double up on swa submain

[pete555]

recently had a test with 2 cables being used for sub main

1st is a 10mm swa 2 core with armourings for earth

2nd is a 6mm swa 3 core with armour and integral earth

both cables doubled up on MCCB 63 a breaker
and both terminated in DB

what code

i think 2

ive told them its grim and needs 1 cable to go but not sure if i should have coded it a 1

 

[yellowvanman]

What reg is applicable?

Not saying its right or wrong but you may need to read Appendix 10

 

[Silly Sausage]

I take it you mean they're in parallel?
If so, it'll contravene the parallel cables reg.
E54's your man for this one.

 

[Knobhead]

All I'll say is one of the cables has to go.

I’ve run many parallel feeds, it’s easier. Both cables of the same length and construction.

 

[Engineer54]

recently had a test with 2 cables being used for sub main

1st is a 10mm swa 2 core with armourings for earth

2nd is a 6mm swa 3 core with armour and integral earth

both cables doubled up on MCCB 63 a breaker
and both terminated in DB

what code

i think 2

ive told them its grim and needs 1 cable to go but not sure if i should have coded it a 1

There is absolutely nothing wrong with parallel supply cabling, in fact they can give many advantages over the use of a single larger cable. So if correctly installed, it's certainly not Grim!!!

This set-up however, contravenes at least two of the parallel cable rules, one being that both cables must be of the same CSA, and the other, must be of the same construction!! Also they should physically follow the same route (same length)...

 

[loop3]

54, you know your ****. Cables in parallel must be of the same length construction and cross sectional area. And to ensure some of the above - run the same path. And just to reiterate for swa. You can use the armour as the cpc as long as you calculate it to be worthy!

 

[yellowvanman]

There is absolutely nothing wrong with parallel supply cabling, in fact they can give many advantages over the use of a single larger cable. So if correctly installed, it's certainly not Grim!!!

This set-up however, contravenes at least two of the parallel cable rules, one being that both cables must be of the same CSA, and the other, must be of the same construction!! Also they should physically follow the same route (same length)...

Not wanting to disagree as I've not done this myself but in Appendix 10 (BRB - maybe a different appendix number in BGB) does say:

"Where the difference in impedance between parallel conductors causes unequal current sharing, for example greater than 10% difference, the design current and requirements for overload protection each conductor should be considered individually"

The maths for this looks a bit complicated but its certainly not saying its prohibited. In practise because of the maths it maybe a bit more difficult, but if a 'serious' observation (potential high cost to rectify) is going to be made on a EICR then the author may need to back up his recommendation.

 

[Silly Sausage]

Eeeee...I like a challenge!
It's an area I'm not, and ever likely to be, involved in, but stuck inside tonight with kids (she's off on the razz)...

 

[Engineer54]

Not wanting to disagree as I've not done this myself but in Appendix 10 (BRB - maybe a different appendix number in BGB) does say:

"Where the difference in impedance between parallel conductors causes unequal current sharing, for example greater than 10% difference, the design current and requirements for overload protection each conductor should be considered individually"

The maths for this looks a bit complicated but its certainly not saying its prohibited. In practise because of the maths it maybe a bit more difficult, but if a 'serious' observation (potential high cost to rectify) is going to be made on a EICR then the author may need to back up his recommendation.

Why do you think the rules for installing parallel cables stipulate all cables MUST be of the same CSA, MUST be of the same overall LENGTH, and MUST be of the same construction. Complying with these conditions, will totally negate the above paragraph you posted!! It's ''Standard Practice'' in both industrial and larger commercial installations...

I would guess that over 90% of all the main distribution supplies on this project is by multiple Parallel cabling (not just 2 cables in parallel). I would like your suggestion how you would supply, say a 3200A main switchboard from a distribution transformer using single core cables, or if you like SWA cables?? ...lol!!

 

[Darkwood]

I run parallel cable sets all the time and as already pointed out its a great way to cut costs and make install easier but as for comments i think all that needs to be said has already been...

 

[Engineer54]

54, you know your ****. Cables in parallel must be of the same length construction and cross sectional area. And to ensure some of the above - run the same path. And just to reiterate for swa. You can use the armour as the cpc as long as you calculate it to be worthy!

Most of the larger SWA cables, steel wire CSA will not meet those requirements.

One of the biggest advantages of running a parallel supply is that you can use much smaller CSA cables to supply loads that would require a relatively much bigger cable if being supplied with a single cable.

 

[i=p/u]

so 5x10mm or 1x50mm...... comon

 

[yellowvanman]

Why do you think the rules for installing parallel cables stipulate all cables MUST be of the same CSA, MUST be of the same overall LENGTH, and MUST be of the same construction. Complying with these conditions, will totally negate the above paragraph you posted!! It's ''Standard Practice'' in both industrial and larger commercial installations...

Where are those rules as they are not in BS7671 as far as I'm aware? I agree its good practise to make sure all things are equal, but in this case they are not and the BGB basically says thats Ok with special consideration! Hence the guy doing the EICR needs to be sure of his facts if he is going to recommend additional work.

I would guess that over 90% of all the main distribution supplies on this project is by multiple Parallel cabling (not just 2 cables in parallel). I would like your suggestion how you would supply, say a 3200A main switchboard from a distribution transformer using single core cables, or if you like SWA cables?? ...lol!!

What's that got to do with this thread?

 

[Engineer54]

Where are those rules as they are not in BS7671 as far as I'm aware? I agree its good practise to make sure all things are equal, but in this case they are not and the BGB basically says thats Ok with special consideration! Hence the guy doing the EICR needs to be sure of his facts if he is going to recommend additional work.




What's that got to do with this thread?

You obviously totally rely on BS7671 for all the answers, and if it's not in there, then something has just got to be wrong!! lol!! Well i have some devastating news for you, BS 7671 (and it's OSG's) is/are only a guide it is not and never has been the definitive bible you seem to think it is. Broadly speaking it's scope is also somewhat limited and would be almost impossible to cover everything. It also assumes, that the reader has a broad understanding of electrical technology.

Wrong the guy undertaking an ECIR needs to be experienced, knowledgeable and competent enough to be able to conduct and assess the installation he is inspecting/testing and be capable of advising on any remedial work that may be necessary, without having to always confer with a guide book of Reg's!!

What has my last paragraph got to do with this thread?? Well if you think about it for a second or two, ...what single cable size would be capable of supplying 3200A?? NONE, this is/was a perfect example to show you that parallel cabling is the only way that it would be possible to interconnect between the source and the main switchboard... Yes there are other methods, but we are talking about cabling solutions...

 

[Simlec]

Where are those rules as they are not in BS7671 as far as I'm aware? I agree its good practise to make sure all things are equal, but in this case they are not and the BGB basically says thats Ok with special consideration! Hence the guy doing the EICR needs to be sure of his facts if he is going to recommend additional work.

Is that not 523.7 -(i)