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

[Rob Taylor]

Looking into a site today, just changing an isolator, i found multiple motors are supplied via a 27 core SWA cable.
The motors are only 1-3kw and are supplied as follows...
Main panel ---(27core SWA)----JB----(7core SWA)----Motor
The 7 core includes an isolator signal for control purposes.

I was just wondering how you would go about cable sizing, this was done in 27 x 2.5mm.
Is there anywhere that you can find out 27core correction factor for SWA, or is it a contact supplier job.

Some of the wires will pull minimal amps and others for phase etc will obviously have a significant load, but just wondered how people would work out there cable size.

I've been shadowing a sparks for a couple of years, done a sh*t, waste of time, domestic course, got my 2394 & 2395, although I still need LOTS more experience in periodics to carry them out on my own!!
Never come across this before and chap I'm with wasn't really interested.

New to the site, so sorry if this is in the wrong group etc!

 

[davesparks]

The manufacturers will give you a current rating based on a 2 core cable, you then have to allow for grouping factor the same way you would if using singles in conduit for the same circuits.

 

[Rob Taylor]

OK, thanks for quick response, just struggling to find Cg for 27 cores

 

[D Skelton]

If the current is less than a third of 100% of rated current then grouping factors won't apply. Also, the grouping factors stated in the bgb only apply to cables pulling full whack constantly. if the loads are off and on, or some are staggered, this also needs to be taken into account. I'd be surprised if you'd need to apply a grouping factor at all, and even if you did, it would most likely be minimal.

 

[Darkwood]

My first thought is how are the motors individually overload protected or are we not savvy to that info yet?

 

[D Skelton]

My first thought is how are the motors individually overload protected or are we not savvy to that info yet?

You know the drill by now mate! lol.

"I've got a motor, what size cable do I need?"

 

[Engineer54]

I don't think i've ever seen a 27 / 2.5 core SWA cable, must be a hell of a size. lol!!

It's surprising how many don't fully grasp or understand how to use the grouping factor table in BS7671......

 

[jon6oy]

Id say your looking at this all wrong. When applying grouping factors, you look at number of circuits or multicore cables. Assuming the maximum number you can have on 27 cores is 6 circuits and treating them as being bunched and enclosed a grouping factor of 0.57 would be used.

That's my take anyways

 

[davesparks]

I don't think i've ever seen a 27 / 2.5 core SWA cable, must be a hell of a size. lol!!

It's surprising how many don't fully grasp or understand how to use the grouping factor table in BS7671......

It is quite big yes, and very very boring when someone buys the cheap stuff where all the numbers are illegible/rub off when you strip it!

I use a lot of 12 and 19 core with the occasional bit of 27 core doing stage lighting installs. Usually when some other --- designs the install as most of the ones I design are steel conduit/trunking installs.

The most annoying thing is the bunch I have been working for recently insist on using 19core 2.5 to carry 6x 10A dimmer circuits, absoloutley ridiculous when 12core 1.5 would be fine!

 

[Rob Taylor]

If the current is less than a third of 100% of rated current then grouping factors won't apply. Also, the grouping factors stated in the bgb only apply to cables pulling full whack constantly. if the loads are off and on, or some are staggered, this also needs to be taken into account. I'd be surprised if you'd need to apply a grouping factor at all, and even if you did, it would most likely be minimal.

Its a processing plant roughly 40T per hour, I think, that runs for 8hrs a day. So they are constant loads. Upon start up the current is obviously higher, but reduces when less torque is required. The plant is designed to start from END to START of plant, to avoid build up of material, so the multicore circuits won't all be at max load at the same time.
Cheers

 

[Rob Taylor]

My first thought is how are the motors individually overload protected or are we not savvy to that info yet?

All motors are protected by thermal overloads suited to motor and controlled by contactors

 

[Engineer54]

Id say your looking at this all wrong. When applying grouping factors, you look at number of circuits or multicore cables. Assuming the maximum number you can have on 27 cores is 6 circuits and treating them as being bunched and enclosed a grouping factor of 0.57 would be used.

That's my take anyways

Ah, ...but would it??

 

[D Skelton]

Ah, ...but would it??

Not in a million years would be the answer

 

[Knobhead]

I don't think i've ever seen a 27 / 2.5 core SWA cable, must be a hell of a size. lol!!

It's surprising how many don't fully grasp or understand how to use the grouping factor table in BS7671......

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.

 

[jon6oy]

Ah, ...but would it??

What do you propose as an alternative?

 

[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.