used (but fill and ambient temperature are).

Reread the last section:. The Conductor Must Always Be Protected.

"As a final check, be sure the size of the overcurrent protective device selected to accommodate continuous loads protects the conductor....if it does not you will NEED TO INCREASE the conductor size accordingly."

In your example, a 300mcm is not properly protected by a 200A ocpd. You will need to increase the conductor to 400mcm.

 

[JuliaTruchsess]

I call your attention to Informative Annex D, Example D3(a), of the 2017 NEC, "Industrial Feeders In a Common Raceway".

First, it discusses the fact that the OCPD must "accommodate" 125% of the continuous load plus the non-continuous load. This is for thermal protection of the breaker itself.



Next, it talks about the feeders themselves, and specifically says that "the conductors must independently meet requirements for (1) terminations, and (2) conditions of use throughout the raceway run." (emphasis mine). Most importantly, note that in this calculation of conductor ampacity, the 125% factor is not used; only fill and ambient temperature derating are.



Finally, it discusses overcurrent protection, and again in that calculation the 125% factor does not appear. Ampacity for the purpose of breaker sizing is derived from 310.15(B)(16) with fill and temperature derating.



All of this confirms what F. P. Hartwell has been saying for years, as in the articles I linked to.

Back to the topic of the thread, I think this example clearly demonstrates that the 125% factor is entirely concerned with protecting terminations and devices (breakers) from exceeding their thermal limits. A conductor in a raceway is in no danger from running at 100% of its derated ampacity for any length of time, and this is borne out by the fact that the 125% factor is not used in the example in calculating ampacity of the conductor or in calculating the required overcurrent protection. My position is that my 200A breaker is capable of handling more than 125% of the continuous current plus non-continuous, and that setting its trip point to 175A adequately protects a conductor of 160A ampacity. Whether or not I could convince an AHJ of that is another question entirely

 

[david luchini]

I call your attention to Informative Annex D, Example D3(a), of the 2017 NEC, "Industrial Feeders In a Common Raceway".

The Example D3(a) provides the solution in the manner we have been describing.

First, it discusses the fact that the OCPD must "accommodate" 125% of the continuous load plus the non-continuous load. This is for thermal protection of the breaker itself.

This is the MINIMUM ocpd size.

150A in the Example D3(a).

200A in your example.

The OCPD cannot be smaller. You cannot reduce the OCPD size to 175A in your example.

All of this confirms what F. P. Hartwell has been saying for years, as in the articles I linked to.

F.P. Hartwell's article provides the solution in the manner we have been describing. "As a final check, be sure the size of the overcurrent protective device selected to accommodate continuous loads protects the conductor....if it does not you will NEED TO INCREASE the conductor size accordingly."

Back to the topic of the thread, I think this example clearly demonstrates that the 125% factor is entirely concerned with protecting terminations and devices (breakers) from exceeding their thermal limits. A conductor in a raceway is in no danger from running at 100% of its derated ampacity for any length of time, and this is borne out by the fact that the 125% factor is not used in the example in calculating ampacity of the conductor or in calculating the required overcurrent protection. My position is that my 200A breaker is capable of handling more than 125% of the continuous current plus non-continuous, and that setting its trip point to 175A adequately protects a conductor of 160A ampacity. Whether or not I could convince an AHJ of that is another question entirely

Setting the trip point to 175A sets the OCPD rating below the minimum required rating. That is a direct violation of NEC Section 215.3. It is not code compliant. You would have to increase the conductor size to 400mcm, as F.P. Hartwell indicated in his article.

 

[JuliaTruchsess]

You cannot reduce the OCPD size to 175A in your example.

I didn't reduce its size - it's still a 200A breaker for purposes of "accommodating" 125% of the continuous load plus the non-continuous load to ensure that it won't overheat. All I did was set its trip point to the next-higher standard size above the conductor ampacity to protect the conductor, as the code requires.

 

[david luchini]

I didn't reduce its size - it's still a 200A breaker for purposes of "accommodating" 125% of the continuous load plus the non-continuous load to ensure that it won't overheat. All I did was set its trip point to the next-higher standard size above the conductor ampacity to protect the conductor, as the code requires.

You changed the rating of the ocpd using 240.6(c). The new rating is smaller than the minimum required rating. That is a violation of 215.3.