Swa main incomer size

[Mwoods4757]

Hi just after some help. It's a high rise block new install. Dno are wiring up to the Ryefield board then to the meter. Then there will be an isolator and then run to each flat cu. question is what size cable. I've been told 3 core 25 mm swa but this seems a bit excessive for 100a dp main switch in cu. the cu only has 9 ways with two 100a 30 ma rccb protection. 25 mm earth in cu will be hard to terminate too.
Also how would a 3 core terminate in the isolator. Would it not be better to use 16mm 2 core and separate earth?

 

[telectrix]

gland the SWA into an galv steel box directly under the CU. take the L and N into the CU main switch and split the E out of the box into a MET.

 

[davesparks]

You'll need a pin lug to get 25mm in to most CU earth bars

 

[Guest111]

As Tel said,gland into a box then go from there,regarding cable size this depends upon length of run and other factors,who has told you 25mm,is it the people ordering the work? If so it may be something on their spec,Around where I live the council insist on a 10mm for every electric shower installed in their properties,even if it's 7.5Kw on a short run.

 

[davesparks]

Why not just gland straight into the CU? It's what I've always done with SWA.

 

[Guest111]

Why not just gland straight into the CU? It's what I've always done with SWA.

On a metal CU fine but not on a plastic one.

 

[telectrix]

glanding into a plastic CU can be OK as long as there's no strain on the enclosure cause d by the SWA. if dressin in and cleated, should be OK.

 

[davesparks]

Yes, but you wouldn't fit a plastic CU when you know you've got an SWA to terminate would you?

 

[Mwoods4757]

Yes the people ordering the work said 25mm 3 core. How ever I think that's a general spec rather than specific which we are struggling to get hold off.
Also if it's a 3 core and terminate the cpc in the isolator once this is isolated there would be no earth at the cu. would you install spectate met at isolator end too?

 

[Guest111]

Yes, but you wouldn't fit a plastic CU when you know you've got an SWA to terminate would you?

These days Dave nothing would suprise me.

 

[Mwoods4757]

I actually think they are plastic in the spec. Yes the people ordering have said 25mm how ever this is a general spec we are struggling to get specific at the moment. Also why use a 3 core if the earthing system provided is SNE.

 

[davesparks]

3 core because the armour almost certainly won't meet the requirements for a main bond

 

[Engineer54]

3 core because the armour almost certainly meet the requirements for a main bond

Think you meant to say ''Won't'' meet the requirements for main bonding!! lol!!

 

[davesparks]

Think you meant to say ''Won't'' meet the requirements for main bonding!! lol!!

Quite right, I'll just make a quick edit and nobody will notice

 

[Engineer54]

I actually think they are plastic in the spec. Yes the people ordering have said 25mm how ever this is a general spec we are struggling to get specific at the moment. Also why use a 3 core if the earthing system provided is SNE.

Anything in the general contract specifications must be complied with!! You or you're company have priced the work to include the 25mm 3 core SWA, so any change will require a change order being raised and will without doubt include adjustments to the price of the job. In this case a reduction against you're companies contract account...

 

[E.E.I]

I wouldn't terminate the cpc through the isolator switch, make sure the isolator has a separate earth terminal/earth bar. Having a separate MET would look messy IMO.

 

[Engineer54]

Yes the people ordering the work said 25mm 3 core. How ever I think that's a general spec rather than specific which we are struggling to get hold off.
Also if it's a 3 core and terminate the cpc in the isolator once this is isolated there would be no earth at the cu. would you install spectate met at isolator end too?

Are you actually being serious here??

You follow the Spec that was issued as part of the contract details the job was priced against, any retrospect amendments to that spec (eg, any so-called specific spec) then both contractual parties would need to agree on any changes made, along with any increase/decrease in the contract price...

 

[Knobhead]

Personally I’d let him go ahead and change the spec. I used to get a perverse pleasure out of telling a contractor to rip it out and do it again at their expense.

 

[shanky887614]

Yes the people ordering the work said 25mm 3 core. How ever I think that's a general spec rather than specific which we are struggling to get hold off.
Also if it's a 3 core and terminate the cpc in the isolator once this is isolated there would be no earth at the cu. would you install spectate met at isolator end too?

isolators have an earth+neutrel terminal mate that isnt isolated by isolator

 

[davesparks]

Not always, a Double pole or a TPSN/four pole isolator isolates the neutral.
And most plug and socket arrangements used for isolation will isolate the earth.

 

[shanky887614]

Not always, a Double pole or a TPSN/four pole isolator isolates the neutral.
And most plug and socket arrangements used for isolation will isolate the earth.

our 4pole isolators early make, late break

have a seperate earth and n connection as well mate.

buy a different brand then mate

 

[davesparks]

our 4pole isolators early make, late break

have a seperate earth and n connection as well mate.

buy a different brand then mate

Why would you need a seperate N in a four pole isolator? That's what the fourth pole is there for!

And I am quite happy with the brand if isolator I use, MEM are tried and tested.

 

[shanky887614]

Why would you need a seperate N in a four pole isolator? That's what the fourth pole is there for!

And I am quite happy with the brand if isolator I use, MEM are tried and tested.

really? we have controls going through our isolators as well sometimes depending on spec

 

[davesparks]

Having controls through a pole of an isolator sounds quite unusual.
Id expect to see control circuits use auxiliary contacts, but then I guess it depends on the equipment and budget.

 

[Knobhead]

For Christ’s sake!

You use 3P+N or SP+N never 4P for distribution!

 

[Knobhead]

It would be great if people learnt the difference between 3P+N and 4P. As Shanky rightly said in a 3P+N the neutral is early make late break. The reason, because you will get voltage imbalance akin to a floating neutral.




I’ve only ever specified four pole ACB/MCCB’s twice in 40 years.

Both breakers were for a 2.6MW 660V two wire earth free system. Four pole because I put two poles in series to handle the fault current. If you look at the manufacturer’s maximum fault current tables for 400V against 690V it drops dramatically!

 

[Engineer54]

It would be great if people learnt the difference between 3P+N and 4P. As Shanky rightly said in a 3P+N the neutral is early make late break. The reason, because you will get voltage imbalance akin to a floating neutral.
I’ve only ever specified four pole ACB/MCCB’s twice in 40 years.

Both breakers were for a 2.6MW 660V two wire earth free system. Four pole because I put two poles in series to handle the fault current. If you look at the manufacturer’s maximum fault current tables for 400V against 690V it drops dramatically!

Yep, all our 2.5-3.2 KA main switchboard TX ACB incomers and the bus couplings are 3 Pole + N with the early make late break function.

 

[davesparks]

For Christ’s sake!

You use 3P+N or SP+N never 4P for distribution!

Sorry Tony, I'm mixing manufacturers terminology with the correct terminology.