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Joined: Mar 2011
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Originally Posted by Tesla
We're not allowed to use the 25 Amp rating for this calculation. It must start at 20 Amperes.

The 25 Amp rating can only be used for derating calculations such as multiple conductors and WRT certain motor load calculations.


This is simply not true, I have worked for and with EE's for 25 years and this has NEVER been the case. Only OCP must be limited to 20 amps for #12's. The derating comment would be true if the 25 amp allowable ampacity was in the 75d or 90d column, but it's in the 60d column.

Originally Posted by Tesla
One aspect of this that is normally overlooked is that these branch conductors are typically installed where shedding heat is not favorable. ( It's a lot warmer up high and when sandwiched within walls. That's why the Code kicks in an extra derating.)

Another aspect is economics. Taking the conductor up to its limit is VERY poor economics for the building's owner.


I am not saying it's good engineering practice to do otherwise, what I'm saying is that it is allowed (by code).

Originally Posted by Tesla
In all of the commercial contracts that I've dealt with it has been mandatory for the EC to bump up all homeruns to #10 ... and to #8 for site lighting runs when specified.


We do nothing but commercial work, most in the 7-8 digit range and it is not normal to make homeruns #10 by default. The length of the circuit determines the size and inside the building, anything larger than #12 for a 20 amp circuit is the exception. For site lighting Vd calcs are a necessity based on circuit length and load and many times even #8's or #6's are inadequate.

As far as 19 amps continuously on #12's, it isn't wise, but it IS allowed by code when fully rated OCP is provided. If you disagree, please prove me wrong (provide relevant code sections).

Joined: Jun 2004
Posts: 1,273
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No one makes a fully rated C/B... they are ALL 80%...

The cram down to 20 Amp is right in the NEC...

And has been there for years and years.

See Uglys...

Safeway, Albertsons an Walmart are as cost oriented as you can get.

They all spec #10s on home runs.



Tesla
Joined: Mar 2011
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Originally Posted by Tesla
No one makes a fully rated C/B... they are ALL 80%...


ALL major manufacturer's do and HAVE for a LONG time.

Originally Posted by Tesla
The cram down to 20 Amp is right in the NEC...

And has been there for years and years.


B.S., provide a code section

Originally Posted by Tesla
See Uglys...


NOT a code

Originally Posted by Tesla
Safeway, Albertsons an Walmart are as cost oriented as you can get.

They all spec #10s on home runs.


AND clueless as to what they're paying for as well, but they are big box stores and as such usually have long homeruns. Regardless, a blanket spec of all homeruns being #10's is wasteful.

"Because so and so said so is NOT an answer", if it's not prohibited by the code it is allowed.


Last edited by Vindiceptor; 11/04/11 01:06 AM.
Joined: Nov 2007
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As I understand it, the NEC only allows a complete listed assembly to be operated at 100% of its rating. This would mean not just the circuit breaker, but all of the attached components, devices and conductors would have to be tested together and listed as an assembly. This is probably not something that you can design and put together on your own without a field visit from UL.
Take a look at the wording in the first sentence of [NEC 2011] 210.19[A],1, Exception regarding branch circuit conductor ratings and then 210.20[A], Exception regarding overcurrent protection device ratings. They are exactly the same with regard to listed assemblies rated for operation at 100% of their rating.
It looks that according to 210.20[A], a circuit breaker would normally be sized for 100% of the non-continuous load, plus 125% of the continuous load, just like branch circuit conductors.
So, if in this case you have a 19A calculated continuous load, then it appears that both the circuit breaker and branch circuit conductors would need to be sized for 19A X 125%= 23.75A.
Since #12 conductors for all temperature ratings in Table 310.15[B],16 are limited to a 20A maximum OCP by 240.4[D],5 it seems you basically have two choices. Reduce the continuous load on the circuit or increase the size of the circuit breaker to 25A and the conductors to #10.

IMO, in this situation the 25A rating for #12 in table 310.15[B], 16 would not help because the circuit breaker also has to be considered, but also IMO, when you get into #12 being used as a neutral conductor where OCP is not involved, then that is when the 25A rating can sometimes come into play.


Joined: Nov 2007
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I probably should have said listed assemblies for continuous operation at 100% and that a circuit breaker can be sized for use at 100% for non-continuous loads.

