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!

 

[Guest55]

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

easy , its zero.
;-)

 

[PRR]

> how do you work out the square root of zero?

Use a calculator. Mine says zero.

Never trust a calculator. Check: 0*0 equals zero. The 4-bit idiot may be right.

> surely that neutral conductor is now underrated?

3 is a big number. Look at a "2-phase" (actually split single-phase) supply. When current flows one way in loop A it flows the other way in loop B. If loads A and B are equal, the neutral has NO current. If A and B don't balance, the *difference* (not sum) flows in "neutral". If B has been evicted and only A is pulling power, the A-line and "neutral" currents are equal.

The neutral current is never(*) more than either line current. Nominal max load: A-line and B-line flow 100 Amps, neutral flows zero (why we call it neutral): Worst-case A-line and "neutral" flow 100 Amps, B-line flows zero Amps. Either way we have two conductors flowing 100A and one loafing.

3-phase theory is trickier but the result is the same. The neutral current is never(*) more than any of the line currents.

Often "it may be assumed" to be much less. A factory may be a lot of 3-phase motors. These will always draw equally from each phase, and the neutral current will sum to zero. Ah, but you may have a 1-Ph tea-pot. So maybe 600A for motors and 10A for tea-pot. The neutral is more than decoration but can be much smaller than the lines. Nevertheless, most codes require it be 50%-80% of line capacity.

So even if this were a factory converted to flats, it is un-likely the tenant loads would be both large and SO un-balanced as to burn the "neutral". (If loads are not-large, which in residences is true 95% of the time, then even a large percent unbalance is a small current and not a problem.)

A typical residence may pull 0A-5A for lights, 0A-5A for TV etc, and 25A for holiday cooking. If A B and C are busy in the kitchen, the loads near-balance and the neutral current is small. If A is cooking but B and C went out, that's 25+A in A-line but maybe 2A in B and C. The "neutral" has strong current but B and C lines are cool, the cable won't overheat. While the "neutral" will run warm, the negligible heat in B and C conductors means it is probably safe; anyway cooking loads may run 2 hours but not a full 8 hours steady.

(*) There is a special case when MOST loads are DC power supplies. Capacitor-input rectifiers draw large spikes which violate the "sine current" assumptions of classic theory. The rectifier spikes do NOT cancel in the neutral, and can exceed current in any line. When LARGE computer systems grew, some neutrals burned off. The first fix is an oversize neutral. Even so the cost and spikes are troubling. Recently most large DC supplies are "PFC" to draw a sine-like current waveshape which is less nasty, and does mostly cancel to the neutral.

 

[Silly Sausage]

Hahaha... I'll let Tony rip that reply to pieces. I can't be bothered.

 

[electric will]

......

 

[electric will]

Hahaha... I'll let Tony rip that reply to pieces. I can't be bothered.

where is the old bugger tonight? I hope he's not in that bloody pub again!