1 cable with 3 circuits

[dh5306]

hi all

a new design i am looking at uses a 5 core swa to feed 3 groups of 4 400w son lowbay luminaires,(via local jb) each phase is seperatly protected.

i cannot see anything in the regs to see if this is allowed or not.

(it is not easy to run singles in conduit or trunking as the route is difficult, 50 metres + obstacles)

the circuit isolation will be clearly labelled at the jb

any thoughts would be appriciated

 

[Andy™]

shared neutral?

it would work fine, but isolation would be a problem. unless all phases (/lights) are on the same switch and come on at the same time. using seperate switches would work, but make isolation dangerous if only 1 phase is isolated

 

[Northern Electric]

Not too sure what your set up will be, all I can advise is make sure the multicore SWA is rated high enough to cope with the current on the neutral from all 3 phases. 4*400=1600w per phase. Times 3= 4800w.

4800/240=20A returning on the neutral, inductive load too, which, depending on the fittings and internal power factor correction, will give you a higher demand.

Personally, I don't like the sound of it.

 

[sellers]

had a little look in the BRB

559.6.2.3

Groups of luminaires divided between the three line conductors of a three phase system with only one common neutral conductor shall be provided with at least one device that simultaneously disconnects all the line conductors.

Triple pole MCB me thinks

 

[NozSpark]

And if you have 4 lights on each phase and have them all on at the same time then you will have a balanced 3 phase load and ....... no neutral current!!

 

[Northern Electric]

had a little look in the BRB559.6.2.3

Groups of luminaires divided between the three line conductors of a three phase system with only one common neutral conductor shall be provided with at least one device that simultaneously disconnects all the line conductors.

Triple pole MCB me thinks

Well, there you go, I haven't come across that one before.

 

[Northern Electric]

And if you have 4 lights on each phase and have them all on at the same time then you will have a balanced 3 phase load and ....... no neutral current!!

Not quite, the current has to get back home somehow! ROTFWL

 

[sellers]

Well, there you go, I havn't come across that one before.

Me neither, I would have thought it would have been a big no no, but obviously not.

 

[Andy™]

Not quite, the current has to get back home somehow! ROTFWL

noz is correct. if balanced, there will be no neutral current. they probably wont be perfectly balanced, so there would probably be a little current on neutral (probably a lot less than 1A)

 

[Andy™]

had a little look in the BRB559.6.2.3

Triple pole MCB me thinks

saved me finding that one!

 

[NozSpark]

Not quite, the current has to get back home somehow! ROTFWL

It's just like a 3 phase motor (star).... no neutral!!

 

[sellers]

[quote name='Andy

 

[Andy™]

This is something I don't get, heard it before. So say you have 1 light per phase. You connect each phase to one side of the lamp. You don't need a neutral. So nothing on the other side. How does current pass through the lamp?

it passes through the lamp and straight through the other 2 lamps.

really, its the exact same way as houses are wired. each house is a phase. if they were all balanced, there would be no need for a neutral all the way to the star point, only between houses (or in your case, between the 3 lamps)

 

[NozSpark]

Connect all the neutrals from the lamps together, but not back to the DB...

It will work perfectly until one of the lamps blow!!

 

[sellers]

Connect all the neutrals from the lamps together, but not back to the DB...It will work perfectly until one of the lamps blow!!

That makes sense!

 

[Andy™]

and make sure all lamps are the same power, otherwise you will blow 2 (possibly 3) lamps!

 

[Northern Electric]

Bloody hell, you have had me looking through my old text books. Yes a balanced 3 phase load will cancel out neutral current, thus allowing you to use a reduced size neutral conductor. I must have been thinking in single phase world. Please ignore my previous b*****s. Sorry guys, quick 3 phase refresh for me there. Twice today.

 

[Badger]

2.5 conductor size

 

[sellers]

Could you really rely on a reduced neutral conductor? Say over time a couple of lamps blow thus the load isn't ballanced causing more current down the neutral.

Also if the rows were switched indepentently you would have the same problem.

 

[Andy™]

Could you really rely on a reduced neutral conductor? Say over time a couple of lamps blow thus the load isn't ballanced causing more current down the neutral.Also if the rows were switched indepentently you would have the same problem.

can you even get SWA with a smaller neutral size?

even though a smaller sized neutral could be used, would it be wise to in these circumstances? after all, it could be open to higher loads than one phase would be