Eddys

[securespark]

Talking to someone today who saw an installation where two 3 core and earths ran between switches. One switch had a L&N feed so the spark had run two strappers and a live in one three core and two strappers and a neutral in the other.

He was worried this was wrong. I reckoned it would be OK, as long as the two cables entered the (metal) switchbox through the same knockout.

What do you reckon?

 

[JohnW2]

Talking to someone today who saw an installation where two 3 core and earths ran between switches. One switch had a L&N feed so the spark had run two strappers and a live in one three core and two strappers and a neutral in the other.

I'm a bit confused - why did he run strappers in both cables? Was this just an attempt to 'balance' things?

He was worried this was wrong. I reckoned it would be OK, as long as the two cables entered the (metal) switchbox through the same knockout. What do you reckon?

Quite honestly, the currents involved will be so small that it wouldn't matter a jot whether they went through the same or different holes!

Kind Regards, John

 

[securespark]

Two pairs of strappers because there are two two ways.
It would appear that the switch wire of fitting A is at one switch and the switch wire of fitting B at the other.
It's a commercial lighting circuit in 2.5.

 

[JohnW2]

Two pairs of strappers because there are two two ways.

Do you mean two gangs, for two light fittings. Maybe a diagram would help us (at least, me!) to understand.

It's a commercial lighting circuit in 2.5.

They found 2.5mm² 3C+E ?

Kind Regards, John

 

[rsgaz]

They found 2.5mm² 3C+E ?

https://www.edwardes.co.uk/en/products/2-5mm-6243y-grey-3core-38-earth-cable-100mt-reel

In grey or white 6243B.

 

[JohnW2]

https://www.edwardes.co.uk/en/products/2-5mm-6243y-grey-3core-38-earth-cable-100mt-reel .... In grey or white 6243B.

Fair enough - but I can't say I've ever come across it! I'm a little surprised that there is a big enough market to justify making it (particularly since many of the commercial/industrial situations in which it might be used will probably be wired in singles, anyway)!

Kind Regards, John

 

[securespark]

From what he is saying, it is like this:

 

[securespark]

Hmm...image disparu!

 

[securespark]

 

[JohnW2]

From what he is saying, it is like this:

...Fair enough - that's what I was expecting.

Taking each of the lights/cables separately, I can see nothing unusual or wrong with that. Is the only concern about the two cables going through one hole in a box, or what?

What sort of current are we talking about?

Kind Regards, John

 

[securespark]

Yes, that was my only concern. Dunno about design current but AFAICT, it's a 20A circuit.

 

[JohnW2]

Yes, that was my only concern. Dunno about design current but AFAICT, it's a 20A circuit.

Presumably even in a commercial environment, and even if it is a 20A lighting circuit (which, presumably, would be non-compliant with BS7671?), neither of the lights would be taking anything remotely like 20A?

In any event, having looked/thought a little more carefully, in terms of just what you have drawn (i.e. ignoring anything which might happen to the left of your diagram!), I don't think there would be a problem with separate holes, would there, since both of the 3C cables appear to be 'balanced'?

Kind Regards, John

 

[securespark]

Yeah, 20A bad, 16A good. Each "light" on my drawing is a bank of fittings, possibly metal halide high bay.

I didn't think there was much to worry about, save for the eddy issue.

 

[JohnW2]

Yeah, 20A bad, 16A good. Each "light" on my drawing is a bank of fittings, possibly metal halide high bay. ... I didn't think there was much to worry about, save for the eddy issue.

As I said, if there are no loads off to the left of your diagram (which would get their L from one of the 3C cables and their N from the other one), I don't think that there are any 'eddy issues', no matter how much current the two lights (or banks of lights) are drawing. Don't you agree?

Kind Regards, John

 

[securespark]

Pretty much. But, and excuse the pun, I'd bank on there being loads off to the left, I'm sure of it. Otherwise, why go to the trouble of designing it that way?
But even if there were loads to the left, there wouldn't be eddy probs (excuse another pun) as long as the two 3 cores run through the same knockouts.