Nec 240.4

[AggieChris]

Based on 240.4 (b)-2 It appears that under normal circumstances, that the NEC allows the protection of a conductor to allow the conductor to be overloaded.

Assuming under 600 volts and ideal conditions ( completely continuous load, no derating, no voltage drop etc.)
If after the load calculations and conductor sizing calculations result in a 255 amps cable. Based on the conditions in 310.16 75 C terminations and insulation, I would pick a 250 MCM cable (like I said this includes the sizing factors i.e. 125% times full load).
The Protection sizing would result in a 255 amp protection( 125% times full load).
Based on 240.4(b) 2, I would then choose a 300 amp breaker, meaning that there is a possibility (between 255 and 300 amps) where the cable could be continuously overloaded without tripping the breaker.


SUMMARY:
Which takes precedent, sizing the conductor just for the load?
Or, sizing the conductor to ensure that the protection will always protect the conductor?


Conservative design says the latter, but it looks like the code may support the former.

Thanks,

Chris

 

[charlie b]

If you place a load of (let us say) 290 amps on a conductor that has an ampacity of 255, it may well overheat, and its insulation system could experience an early failure. And yes, a 300 amp breaker would not be able to protect against that event. But you wouldn't be putting that load on the wire, in your example. You selected a conductor that has sufficient ampacity for the calculated load. So how would it come to pass that the load exceeded that value? If it is because of a load addition, then whoever did that load addition was responsible for making sure the existing conductor could handle it.

The probability of getting a current between the rated 255 and the trip setting of 300 is very low, as it would require a very specific range of resistances in the ground fault path. So the code considers it to be an acceptable risk.

Welcome to the forum.

 

[Dennis Alwon]

Chris ,if you use the next higher size breaker the load must not ever be higher then the ampacity of the wire. Thus you can use a 300 amp breaker on the 250 KCM cable but the calculated load must not ever be higher than 225amps. Once the load changes then the wire size must be increased.

This is one of the code areas I think can be bad as the next guy may not pay attention to the calculated load but that is how it is.

 

[AggieChris]

Thanks for the replies.
It seems that the only time this could happen is if there was a partial failure of the load ( i.e. I high resistance short to ground).
Then at this point, an overload could effective sit on the cable forever until failure.

But I guess the point is, even though there is a slim possibility of this kind of failure, I would still be within the code to size my conductor this way.

 

[Dennis Alwon]

Chris, I don't think that there would be a problem with a ground fault condition. In the event of a ground fault the 300 amp breaker will trip. It would be unusual to not trip the 300 amp breaker.

 

[steve66]

Thanks for the replies.
It seems that the only time this could happen is if there was a partial failure of the load ( i.e. I high resistance short to ground).
Then at this point, an overload could effective sit on the cable forever until failure.

But I guess the point is, even though there is a slim possibility of this kind of failure, I would still be within the code to size my conductor this way.

The NEC conductor ampacity is pretty conservative, so even if there was an overload in the 250-300 amp range, there is a good chance it wouldn't cause a real problem anyway. The wire might get hotter than its insulation rating, and it might even fail eventually. But the breaker should trip then.

 

[Pierre C Belarge]

You selected a conductor that has sufficient ampacity for the calculated load. So how would it come to pass that the load exceeded that value? If it is because of a load addition, then whoever did that load addition was responsible for making sure the existing conductor could handle it.

The probability of getting a current between the rated 255 and the trip setting of 300 is very low, as it would require a very specific range of resistances in the ground fault path. So the code considers it to be an acceptable risk.

Welcome to the forum.

This is key when additional work is performed. Professionals are suppost to do the work, but we all know about that...

Chris, I don't think that there would be a problem with a ground fault condition. In the event of a ground fault the 300 amp breaker will trip. It would be unusual to not trip the 300 amp breaker.

Dennis
Chris mentioned a "high resistance" groundfault. Which could become an issue, but not an issue of sizing the conductor. High resistance groundfaults are generally a big issue.