conductor ampacity

[kwired]

Hmmm.... can't agree. You have to use 125% continuous load in your step 3 as you are determining conductor ampacity.

    0        100% non-continuous load
+    [U]125[/U]        125% continuous load
    125        Minimum Circuit Ampacity
?    [U]0.76[/U]        Ambient factor for 75/90? conductor
    164        
?    [U]100%[/U]        More than 3 CCC derating factor
    164        Minimum 75/90?C Table Ampacity

Looking at 1/0, 90?C-rated @ 170A here...

steps 1 and 2 size the conductor for the terminal (60 or 75 deg)

steps 3 thru 5 size the conductor for the insulation on the conductor.

You use 125% of continuous load for the terminals not the insulation therefore you only need to start at 100 percent of load for your insulation calculations.

If you have a fuse or breaker terminal rated for 100% continuous operation you do not have to increase ampacity for continuous load because the device terminal can handle this at 100%.

The reason we increase conductor size for continuous load is because the device depends on the first few inches of wire as a heat sink for the overcurrent device.

No extra heat is introduced in the conductor insulation because of continuous load. no need to derat for this Extra heat is introduced into the terminals because of continuous load. we do need to derate for this.

The minimum conductor size then becomes the larger of either the size required for the terminations or the size required for the insulation, then both conditions are covered.

 

[Smart $]

steps 1 and 2 size the conductor for the terminal (60 or 75 deg)

steps 3 thru 5 size the conductor for the insulation on the conductor.

You use 125% of continuous load for the terminals not the insulation therefore you only need to start at 100 percent of load for your insulation calculations.

If you have a fuse or breaker terminal rated for 100% continuous operation you do not have to increase ampacity for continuous load because the device terminal can handle this at 100%.

The reason we increase conductor size for continuous load is because the device depends on the first few inches of wire as a heat sink for the overcurrent device.

No extra heat is introduced in the conductor insulation because of continuous load. no need to derat for this Extra heat is introduced into the terminals because of continuous load. we do need to derate for this.

The minimum conductor size then becomes the larger of either the size required for the terminations or the size required for the insulation, then both conditions are covered.

There are a couple (though I'm not really counting, nor will I cover them in their entirety ) flaws in your premise. No matter how you calculate your terminal temperature limitations under 110.14(C)(1), you still have to comply with the following:

210.19 Conductors— Minimum Ampacity and Size.
(A) Branch Circuits Not More Than 600 Volts.
(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.
Exception: If the assembly, including the overcurrent devices protecting the branch circuit(s), is listed for operation at 100 percent of its rating, the allowable ampacity of the branch circuit conductors shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.


215.2 Minimum Rating and Size.
(A) Feeders Not More Than 600 Volts.
(1) General. Feeder conductors shall have an ampacity not less than required to supply the load as calculated in Parts III, IV, and V of Article 220. The minimum feeder-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.
Exception No. 1: If the assembly, including the overcurrent devices protecting the feeder(s), is listed for operation at 100 percent of its rating, the allowable ampacity of the feeder conductors shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load


IV. Service-Entrance Conductors
230.42 Minimum Size and Rating.
(A) General. The ampacity of the service-entrance conductors before the application of any adjustment or correction factors shall not be less than either (A)(1) or (A)(2). Loads shall be determined in accordance with Part III, IV, or V of Article 220, as applicable. Ampacity shall be determined from 310.15. The maximum allowable current of busways shall be that value for which the busway has been listed or labeled.
(1) The sum of the noncontinuous loads plus 125 percent of continuous loads
Exception: Grounded conductors that are not connected to an overcurrent device shall be permitted to be sized at 100 percent of the continuous and noncontinuous load.
(2) The sum of the noncontinuous load plus the continuous load if the service-entrance conductors terminate in an overcurrent device where both the overcurrent device and its assembly are listed for operation at 100 percent of their rating

My calculation shows the minimum 90?C-rated conductor ampacity under the general statements of all three sections listed above. Yes you can use 100% continuous values if the equipment is 100% duty rated, but that equipment is typically not available, and so indicated by being the exception rather than the general rule for feeders and branch circuits.

As for heat sinking, more than the first few inches act as a heat sink. The Code, though not explicit in its usual ways, considers this heat sinking effect to be the lesser of 10' or 10% of the circuit length, as so indicated by 310.15(A)(2) Exception.

 

[hardworkingstiff]

If you have a copy of the NEC handbook (2008), read the application example that starts on page 309 (the last paragraph on the page). The example really gets interesting on page 310 in step 2.

It seems to support what KP2 has posted.

 

[kwired]

There are a couple (though I'm not really counting, nor will I cover them in their entirety ) flaws in your premise. No matter how you calculate your terminal temperature limitations under 110.14(C)(1), you still have to comply with the following:

​

My calculation shows the minimum 90?C-rated conductor ampacity under the general statements of all three sections listed above. Yes you can use 100% continuous values if the equipment is 100% duty rated, but that equipment is typically not available, and so indicated by being the exception rather than the general rule for feeders and branch circuits.

As for heat sinking, more than the first few inches act as a heat sink. The Code, though not explicit in its usual ways, considers this heat sinking effect to be the lesser of 10' or 10% of the circuit length, as so indicated by 310.15(A)(2) Exception.

Please tell me the flaws. I recently attended a CEU and this was a big topic from the instructor who is a national speaker on code and has some of his own published materials.

The five step method I described comes directly from one of his publications and we did many exercises in the class to put the five steps to use.

Steps 1 and 2 meet all 3 sections you quoted. The result of these two steps is the minimum allowed conductor no matter what other sizes could possibly be allowed. If steps 3, 4, and 5 would allow a smaller conductor you still need to use the larger conductor from steps 1 and 2.

210.19 Conductors ? Minimum Ampacity and Size.
(A) Branch Circuits Not More Than 600 Volts.
(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.
Exception No. 1: Where the assembly, including the overcurrent devices protecting the branch circuit(s), is listed for operation at 100 percent of its rating, the allowable ampacity of the branch circuit conductors shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.
Exception No. 2: Grounded conductors that are not connected to an overcurrent device shall be permitted to be sized at 100 percent of the continuous and noncontinuous load.

215.2 Minimum Rating and Size.
(A) Feeders Not More Than 600 Volts.
(1) General. Feeder conductors shall have an ampacity not less than required to supply the load as calculated in Parts III, IV, and V of Article 220. The minimum feeder-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.
Exception No. 1: Where the assembly, including the overcurrent devices protecting the feeder(s), is listed for operation at 100 percent of its rating, the allowable ampacity of the feeder conductors shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.
Exception No. 2: Grounded conductors that are not connected to an overcurrent device shall be permitted to be sized at 100 percent of the continuous and noncontinuous load.

230.42 Minimum Size and Rating.
(A) General. The ampacity of the service-entrance conductors before the application of any adjustment or correction factors shall not be less than either (A)(1) or (A)(2). Loads shall be determined in accordance with Part III, IV, or V of Article 220, as applicable. Ampacity shall be determined from 310.15. The maximum allowable current of busways shall be that value for which the busway has been listed or labeled.
(1) The sum of the noncontinuous loads plus 125 percent of continuous loads
(2) The sum of the noncontinuous load plus the continuous load if the service-entrance conductors terminate in an overcurrent device where both the overcurrent device and its assembly are listed for operation at 100 percent of their rating

Why are all the blue highlighted items in the code? It is because they all involve something other than standard OCPD's. Standard OCPD's need the conductor to be a heat sink and therefore require 125% on continuous load. It has nothing to do with conductor insulation and everything to do with termination temperature on non standard OCPD's

 

[Smart $]

Please tell me the flaws.[/COLOR]

The major flaw is your result(s). The ampacity value resulting from your steps 1 through 2 is the minimum 75?C-rated conductor ampacity after correction and adjustment. Steps 3 though 5 yields not only the minimum conductor ampacity after correction and adjustment, it does not account for the increased conductor ampacity required for continuous loads per the above cited Code sections. Note 110.14(C)(1) does not say you can determine the ampacity before the application of correction and adjustment factor as in the above cited references. So its not that your calculation is faulty, per se, it is more that they are incomplete in providing a conductor ampacity for direct Table comparison. The goal is determining the minimum size conductor of a particular type that we are going to use, not just the minimum per 110.14(C)(1) and taking into account no other factors of installation.

