Short Circuit Characteristics of Insulated Cables

[yesterlectric]

Wondering about this fine print note below 240.4:
- See ICEA P-32-382 2007 (R2013), Short Circuit Characteristics of Insulated Cables, for information on allowable short-circuit currents for insulated copper and aluminum conductors.

Is this to suggest that the standard would require more than ampacity consideration for conductors and overcurrent, and would require one to consider the momentary current that a conductor will carry before the breaker operates during a short circuit event, and possibly upsize conductors accordingly? Never had to do this before. Wouldn't think it's standard on less than 1KV applications. And if I only could afford to buy each standard the informational notes suggest..........................................................................................................

 

[jim dungar]

UL 489 listing for molded case circuit breakers requires them to be connected to 4' of conductors per the NEC. Among other reasons is to prove the insulated conductors are fully protected over the breakers entire short circuit rating.

All you need to do is apply conductors per NEC tables and protect them with UL listed devices

 

[yesterlectric]

UL 489 listing for molded case circuit breakers requires them to be connected to 4' of conductors per the NEC. Among other reasons is to prove the insulated conductors are fully protected over the breakers entire short circuit rating.

All you need to do is apply conductors per NEC tables and protect them with UL listed devices

Yes that is all I have ever done. But I wonder what the purpose of the note is. Is it just a commercial break to try to sell another standard that most electricians will never need?

 

[mbrooke]

UL 489 listing for molded case circuit breakers requires them to be connected to 4' of conductors per the NEC. Among other reasons is to prove the insulated conductors are fully protected over the breakers entire short circuit rating.

All you need to do is apply conductors per NEC tables and protect them with UL listed devices

UL 489 doesn't spec short circuit currents above a conductor's withstand or fusing constant.

 

[mbrooke]

Yes that is all I have ever done. But I wonder what the purpose of the note is. Is it just a commercial break to try to sell another standard that most electricians will never need?

All conductors are only capable of handling short circuit current for a certain period of time before they exceed 150*C- the typical limit of insulation.

240.92B while applicable to only tap conductors, technically still holds valid for all conductors.

This paper has an excellent discussion on the subject:

https://www.eaton.com/content/dam/e...ductor-protection-wire-cable-protection-2.pdf


EGC withstand:

https://www.eaton.com/content/dam/eaton/products/electrical-circuit-protection/fuses/solution-center/bus-ele-tech-lib-conductor-protection-wire-cable-protection-1.pdf

 

[jim dungar]

Yes that is all I have ever done. But I wonder what the purpose of the note is. Is it just a commercial break to try to sell another standard that most electricians will never need?

Sometimes conductors are applied outside of the NEC tables and a resource such as this becomes important, but a typical electrician would never need it.

It has probably been some 30 years since I have looked at it.

 

[don_resqcapt19]

UL 489 listing for molded case circuit breakers requires them to be connected to 4' of conductors per the NEC. Among other reasons is to prove the insulated conductors are fully protected over the breakers entire short circuit rating.

All you need to do is apply conductors per NEC tables and protect them with UL listed devices

Jim, do you have any idea of what section of 489 says that? I just looked at the testing part of that standard and I see nothing that talks about looking at the condition of the test leads as part of the breaker testing process. Of course, given the difficultly of using their free view, I could have easily missed the section.

 

[jim dungar]

Don,
I haven't read the standard in years.
I remember it being discussed in the 80s when a fuse manufacturer was trying to 'dismiss' molded case breakers and their Amps Interrupting Capacity.

Another reason is to prove the conductors do not pull free from the breaker lugs during a short circuit.

 

[don_resqcapt19]

Don,
I haven't read the standard in years.
I remember it being discussed in the 80s when a fuse manufacturer was trying to 'dismiss' molded case breakers and their Amps Interrupting Capacity.

Another reason is to prove the conductors do not pull free from the breaker lugs during a short circuit.

I did find that the conductors have to stay in place, and that you cannot tighten the terminals to a torque greater than specified, but could not find anything on the conductors not being damaged.

 

[jim dungar]

I did find that the conductors have to stay in place, and that you cannot tighten the terminals to a torque greater than specified, but could not find anything on the conductors not being damaged.

I just did a quick look through my files and I found an article from EC&M magazine dated October 1992. This was a round table discussion with the panel members including editor Fred Hartwell as well as manufacturer reps. The point was made by George Gregory, from Square D, that the UL inspector visibly checks the conductor insulation after a short circuit test. Hartwell raised some concerns about this, but then went on to say that ICEA acknowledged it's short circuit withstand tables were not suitable when conductors were applied per the NEC, as well as mentioning that other research has shown the IECA values maybe conservative.

 

[mbrooke]

Respected members, may I introduce the "bath tub slide" let through energy of a molded case circuit breaker?

 

[bwat]

Respected members, may I introduce the "bath tub slide" let through energy of a molded case circuit breaker?


View attachment 2557340

Sure, but what are you demonstrating here with this chart that's connected to the discussion? Interested.

Looks like a good chart that (I think) shows that you can have more let-through energy in a lower magnitude fault (like in the thermal region of a breaker) than if it would have been in the instantaneous region. The relative overlapping of the energy in the instantaneous region between difference size breakers is also notable. I assume this is for fixed trip TM?

 

[jim dungar]

Sure, but what are you demonstrating here with this chart that's connected to the discussion? Interested.

The curves are for IEC circuit breakers. They have no bearing on applying UL listed conductors and molded case circuit breakers per the NEC. Decades of testing by UL489, as well as thousands of installations, demonstrate that these breakers can protect insulated conductors. But of course statistically there will be outlier examples which can be cited.

 

[mbrooke]

Sure, but what are you demonstrating here with this chart that's connected to the discussion? Interested.

Looks like a good chart that (I think) shows that you can have more let-through energy in a lower magnitude fault (like in the thermal region of a breaker) than if it would have been in the instantaneous region. The relative overlapping of the energy in the instantaneous region between difference size breakers is also notable. I assume this is for fixed trip TM?

The more current flows through a breaker, the faster it will trip, the less heating and incident energy is observed during a fault.

Once you hit the magnetic trip threshold the incident energy let through drops dramatically.

However, as the available fault current climbs above the magnetic pickup, the incident energy gradually increases- due to the fact a molded case breaker can not clear faster than 2 cycles.

If faster clearing is needed, than fuses must be selected.

Going by ICEA, a standard thermal magnetic THQL, QP or QO breaker will protect 12 gauge wire up to 2000 amps of fault current.




If total conductor protection is desired, fuses are required where panelbaord fault current exceeds 2000 amps.

However, in most cases the risk of #12 conductor burning up is small in that:

1) the available fault current in the panelboard must be over 2000amps.

2) The fault must be bolted, with negligable impedance.

3) the fault must take place in the first 15 feet of the run at 120 volts, 35 feet at 277 volts. Beyond that it is impossible for any thermal damage to take place on the conductor.

 

[mbrooke]

Sure, but what are you demonstrating here with this chart that's connected to the discussion? Interested.

Looks like a good chart that (I think) shows that you can have more let-through energy in a lower magnitude fault (like in the thermal region of a breaker) than if it would have been in the instantaneous region. The relative overlapping of the energy in the instantaneous region between difference size breakers is also notable. I assume this is for fixed trip TM?

This video demonstrates clearing time impacts perfectly on small conductors with high available fault current starting at 4:33: