3ph and neutral

[brizospark]

If there is a 3 phase 6 way d.b. lets say all circuits are single phase.

How come it is ok to have the phase and neutral feeds to d.b. the same size when the neutral can be carrying load of all 3 phases?

 

[davelerave]

the max neutral current in a 3ph system will be maximum the value of one phase. this would only happen if only one phase was loaded and the otherphases not pulling anything

could be higher with harmonic currents-in theory

 

[Electro-tech]

Where the neutral current is expected to be higher than the line current, then the cable size should be selected on the basis of the neutral current. Appendix 11 BS7671. Also 523.6.3.

 

[topquark]

Where the neutral current is expected to be higher than the line current, then the cable size should be selected on the basis of the neutral current. Appendix 11 BS7671. Also 523.6.3.

Curious (and useful to keep in mind). Not sure I'd "expect" it to be though. I guess it depends on where your 3 phase supply is coming from and whether any harmonics have been sufficiently filtered. Looks like someone has decided to add VAT to be on the safe side (well at the old rate anyway).

Is this something you have come across frequently (pun intended )? Do you take this into account with fault scenarios, for example?

 

[Electro-tech]

Maybe over the last decade, occassionally.Fault scenarios where you have over-heating of neutral conductors.Non-linear loads generate high levels of harmonic currents. Typical non-linear loads include desktop computers, ac variable speed drives, inverters, and welders. Of these non-linear loads the main cause for concern is the switch mode power supply found in desktop computers and other office equipment.

 

[topquark]

Ah, so more load side than supply side related (must admit my thought track was on the supply side). I could see 100's of PC's being a consideration (the current's involved would be so small for the harmonics for an individual PC) ... but they should normally be divided amoungst several circuits anyway.

Welders, garage lifts etc I can see being a problem and potentially difficult to balance the load on 3 phase in that type of environment.

Not something I get involved with ... everyday's a school day, thanks!

 

[davelerave]

yes load side

 

[telectrix]

no i'm confused. my van has 2 side loading doors, so does that mean i've got 2 load sides? or is it 3 with the back door, or is the back classed as an end and not a side?

 

[topquark]

It's 3 and they're all out of phase (but in this case by 90deg and 180deg) with each other.

 

[telectrix]

things are becoming clear now, or not, as i don't have side windows.

 

[jeffducati]

Wow its frightening question??
dont go there if you do not understand it !!
these are course work questions.
its ok though had student put his test leads on PFC across 2 sad to say it never worked again!!!

 

[topquark]

Wow its frightening question??
dont go there if you do not understand it !!
these are course work questions.
its ok though had student put his test leads on PFC across 2 sad to say it never worked again!!!

Don't take the last few bits in here toooo seriously jeffducati We do like to have a little fun (once the original posters question has been answered of course).