Also, the exception or option to use 100% for continuous loads while using 100% duty rated equipment is just that. You can't calculate as if and use that result when actually using standard, non 100% duty rated equipment.

As for why your authoritative figure does not apply 125% continuous load factoring to the higher temperature-rated conductor's calculation, I do not know. It is not written in the Code that you can... but then again it is not written that you can't. It is therefore only my opinion that one should... for ultimately it will still have to be factored into the final conductor size determination (except for the mentioned, and quite rare, 100%-duty-rated installations, or if one decides to go with the 100% continuous plus non-continuous ungrounded, non-OCPD'd conductor... which, though commonly applicable, also seems to be a rare occurrence in practice).

 

[kwired]

Exception No. 1: Where the assembly, including the overcurrent devices protecting the branch circuit(s), is listed for operation at 100 percent of its rating, the allowable ampacity of the branch circuit conductors shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.
Exception No. 2: Grounded conductors that are not connected to an overcurrent device shall be permitted to be sized at 100 percent of the continuous and noncontinuous load.

These exceptions are written for the reason that the non 100 percent terminal must have additional ampacity added to conductors for a continuous load (for heat sinking purposes)

Exception 2 says that a grounded conductor not connected to an overcurrent device is permitted to be sized at 100 percent of any load, why? Because there is no terminal that needs an increased conductor for heat sinking.

If a grounded conductor can carry its 90 degree ampacity why not an ungrounded conductor? Because the OCPD device needs the additional size for heat sinking.

deration for temperature is done for conductor insulation and not terminals therefore you use 100 percent load at highest temperature rating of the conductor. (90 deg most of the time) 110.14(C) permits this.

110.14
(C) Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.

By using the larger of the two conductor sizes in the 5 step method I mentioned the conductor selected will never be less than 125% of continuous load plus 100 percent of non continuous load.

210.19 Conductors — Minimum Ampacity and Size.
(A) Branch Circuits Not More Than 600 Volts.
(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

(similar language for feeders and services)

Once again the ampacity adjustment or correction factor applies to conductor insulation.

Minimum size is determined before you apply any adjustment or correction factor, if the adjustment or correction factor results in a larger conductor than the minimum size conductor you must use the larger conductor.

 

[Smart $]

Exception No. 1: Where the assembly, including the overcurrent devices protecting the branch circuit(s), is listed for operation at 100 percent of its rating, the allowable ampacity of the branch circuit conductors shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.
Exception No. 2: Grounded conductors that are not connected to an overcurrent device shall be permitted to be sized at 100 percent of the continuous and noncontinuous load.

These exceptions are written for the reason that the non 100 percent terminal must have additional ampacity added to conductors for a continuous load (for heat sinking purposes)

Exception 2 says that a grounded conductor not connected to an overcurrent device is permitted to be sized at 100 percent of any load, why? Because there is no terminal that needs an increased conductor for heat sinking.

You're using reverse logic, but I'll accept your premise as not needing debate for the time being.

If a grounded conductor can carry its 90 degree ampacity why not an ungrounded conductor? Because the OCPD device needs the additional size for heat sinking.

The wording does not say a grounded conductor can carry its 90?C ampacity. It says the grounding conductor's ampacity may be determined using 100% of continuous plus non-continuous loads. The grounded conductor is still subject to termination temperature limitations, and correction and adjustment factors, as they apply.

deration for temperature is done for conductor insulation and not terminals therefore you use 100 percent load at highest temperature rating of the conductor. (90 deg most of the time) 110.14(C) permits this.

This is where your premise starts to become flawed. You are correct in the deration part, but then you turn around and enter insulation rating back into the picture. Under the same conditions, a 75?C-rated conductor carrying a continuous current which causes it to reach a sustained temperature of 75?C will be the same size as a 90?C-rated conductor (of the same conductor type, e.g. copper). That is, the insulation rating has no bearing on the conductor temperature for a given amount of current and installed under identical operating conditions.

110.14
(C) Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.

It is quite easy to determine the minimum size conductor for terminal temperature limitation, other factors withstanding. Simply go to the Table and out of the appropriate 75?C column (for 75?C rated terminals) determine the size conductor which has a 75?C ampacity equal to or greater than the actual maximum connected load as calculated under Article 220 (without applying the 125% continuous load buffer). Let's use the OP'er scenario of Posts #1 and #4 as an example. Looking to the Table for a 100A maximum connected load (aka actual load). Thus, the minimum size copper conductor would be a copper-type #3, period. That's it, we're done with this part. A #3 conductor is the minimum size that can be used for this circuit.. But that is under ideal conditions of usage. So, any "selection and coordination" beyond this is for other, non-ideal conditions which must be considered.

By using the larger of the two conductor sizes in the 5 step method I mentioned the conductor selected will never be less than 125% of continuous load plus 100 percent of non continuous load.

210.19 Conductors ? Minimum Ampacity and Size.
(A) Branch Circuits Not More Than 600 Volts.
(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

(similar language for feeders and services)

Once again the ampacity adjustment or correction factor applies to conductor insulation.

Minimum size is determined before you apply any adjustment or correction factor, if the adjustment or correction factor results in a larger conductor than the minimum size conductor you must use the larger conductor.

I fail to see any benefit of making this a five step process.

The rest is quite simple (as the code is currently written). Simply add in any "125%" buffering to the load value, and divide by the factors for correction and adjustment to determine the minimum conductor size by the resulting ampacity value (note this value is not the circuit ampacity or any other relevant ampacity under Code; it is simply a number to compare with Table values). Going back to the example... we know all of the load to be continuous, So 100A ? 125% + 0 ? 100% = 125A. At this stage, we could look at the Table and see we could use copper conductor types sized at 1/0 60?C-rated, #1 75?C-rated, or #2 90?C-rated. All three of the sizes are larger than the #3 minimum we determined above for the terminal temperature limitation.

From there, we also know that if the ambient temperature is higher than 30?C and/or any derating for more than three conductors come into play, that the conductor sizes that we would have to use may even be larger yet, but defintely not smaller. At this point I am simply belittling these aspects to shorten my post. Yes there could be a condition where the ambient is less than 30?C allowing one to use a smaller conductor, or the load is non-continuous. The point I am trying to make is that it is not necessary to draw the process out to five steps just to find the minimum size conductor for terminal temperature limitations, and even then, you still have to include the other size-altering factors before a total correlation can be achieved. It truly is a two-step only process

1. Determine minimum size conductor based on maximum connected load current as calculated under Article 220 (i.e. no 125% continuous load buffering).
2. Determine minimum size conductor for insulation used by factoring in for continuous loading, ambient temperature correction, and multiple conductor adjustment.
3. Step 2 shall not be smaller in size than Step 1.

OOOps! I guess my counting is off this morning. As you can see, it is a three step process

 

[kwired]

You're using reverse logic, but I'll accept your premise as not needing debate for the time being.


The wording does not say a grounded conductor can carry its 90?C ampacity. It says the grounding conductor's ampacity may be determined using 100% of continuous plus non-continuous loads. The grounded conductor is still subject to termination temperature limitations, and correction and adjustment factors, as they apply.


This is where your premise starts to become flawed. You are correct in the deration part, but then you turn around and enter insulation rating back into the picture. Under the same conditions, a 75?C-rated conductor carrying a continuous current which causes it to reach a sustained temperature of 75?C will be the same size as a 90?C-rated conductor (of the same conductor type, e.g. copper). That is, the insulation rating has no bearing on the conductor temperature for a given amount of current and installed under identical operating conditions.


