110.14(C)(1) again!!!!!!!

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San -Brooke

San -Brooke

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There seems to be so much confusion with derating calculations I thought I would ad to it. My question is why would we calculate the termination at for example a circuit breaker based on the wire sized used rather than the maximum allowable wire size? For example I am looking at a 15 amp breaker with a 75 C rating however according to the torque setting I am extrapolating that the manufacturers listing allows up a size #4 AWG to be landed on the lug of this breaker. So could we not utilize the maximum allowable listed wire size to calculate the amperage rating of that lug rather than the wire size used to terminate on that breaker?
 
San -Brooke said:
There seems to be so much confusion with derating calculations I thought I would ad to it. My question is why would we calculate the termination at for example a circuit breaker based on the wire sized used rather than the maximum allowable wire size? For example I am looking at a 15 amp breaker with a 75 C rating however according to the torque setting I am extrapolating that the manufacturers listing allows up a size #4 AWG to be landed on the lug of this breaker. So could we not utilize the maximum allowable listed wire size to calculate the amperage rating of that lug rather than the wire size used to terminate on that breaker?
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You sort of lost me here.

What does the wire size you are using have to do with the wire size you could be using?

You don't calculate the amperage rating of a lug, it is marked on the lug.
 
We don't calculate the amperage rating of the lug. The manufacturer just tells us that number. What we calculate is the ampacity (i.e., ability to carry current under the conditions of use) of the wire we plan to install. That calculation takes into account the impact of heat (i.e., generated by current in the wire) upon the lug.
 
charlie b said:
We don't calculate the amperage rating of the lug. The manufacturer just tells us that number. What we calculate is the ampacity (i.e., ability to carry current under the conditions of use) of the wire we plan to install. That calculation takes into account the impact of heat (i.e., generated by current in the wire) upon the lug.
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I guess where I am getting confused is due to people thinking that you apply derating factors to the lug. For example if you were to have up to 9 # 12 thhn current carrying conductors in a conduit if I am correct the ampacities of these wires are calculated at being 70% of the 90 degree C table but what I am running into are people thinking that you should calculate at 75 degre C due to the lug that you are landing on. What I am saying is even if you were to calculate this way which I believe is incorrect wouldn't the lug being able to accept the larger wire not actually be able to accept a higher ampacity thus making the argument moot?
 
Larger wire, where properly sized, may lower temperature at the lugs.
Engineering formulas exist for sizing wire, more reliably, according to temperature.

Unfortunately, the NFPA will not use superior methods if publishing royalties are required.

The result is error-prone de-rating schemes and proprietary NEC tables for idiots, more cumbersome than the engineering it's intended to replace.
 
San -Brooke said:
What I am saying is even if you were to calculate this way which I believe is incorrect wouldn't the lug being able to accept the larger wire not actually be able to accept a higher ampacity thus making the argument moot?
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So instead of convincing people to perform the adjusments correctly, you think it will be easier to integrate more variable information into the process?
 
It's about the heat. 15-amps on a #14 wire generates more heat than 15-amps on a #4 wire. Plus, a #4 wire can dissipate more heat than a #14 wire.

The breakers are only tested with a 4' piece of wire to test for heat dissipation (not sure what that may have to do with your question, but I find it an interesting piece of info).
 
San -Brooke said:
There seems to be so much confusion with derating calculations I thought I would ad to it. My question is why would we calculate the termination at for example a circuit breaker based on the wire sized used rather than the maximum allowable wire size? For example I am looking at a 15 amp breaker with a 75 C rating however according to the torque setting I am extrapolating that the manufacturers listing allows up a size #4 AWG to be landed on the lug of this breaker. So could we not utilize the maximum allowable listed wire size to calculate the amperage rating of that lug rather than the wire size used to terminate on that breaker?
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I will try and answer your question.

To me, heat dissipation or heat flow in its different applications per NEC, provide a foundation of protection against fire (over heating). Heat flow from a current carrying item to ambient air temperature (high to low temperature) is a fundamental NEC principle.

In understanding this heat flow, I think you can better reflect upon what the NEC is trying to do.

I believe if you can understand Ohms law, you can understand its parallel (or analog) in thermodynamics often called Ohms Law for thermal circuits. So if you can picture in your mind electric current flow, you can use the same technique in understanding heat flow.

