SkillsUSA preliminary contest Industral Motors Controls Written test Q7

[fmtjfw]

Doesn't mean you couldn't run two feeders in a common raceway and use only one common neutral from what I can see. All conductors have to be in the same racway - this will result in needing to derate for number of current carrying conductors in the raceway and possible need to increase conductor sizes.

There is no requirement for a neutral with an feeder. Consider two feeders: 3-wire 480V delta and 208Y/120V.
Derating required: If might not effect wire size: 6 THHN #8 .8?55 = 44 next larger OCPD 50A

 

[Smart $]

Doesn't mean you couldn't run two feeders in a common raceway and use only one common neutral from what I can see. All conductors have to be in the same racway - this will result in needing to derate for number of current carrying conductors in the raceway and possible need to increase conductor sizes.

Other than the scenario of low maximum unbalanced neutral current for separate feeder neutrals I mentioned earlier, what benefit would there be to run a common neutral? It has to be sized to handle the maximum unbalanced neutral current for both feeder circuits.

 

[fmtjfw]

Other than the scenario of low maximum unbalanced neutral current for separate feeder neutrals I mentioned earlier, what benefit would there be to run a common neutral? It has to be sized to handle the maximum unbalanced neutral current for both feeder circuits.

Existing conduit run...

 

[Smart $]

Existing conduit run...

How so? If you are trying to save on fill area, separate neutrals would have less area (ampacity per cmil decreases as size increases).

 

[kwired]

There is no requirement for a neutral with an feeder. Consider two feeders: 3-wire 480V delta and 208Y/120V.
Derating required: If might not effect wire size: 6 THHN #8 .8?55 = 44 next larger OCPD 50A

I'm well aware there is no requirement for a neutral with a feeder. 3 wire 480 volt feeder is not a good example as it wouldn't have a neutral to share even if you wanted to, and yes derating may not effect conductor size in some instances, others it will.

Be careful with the 8AWG - if you have a continuous load of more than 40 amps the conductor is too small anyway if you have 75C terminations.

 

[Sahib]

I just don't understand the physics of where and why one might use this.

The drawings in your attachment are not MWBC's. So full phase current and not unbalance current flows in the neutral. So size of the common neutral to be increased suitably.

 

[Smart $]

The drawings in your attachment are not MWBC's. So full phase current and not unbalance current flows in the neutral. So size of the common neutral to be increased suitably.

The whole issue is regarding feeders. Common neutrals are no longer permitted for MWBCs. Nonetheless, the common neutral in the feeders diagrammed does carry unbalanced current and only required to be sized for the maximum calculated unbalanced current... but not smaller than the minimum required EGC.

 

[Sahib]

Smart $
You are correct. I did not notice '3P' in the diagram. Thanks.