suppliers neutral size

[dellboy78]

I work in a lot of big old townhouses that are converted into flats. Many of them have a 3 phase head at the intake (3x 100A fuses) which makes me think the suppliers neutral must be quite substantial?? I have always assumed it is <35mm when selecting main bonding conductors but thinking about it, could it be bigger? Your thoughts would be appreciated!

 

[D Skelton]

Worst case scenario (only one phase of three being utilised) the neutral is never going to need to be bigger than 25mm with a 100A main head.

 

[dellboy78]

Some of these houses are converted into between 3 and 6 flats, so often each main fuse will supply 2 flats plus a landlords supply as well. Rarely is only of the fuses utilized.

 

[D Skelton]

There you go then. Neutral is never going to be required to be bigger than 25mm.

Do you know how to calculate neutral conductor current?

 

[dellboy78]

Yep, I get you mate. I don't know why but I was thinking of it as 3 single phase supplies rather than a 3 phase one! Early night tonight I think. Cheers.

 

[Guest55]

i'd put money on the dno neutral being 16mm for 100a whether single or 3 phase.

 

[loop3]

The primary function and only function of the neutral in a three phase system is to carry away out of balance current.

 

[Deleted member 26818]

The primary function and only function, eh.

 

[shocking_eg]

The primary function and only function of the neutral in a three phase system is to carry away out of balance current.

ok so that's correct, but say this 3 phase installation has been converted to single phase loads only, surely that neutral conductor is now underrated ?

 

[Deleted member 26818]

ok so that's correct, but say this 3 phase installation has been converted to single phase loads only, surely that neutral conductor is now underrated ?

No, because the 3 phases are out of sync, therefore the current is out of sync.
If it were thre supplies on one phase, then yes the neutral conductor would be undersized.

 

[shocking_eg]

No, because the 3 phases are out of sync, therefore the current is out of sync.
If it were thre supplies on one phase, then yes the neutral conductor would be undersized.

oh right, of course ! thanks for clearing that one up.

 

[Knobhead]

Try this

In=√((Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic)))

 

[Deleted member 26818]

Oh dear, how do you work out the square root of zero?

 

[Silly Sausage]

Oh dear, how do you work out the square root of zero?

... Que?

 

[Silly Sausage]

Try this

In=√((Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic)))

Nah...try this...

In = sqrt[ (Ia - 0.5(Ib + Ic))[SUP]2[/SUP] + (0.866(Ib - Ic))[SUP]2[/SUP] ]

:wink_smile:

(Ia is the reference)














I hope that's right...haha!