Here is one of many links:

http://www.egr.msu.edu/~raguin/ME812/FinalProjects/Lindberg_FinalProject.htm

So how does heat move away from a lug on a breaker, for example? A simple description would be, the breaker is tested with a conductor connected to sink heat away. The lug alone contributes very little in conducting heat to ambient.

 
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hardworkingstiff said:
The breakers are only tested with a 4' piece of wire to test for heat dissipation (not sure what that may have to do with your question, but I find it an interesting piece of info).
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Heat dissipation is only part of the reason that breakers are tested with a length of conductor.
Another reason is to make sure that the conductor does not pull free from the breaker during a short circuit.

If the testing only uses 4' of conductor then temperature issues with the conductors past that point do not impact the breaker, is this the point where 110.14(C) is not important?
 
So if you have a 400 amp breaker that provides lugs that can accomodate up to 750 MCM and is listed at 75 degree C what is the potential ampacity of those lugs? If you land 500 MCM thhn on those lugs what should the ampacity rating of the wire be?
 
San -Brooke said:
So if you have a 400 amp breaker that provides lugs that can accomodate up to 750 MCM and is listed at 75 degree C what is the potential ampacity of those lugs? If you land 500 MCM thhn on those lugs what should the ampacity rating of the wire be?
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In 110.14(C), the NEC is discussing terminations not lugs. Terminations include the item the lug is mounted to.
In your example the breaker current carrying capacity of the termination has been determined following UL489 procedures, using 400A and the smallest sized conductor with 75?C insulation. If this combination passes the heat rise test, then all larger conductors will also pass, as long as 400A is not exceeded. The largest size conductor is then tested to make sure it remains secured during a short circuit.
 
San -Brooke said:
So if you have a 400 amp breaker that provides lugs that can accomodate up to 750 MCM and is listed at 75 degree C what is the potential ampacity of those lugs? If you land 500 MCM thhn on those lugs what should the ampacity rating of the wire be?
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The NEC has different answers selected from tables 310.15 - 310.60 based upon conductor types, wiring methods, & bundling.

Most NEC tables are invalid if, PF ‡ 1, after derating, or continuous-load adjustments, or beyond 60Hz (Marine & Aircract operate at 400Hz).

NEC tables are invalid where enclosure temperatures can exceed their listing. The industry application of NEC tables often results in overheating.

If correctly applying 2011 NFPA Table 310.15(B)(16) (formerly Table 310.16), 500 mcm shows 380 Amps max in the 75? column.

The engineering formula I use, set to the same table conditions, show 500 mcm at 75? at only 372 Amps.
 
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jim dungar said:
If the testing only uses 4' of conductor then temperature issues with the conductors past that point do not impact the breaker, is this the point where 110.14(C) is not important?
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I have used that argument with an inspection authority and was successful in convincing them to allow me to do that (based on an engineer's approval which I obtained). Basically I was allowed to use #4 type W cable with 100-amp OCP so long as I transitioned to a #3 THHN for at least 4' before the breaker.
 
hardworkingstiff said:
I have used that argument with an inspection authority and was successful in convincing them to allow me to do that (based on an engineer's approval which I obtained). Basically I was allowed to use #4 type W cable with 100-amp OCP so long as I transitioned to a #3 THHN for at least 4' before the breaker.
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Good, that was a correct ruling.

Assuming the #4 W was properly applied.:)
 
Smart $ said:
Here's a link to a Square D Data Bulletin which covers this topic...http://static.schneider-electric.us/docs/Power Management/0110DB9901.pdf
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Smart $,

Thanks for the pdf link. I found the article more direct than the NEC 2011, in emphasizing Table 310.15(B)(16) is device termination centric, as opposed to stand alone lugs terminating a conductor on a tap, for example.

It seems to me, perhaps it would be clearer if Table 310.15(B)(16) included additional terminology about its importance in terminations.

What do you think?

Best regards,

MarineTech
 
MarineTech said:
Smart $,

Thanks for the pdf link. I found the article more direct than the NEC 2011, in emphasizing Table 310.15(B)(16) is device termination centric, as opposed to stand alone lugs terminating a conductor on a tap, for example.

It seems to me, perhaps it would be clearer if Table 310.15(B)(16) included additional terminology about its importance in terminations.

What do you think?

Best regards,

MarineTech
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weakest link in the chain is where it is going to fail. The termination at the overcurrent device is likely to sink more heat into attached conductor than any other point along conductor in most cases so that is weakest link in the chain and the point that will require the largest conductor, before throwing in other adjustment factors anyway.
 
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