Neutral and earth for a 3 phase motor

[AusLee]

Hello,

I have a contractor who wants to use a signle conductor for both earth and neutral, but from what i read here in your posts "neutral shall be connected to earth only at the service entrance" - so is there a breach of the code in him doing so?

Another thing - i don't think the Search service works because for the nth time i try and i get no results, could you please give me an example of a search that gives results?

 

[Skogsgurra]

Why would he use Neutral for a three-phase motor?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[davidbeach]

What is permissible by code depends on where you are and what code applies. Per the NEC rules, neutral and ground are always separate for low voltage (not over 600V in the NEC) systems; some exceptions for higher voltages. On the other hand there is no reason to take a neutral to a 3-phase motor.

Putting the words neutral earth into the search box at the top of the page produced no results. Putting those words in the Google Search for Eng-Tips Forums provided over 200 results, clicking on advanced search and using those key words for the forums in my thread minder list resulted in "hundreds of results matched your query".

 

[AusLee]

Ok - no matter what i want to apply NEC.

Why wouldn't i need a neutral, do all 3 phase motors have a detlta connection? The motors are direct on line, but for star-delta, i will give him only 1 cable to the starter, from there to the motor he has to run 2, is the cable for star delta also 3 cores without neutral?

or is the reason that the neutral current is perfectly zero, therefore no need for the wire?

 

[davidbeach]

Even in wye connected motors the wye point is not brought out for connection. Three terminations, three conductors; plus a safety ground. No place for a neutral.

See thread237-169272.

 

[jraef]

Also, for Start-Delta starting capability you must run 6 leads to the motor because the Star and Delta connections are then made in the contactors, not the motor. But you would still only need one ground conductor.

JRaef.com
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[AusLee]

Ok, thank you very much for the info. In conclusion: 3 phase motors have no terminal for neutral, only for earth, therefore a neutral conductor will have no termination and hence it is not necessary. Thanks again.