NEC 705.40 and 705.42

[Tainted]

There is a building that has cogenerator (CHP), upon loss of power to utility should it comply with 705.40 and 705.42?

What really is the difference between 705.40 and 705.42?

705.40 tells you that if you lose primary power source, that all the electric power production sources shall be automatically disconnected and shall not be reconnected until primary source is restored. Does this also apply to 3-phase systems?

705.42 tells us something a little different regarding to 3 phase systems.

705.42 tells you that if one of the 3 phases opens (not sure if opens means loss of utility power), the power production source shall be automatically disconnected.
Unlike 705.40, why doesn't 705.42 tell you that you cannot re-connect the power production source after it being automatically disconnected? In other words, are you allowed to reconnect the power production source after it's automatically disconnected even though the utility power is still off?

 

[wwhitney]

705.40 tells you that if you lose primary power source, that all the electric power production sources shall be automatically disconnected and shall not be reconnected until primary source is restored. Does this also apply to 3-phase systems?

Certainly.

705.42 tells you that if one of the 3 phases opens (not sure if opens means loss of utility power), the power production source shall be automatically disconnected.
Unlike 705.40, why doesn't 705.42 tell you that you cannot re-connect the power production source after it being automatically disconnected? In other words, are you allowed to reconnect the power production source after it's automatically disconnected even though the utility power is still off?

That's unclear to me, but if you check the 2020 NEC, you will find that the two sections have been combined, perhaps to address these ambiguities. So it would be reasonable to use the 2020 705.40 wording as a guide for the earlier intent. In which case the answer to the last question would be no.

Cheers, Wayne

 

[Tainted]

Certainly.


That's unclear to me, but if you check the 2020 NEC, you will find that the two sections have been combined, perhaps to address these ambiguities. So it would be reasonable to use the 2020 705.40 wording as a guide for the earlier intent. In which case the answer to the last question would be no.

Cheers, Wayne

Good catch! This revised version of the code makes more sense to me, I see the intent now. Unfortunately in NYC we are still stuck with old code so I hope they revise it that way.

 

[Tainted]

Certainly.


That's unclear to me, but if you check the 2020 NEC, you will find that the two sections have been combined, perhaps to address these ambiguities. So it would be reasonable to use the 2020 705.40 wording as a guide for the earlier intent. In which case the answer to the last question would be no.

Cheers, Wayne

So now I have another question that I just thought of... Why does the NEC want you to shut down the electric power production source after primary utility power is offline?