It is quite easy to determine the minimum size conductor for terminal temperature limitation, other factors withstanding. Simply go to the Table and out of the appropriate 75?C column (for 75?C rated terminals) determine the size conductor which has a 75?C ampacity equal to or greater than the actual maximum connected load as calculated under Article 220 (without applying the 125% continuous load buffer). Let's use the OP'er scenario of Posts #1 and #4 as an example. Looking to the Table for a 100A maximum connected load (aka actual load). Thus, the minimum size copper conductor would be a copper-type #3, period. That's it, we're done with this part. A #3 conductor is the minimum size that can be used for this circuit.. But that is under ideal conditions of usage. So, any "selection and coordination" beyond this is for other, non-ideal conditions which must be considered.


I fail to see any benefit of making this a five step process.

The rest is quite simple (as the code is currently written). Simply add in any "125%" buffering to the load value, and divide by the factors for correction and adjustment to determine the minimum conductor size by the resulting ampacity value (note this value is not the circuit ampacity or any other relevant ampacity under Code; it is simply a number to compare with Table values). Going back to the example... we know all of the load to be continuous, So 100A ? 125% + 0 ? 100% = 125A. At this stage, we could look at the Table and see we could use copper conductor types sized at 1/0 60?C-rated, #1 75?C-rated, or #2 90?C-rated. All three of the sizes are larger than the #3 minimum we determined above for the terminal temperature limitation.



From there, we also know that if the ambient temperature is higher than 30?C and/or any derating for more than three conductors come into play, that the conductor sizes that we would have to use may even be larger yet, but defintely not smaller. At this point I am simply belittling these aspects to shorten my post. Yes there could be a condition where the ambient is less than 30?C allowing one to use a smaller conductor, or the load is non-continuous. The point I am trying to make is that it is not necessary to draw the process out to five steps just to find the minimum size conductor for terminal temperature limitations, and even then, you still have to include the other size-altering factors before a total correlation can be achieved. It truly is a two-step only process

1. Determine minimum size conductor based on maximum connected load current as calculated under Article 220 (i.e. no 125% continuous load buffering).
2. Determine minimum size conductor for insulation used by factoring in for continuous loading, ambient temperature correction, and multiple conductor adjustment.
3. Step 2 shall not be smaller in size than Step 1.

OOOps! I guess my counting is off this morning. As you can see, it is a three step process

The five step process does the same thing as the three you mentioned. The details are just presented in separate steps.

The only thing we are disagreeing on is where the continuous load factoring should be used in the calculations. At this class we were taught that insulation ratings are for 100 percent continuous loading and terminals are not. Therefore the 125% continuous load rule applies to to minimum conductor size but not to thermal adjustments in the raceways. And his reasoning was that the overcurrent device terminals (unless 100% rated and that is why the exceptions) are not designed for this.

I have not found a reference yet for overcurrent terminals (I havent looked real hard yet) but the idea makes sense. This instructor has been doing this for a long time and does presentations in several states. His credibility seems to be pretty good and I am sure he has done reasearch to support this idea. Many other items discussed in the class he would bring up reasons behind code requirements and not just this is what it says.

If other forum members have ideas to add please do so, this is so far only a two person debate on a topic all of us run into almost daily.

Please don't reply with an I have always done it that way response. Sometimes we need to look at the things we do all the time and realize we get into such a habit we are overlooking something.

Using 125% before will not hurt anything because worst case you will have a conductor larger than what the minimum may have been allowed to be, so no one ever gets into trouble for doing it that way.

I have always understood the conductor selection process the same way before attending the CEU class I recently attended.

 

[kwired]

The five step process does the same thing as the three you mentioned. The details are just presented in separate steps.

I need to add here: except "(i.e. no 125% continuous load buffering)" that you had at the end of step one is moved to the end of your step 2. Then your three steps accomplish the same as my five steps in a nutshell.

