3ph Smaller neutral than phases?

[festa]

Hi guys
Im a domestic installer and know very little about industrial electrics but yesterday we had some machinery installed and wired in so last nite,as you do i was inspecting the installation and noticed...

3PH ...6mm Insulated single core phases with a 4mm neutral !!!

Is this good practice?

 

[Steve]

Yes. If you know anything about 3 phase theory, you'll know that the 3 phases add up to zero current on the neutral when the load is balanced perfectly.

The neutral is probably there for any single phase control gear. Zero current flows on a neutral when the load is correctly balanced.

 

[festa]

Nice one Steve....

 

[plugwash]

Is this good practice?

Depends, it is far less common than it used to be because of the increase in use of electronics.

Yes. If you know anything about 3 phase theory, you'll know that the 3 phases add up to zero current on the neutral when the load is balanced perfectly.

And if you know anything about three phase theory you should know that that does not apply to currents on harmonics that are multiples of 3.

If the load is mostly three phase motors or three phase rectifiers then reducing the neutral size should not be a problem but if there is a lot of single phase electronic kit on a supply you should really not use reduced neutrals even if the load is fairly well balanced.

 

[festa]

Yes Plugwash its a dedicated 3ph motor so as you and Steve say its not a problem.

The reason i was concerned was the same company had installed a single ph piece of machinary rated at 45amps and used 2.5mm single core cables.

 

[ricicle]

Fri May 12, 2006 6:32 pm Post Subject:

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sorry, whats this about blue red and yellow phases?

 

[Steve]

Fri May 12, 2006 6:32 pm Post Subject:

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sorry, whats this about blue red and yellow phases?

Care to provide a link? So I may defend myself and put this into the context it was intended?

 

[ColJack]

at work we wire industrial air conditioners.. the type on the roof that does the whole store...

we quite often have 70mm singles mains cables from the isolator to the control panel.

they goes into something similar to "henley blocks" and are then split down to 10mm's, 6mm's adn so on to feed the breakers in the control panel..

mostly 3 phase compressors and the like..

sometimes there has been as few as 2 2.5mm neutrals comming out of the neutral block but they still insist on sizing the neutral the same as the phases..