Multiple 3 phase motors fed via a JB supplied with a 27 core SWA

[D Skelton]

What do you propose as an alternative?

A grouping factor not too far off 1 if not 1 itself.

I think you need to read appendix 4 a little closer, then you'll see for yourself what he might be proposing as an alternative.

 

[stevethesparks]

Putting multiple power circuits in the same cable would get you hung, drawn and quartered.

That was my thought too.

 

[shanky887614]

12, 19 and 27 core 2.5 SWA was the standard size for our plant control cables. Many would be over ½ mile in length so something with a bit of meat was needed. As Dave said, it gets boring when you’ve any number to make off.

Putting multiple power circuits in the same cable would get you hung, drawn and quartered.

before people think its a bit small usually .75 is enough for controls for valves etc

 

[jon6oy]

With regards to multiple circuits, reg 521.7 does permit as the voltage across the circuits are the same.

Reg 314 is met as the circuits are individually fused and are electrically separate.

Using table 4B5 to de-rate the cable for multiple conductors i still can't see why this is a problem. It's not particularly good practice, I certainly wouldn't do it myself, but I am struggling to find any evidence to suggest this contravenes regulations.

Can I have some more cryptic responses please? Useful answers would be preferred. :smiley2:

 

[D Skelton]

No one said applying a grouping factor would contravene regulations, only that it probably wouldn't be necessary. Read appendix 4 more thoroughly, especially the section that applies to Cg.

 

[jon6oy]

Ok, agreed, the grouping factor possibly wouldn't be required. But it's not a bad thing to err on the side of caution, that's what I've always been told.

Thanks for letting me stretch my brain, been a while!

 

[D Skelton]

Ok, agreed, the grouping factor possibly wouldn't be required. But it's not a bad thing to err on the side of caution, that's what I've always been told.

Thanks for letting me stretch my brain, been a while!

Or you could just design it properly providing the client with exactly what is required, no more, no less, and saving them money in the meantime.

 

[Knobhead]

before people think its a bit small usually .75 is enough for controls for valves etc

I wish. Before the advent of PLC’s with outstations some of the individual circuits were of ridiculous length, fault finding kept you fit if nothing else.

 

[shanky887614]

I wish. Before the advent of PLC’s with outstations some of the individual circuits were of ridiculous length, fault finding kept you fit if nothing else.

i agree with that, we have generally been lucky that our panels are on the same floor and the longest we have really had to do was around a 300m run

 

[jon6oy]

Or you could just design it properly providing the client with exactly what is required, no more, no less, and saving them money in the meantime.

But this wasn't a design query, it was a validation question was it not?

 

[D Skelton]

But this wasn't a design query, it was a validation question was it not?

Originally by the OP yes, but you started talking about erring on the side of caution when applying grouping factors, which is normally done by an inexperienced electrician at the design stage.

If validation is all the client is after, again, erring on the side of caution could be tantamount to giving them the wrong advice, such as telling them that a suitably sized cable isn't suitably sized.

Either way, there is a time and a place for erring on the side of caution, and applying grouping factors is often not one of those times or places. Do it right.

 

[LeeH]

Putting multiple power circuits in the same cable would get you hung, drawn and quartered.

Why was that Tony?