1 cable with 3 circuits

[sellers]

No what I'm saying is that, you may need a larger neutral to accomodate for the situations where the load wouldn't be balanced. Therefore you may not be able to rely on an swa with all lines and neutral the same size.

 

[Northern Electric]

When I say reduced neutral, I mean the same size as the phase conductors.

I'll tell you where I got confused. A few years ago I went to a job with an overdoor heater. It was designed to be on 3 phase, but some bright spark wired it single phase. Any how it was drawing 27A total current, 9A per element. It was wired in 6mm T&E and the control gear was rated at 20A (supplied with the heater). The neutral side of the control gear burnt out, obviously if it was wired on 3 phase the neutral current would have been very low, thus the control gear would have been suitable, but because it was wired single phase the 27A eventually cooked the switch.

This example is opposite to what we have been talking about.

I'm just trying to dig myself out of a hole now.

 

[sellers]

I think your out of the hole. Someone please correct me if I'm wrong, but..

lets say 3 rows of 6 x 400w luminaires. Each row on a phase.

6x400w = 2400w, 2400w/240v = 10A

but you can't base your cable selection on this as there may be alot more current down the neutral

6x400w = 2400w 2400wx3 = 7200w 7200w/240v = 30A

So your cable selection would need to be capable of carrying 30amps.

 

[Northern Electric]

No neutral currentFor the case of equal loads on each of three phases, no net current flows in the neutral. The neutral current is the sum of the phase current.

Since we have shown that the neutral current is zero we can see that removing the neutral core will have no effect on the circuit, provided the system is balanced. In reality such connections are generally used only when the load on the three phases is part of the same piece of equipment (for example a three-phase motor), as otherwise switching loads and slight imbalances would cause large voltage fluctuations.

see also website http://en.wikipedia.org/wiki/Three-phase_electric_power

OK, I have trawled the net to try and find some answers here, it's been a bloody good refresher on 3 phase theory for me.

So, if we have a three phase circuit going off to a few lights, we know each phase has to supply 4 X 400w fittings. That's roughly 7A a phase. If we base the cable size on that, then we could use 1.5. All fair and well in a balanced system, but lets say one phase of lights developed a problem, you have 2 phases powered, but now it's not balanced, you will have 14A on the neutral.

I am beginning to think my first idea was better. You can't base it on a balanced system unless it is a fixed load.

 

[dh5306]

thanks for help, but forgot to mention SP not 3P as i also found reg on disconnection in circuit and saw it meets reg using DB mcb

i have attached circuit on what is proposed.

(personally i dont like it. i would rather have luminares from 3p supply)

View attachment 323

PS if 6mm connections to big for switches then that is design issue not regs issue

thanks again

3 Circuit + 1 cable.dwg Mode.pdf

 

[Northern Electric]

After all that it's single phase! ROTFWL

Well, a bit of 3 phase theory never did any one any harm.

That's a very bizzare design, I have never done one like that before. Without actually seeing the job it's hard to comment on the selection of cables, though the circuit is fused to 16A and eventually ends up in 1.5, maybe fusing to 10A may be a better idea.

 

[Andy™]

but lets say one phase of lights developed a problem, you have 2 phases powered, but now it's not balanced, you will have 14A on the neutral.

no, you dont have 14A on neutral. you have less. some power will flow direct between phases, so neutral wont carry 2x as much. not sure of the exact maths to get neutral current though, but at a quick guess id say 8-10A

 

[kme]

Would be in agreeance with "our Andy" on this one

 

[Northern Electric]

I know, I should have typed could have up to 14A.

Using vector diagrams I would calculate the resultant neutral current as 12.2A in a purely resistive circuit, but as we have inductive loads it would probably work out a bit more, maybe even a bit less depending on the power factor rating of the fittings.

Seeing as it's single phase, I don't suppose it matters now!