The only thing we are disagreeing on is where the continuous load factoring should be used in the calculations. At this class we were taught that insulation ratings are for 100 percent continuous loading and terminals are not. Therefore the 125% continuous load rule applies to to minimum conductor size but not to thermal adjustments in the raceways. And his reasoning was that the overcurrent device terminals (unless 100% rated and that is why the exceptions) are not designed for this.

I have not found a reference yet for overcurrent terminals (I havent looked real hard yet) but the idea makes sense. This instructor has been doing this for a long time and does presentations in several states. His credibility seems to be pretty good and I am sure he has done reasearch to support this idea. Many other items discussed in the class he would bring up reasons behind code requirements and not just this is what it says.

If other forum members have ideas to add please do so, this is so far only a two person debate on a topic all of us run into almost daily.

Please don't reply with an I have always done it that way response. Sometimes we need to look at the things we do all the time and realize we get into such a habit we are overlooking something.

Using 125% before will not hurt anything because worst case you will have a conductor larger than what the minimum may have been allowed to be, so no one ever gets into trouble for doing it that way.

I have always understood the conductor selection process the same way before attending the CEU class I recently attended.

See added text to my previous post

 

[Smart $]

The five step process does the same thing as the three you mentioned. The details are just presented in separate steps.

I agree to "similar" but not "the same thing". The biggest problem I have is your result in Post #10

The only thing we are disagreeing on is where the continuous load factoring should be used in the calculations. At this class we were taught that insulation ratings are for 100 percent continuous loading and terminals are not. Therefore the 125% continuous load rule applies to to minimum conductor size but not to thermal adjustments in the raceways. And his reasoning was that the overcurrent device terminals (unless 100% rated and that is why the exceptions) are not designed for this.

Just as conductor insulation ratings bear no part, 100% vs. 80% duty-rated OCPD's have no bearing on the minimum wire size for terminal temperature limitations. The terminal is what it is, and rated as such. If there are other factors of the circuit to consider, then by all means they should be considered. It may affect the minimum circuit conductor size... but not regarding the terminal by and of itself.

When we consider terminal temperature limitation at OCPD's, we look at heat being generated on both sides of the terminal as a result of current through the conductive pathway (I?R loss). Heat sinking is also a concept in play here... and we are attempting to determine the point of equilibrium—where the maximum load current causes neither side to exceed the terminal's temperature rating. The consideration for the equipment side of the terminal has been predetermined by the manufacturer and listing agency testing. We only need be concerned with our field installed conductor not exceeding the terminal's temperature rating in the course of normal operation of the circuit... at the terminal. The temperature elsewhere on the conductor is permitted to exceed the terminal's temperature rating... it is just not permitted at the terminal.

I mentioned 310.15(A)(2) Exception before, but did not explicitly include it in any determination step, as many here are skeptical on its permitted use regarding terminal temperature limitations.

I also mentioned Example D3(a) before, but I avoid doing so because I believe it to be in error. When you look at what it says under the Overcurrent Protection of the example, it calculates using 125% continuous load buffering. Yet when we get to the Ungrounded Feeder Conductors it does not use 125% contiuous load buffering to determine conductor size (which is a violation if installed as such). Anyway, I brought it up because it mentions near the end of the 'Feeder' section that the layout precludes the application of 310.15(A)(2) Exception.

I have not found a reference yet for overcurrent terminals (I havent looked real hard yet) but the idea makes sense. This instructor has been doing this for a long time and does presentations in several states. His credibility seems to be pretty good and I am sure he has done reasearch to support this idea. Many other items discussed in the class he would bring up reasons behind code requirements and not just this is what it says.

There are many ways to make something sound reasonable, even untrue ones . I have also found what may sound reasonable today, may not tomorrow, and vice-versa. At times, as you mention, we become complacent in our understanding of an issue and simply move on, accepting our understanding to be ultimately factual, yet in truth it may only be factually-based. What I present to you is the facts as I understand them to be... along with a few opinions. Take it all, or part, or none, for what it is worth to you.