Sizing of conductors at 125 percent

[NSTech]

Hello,
I've been in the game for a couple decades but I'm not reading something right. I'm preparing to teach some CEU's and in Mike's Changes to the NEC 2008 book he mentions a couple times, "The sizing of conductors at '125 percent of the continuous load' is necessary because protection devices are not typically listed to operate at 100 percent of their rating continuously"....

I realize the conductors for continuous loads need to be rated at 125%. But the reason given is because protection devices are not listed to operate at 100 percent continuously. This doesn't make sense to me. If the book is saying the CB is not designed to operate at 100 percent that would seem to make it less likely that conductors would need to be sized larger. It is my understanding that conductors under continuous load are sized larger because of gradual increase of temperature of the conductor and connections. Not sized larger because the CB isn't sized for loads of 125%. Maybe I'm overlooking something obvious? Or maybe he means the CB might trip at higher than its rating? That probably is more likely. I would appreciate any opinions. Thanks.

 

[charlie]

I think you are confusing the code rules and how things work. A circuit breaker will hold 135% of the rating for a couple of hours before tripping unless it is sandwiched between a couple of heavily loaded circuit breakers. 125% will protect the wire itself if the actual overload actually occurs. Additionally, the overcurrent device uses the conductor as a heat sink, especially in the larger sizes of over 800 amperes.

The code rules permit the devices to actually work safely. :smile:

 

[NSTech]

Thanks for the reply regarding 125% conductors

Thanks for the reply regarding 125% conductors

Thanks for the reply. Your answer gave me enough of a hint so I understood what the author had in mind. My misunderstanding is more a question of semantics than confusion of how things work. I've spent hours over the years discussing CB trip curves with engineers and other inspectors as this can become a gray area. The 135% comment means you too have run across FPE breakers. Thanks again.

 

[jmccamish]

not just fpe or breakers

not just fpe or breakers

Many overcurrent protective devices can hold the current at 135% for 2 hours.
A UL 248 listed fuse such as an RK 5 can hold 135% for upto 2 hours and still meet the listing
requirement. Conductors are sized for general continuous load branch circuits for use as a heat sink to help keep the breaker and terminations more cool. Check out he new language in 210.19 A. (2008) ( I think its exception 2, I dont have my code book with me) it does not require the grounded conductor if not connected to an OCPD to be sized at more than 100%. This shows the 125% is for heat sink use, not ampacity.
Conductors can withstand much greater amps than listed in 310.16 if the time is not sufficient to cause damage.

 

[hardworkingstiff]

Check out he new language in 210.19 A. (2008) ( I think its exception 2, I dont have my code book with me) it does not require the grounded conductor if not connected to an OCPD to be sized at more than 100%.

So, if I have a circuit feeding outdoor lights (120-volts) and the load is 19-amps, I need to pull a #10 ungrounded conductor and I can get by with a #12 grounded conductor and put it on a 25-amp breaker.

Got old timers' disease, hard to do that.

 

[infinity]

So, if I have a circuit feeding outdoor lights (120-volts) and the load is 19-amps, I need to pull a #10 ungrounded conductor and I can get by with a #12 grounded conductor and put it on a 25-amp breaker.

Got old timers' disease, hard to do that.

How would you get around 240.4(D)(5)?