neutral sizing

[sarge]

when using a 3phase sub main for 3 single phases (230v) what size should the neutral be (supplying the sub main).
Mr technical from a cable manufacturer told me that the neutral would be the same size as the phase conductors so i could use a 4c swa. so the neutral wuld be shared.
My boss tells me he thinks it should be 3 times the size of the phase conductors.
Any one got the correct answer.
Thanks for reading

 

[Darkwood]

Same size as phase as the maximum load on the neutral would only occur if you fully loaded one phase and remaining phases were zero load, by loading up the other phases too it has a negative effect on the load of the neutral thus reducing it, if all 3 phases are fully loaded and pulling the same current then no current will flow in the neutral, tell your boss to go back to college and learn his theory.

 

[Darkwood]

also note that any three phase load that is balanced will have no neutral current hence motors which are 3ph dont need a neutral to work but may have a neutral supplied if their control circuit requires one.
Supplying a 3ph submain with 4core cable with neutral matching phase size is standard practice, i would like to see your bosses solution to wiring in 120mm swa submains supply and needing by his theories total csa of 360mm for neutral ??????? How would he fit this in to the termination to start with???

 

[sarge]

thanks darkwood, didnt agree with him to start with, which is why i phoned the AEI technical, will have great pleasure in re-educating the ****** tomorow. Did learn this theory ages ago but when your boss thinks he knows better, knocks your confidence a bit.. thanks again

 

[telectrix]

Bosses, who wants them? perhaps he's got shares in a copper mine

 

[davelerave]

the only reason for oversizing the neutral -is possible harmonics causing overcurrent in the neutral

http://www.tlc-direct.co.uk/Book/4.3.12.htm

 

[Marvo]

the only reason for oversizing the neutral -is possible harmonics causing overcurrent in the neutral

http://www.tlc-direct.co.uk/Book/4.3.12.htm

This would usually go hand in hand with very poor power factor. Correction would be an alternative solution and possibly cheaper for the consumer depending on how they're being billed for their power.

 

[bugsy]

Ask you boss to explain his therory , should be intersting!

 

[sarge]

his theory was that 3 phase being used at 230v (x3) would need a nuetral three times the size in case full current from each flowed through the nuetral. however when i explained the correct theory he then said it would only need to be twice the size. So i drew out some sine waves and showed him how the sum of the current flow in each would cancel eachother out (fully balanced of course). With a red face he admitted hed forgotten about that!!!! he also agreed 360mm would need a big terminal as darkwood mentioned. he was clearly embarrased..

 

[ringer]

At least he was big enough to admit to being in the wrong - many people I know would have tried to bluff their way out of it. I imagine you got some good kudos out of that conversation. Well done for sticking up to him and making your point.

 

[mogga]

couldnt he just run 3no cores the same size as the csa of the sum of the singlecores of the phase conductors? if hes the "Boss" god help you M8

 

[sarge]

Well ive not been with him long and he hasnt done much 3phase work, mainly domestic, i think its just one of those things that gets forgotten when you dont deal with it much..but he heard me out. he should've looked it up first before if he wasnt so sure. At least he shouldnt forget again!!

 

[olly w]

Fair play he listened and learned from it.
Don't ever be afraid to be wrong, if you don't understand ASK.
It's all about learning.(but I admit I hated school!)