I found this article which does a pretty good job explaining things. It’s from 1996, so the NEC articles probably have changed since then.
Sizing Circuit Breakers

Joined: Mar 2011
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Originally Posted by KJay
As I understand it, the NEC only allows a complete listed assembly to be operated at 100% of its rating. This would mean not just the circuit breaker, but all of the attached components, devices and conductors would have to be tested together and listed as an assembly. This is probably not something that you can design and put together on your own without a field visit from UL.


It is done all the time, I have seen jobs where 100% of the distribution gear was 100% rated.

Originally Posted by KJay
Take a look at the wording in the first sentence of [NEC 2011] 210.19[A],1, Exception regarding branch circuit conductor ratings and then 210.20[A], Exception regarding overcurrent protection device ratings. They are exactly the same with regard to listed assemblies rated for operation at 100% of their rating.
It looks that according to 210.20[A], a circuit breaker would normally be sized for 100% of the non-continuous load, plus 125% of the continuous load, just like branch circuit conductors.
So, if in this case you have a 19A calculated continuous load, then it appears that both the circuit breaker and branch circuit conductors would need to be sized for 19A X 125%= 23.75A.
Since #12 conductors for all temperature ratings in Table 310.15[B],16 are limited to a 20A maximum OCP by 240.4[D],5 it seems you basically have two choices. Reduce the continuous load on the circuit or increase the size of the circuit breaker to 25A and the conductors to #10.


Again, if the breaker and assembly is fully rated (100%) it can carry the 19 amp load and the wire is not an issue because it's allowable ampacity is 25 amps.

Originally Posted by KJay
IMO, in this situation the 25A rating for #12 in table 310.15[B], 16 would not help because the circuit breaker also has to be considered, but also IMO, when you get into #12 being used as a neutral conductor where OCP is not involved, then that is when the 25A rating can sometimes come into play.


See previous comment.

This to me is a simple, apparently widespread misunderstanding of what the code allows. In my world this has been commonplace for decades (fully rated panels and breakers, etc.), so when someone says you have to upsize #12's to 10's because the load exceeds 16 amps and others are taking that for granted I see a need to speak out.

"Just because" or "because I've always done it that way" doesn't fly with me, ever.

The 20 amp limitation for #12's is for OCP only and always has been.

It isn't wise, but it is OK (by code).

Joined: Nov 2007
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Okay, but I think it would be a misunderstanding to consider that only the panel, breakers and conductors constitute an assembly that is listed for continuous use at 100% its rating, regardless of what someone is used to doing.
IMO, an assembly listed for this type of use would also include all the equipment and devices attached to the circuit in addition to a circuit breaker and conductors. As I understand it, this would require all components, conductors and connected equipment, etc., to be designed with terminals and connections rated at 90-Degree C.
Even though the circuit breakers are listed for use at 100% of their rating for non-continuous loads, aren’t most CB terminals only rated at 40-degree C ?

Joined: Mar 2011
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Originally Posted by KJay
Okay, but I think it would be a misunderstanding to consider that only the panel, breakers and conductors constitute an assembly that is listed for continuous use at 100% its rating, regardless of what someone is used to doing.
IMO, an assembly listed for this type of use would also include all the equipment and devices attached to the circuit in addition to a circuit breaker and conductors. As I understand it, this would require all components, conductors and connected equipment, etc., to be designed with terminals and connections rated at 90-Degree C.
Even though the circuit breakers are listed for use at 100% of their rating for non-continuous loads, aren’t most CB terminals only rated at 40-degree C ?


All circuit breaker terminals are rated at 60d C minimum. Many are also rated at 100% continuous load ratings at 40d C ambient, which should not be confused with the 60d C insulation rating.

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We are getting into an area about the difference between safe and legal. 240.4(D) was put there for safety and it is really there because the NFPA people know 14/12/10 is used on circuits where the loads are likely to be cord and plug connected.
The assumption is that the installer will not know what the user will actually plug in. People will keep plugging in things until the breaker trips and then unplug the clock. They build the 80% into the maximum breaker size. That still does not change the requirement that hard wired continuous loads are still limited to 80% of the breaker size, as KJay points out.
Whether the NFPA should put the same kind of exemptions for lighting loads into the code that they have for motors is conjecture but you still have a few weeks to write a proposal and see what they say.


Greg Fretwell
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The 40C has to do with the trip curve, not the terminal rating.

Here are some typical breakers and the terminal rating.

[Linked Image from gfretwell.com]

Last edited by gfretwell; 11/04/11 01:43 PM. Reason: add picture

Greg Fretwell
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