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Posted By Topic: Common sub main

Nickg
Jun 06 2018 20:56

An electrician friend recently came across a job he was telling me about, where a 300mm2 4 core + earth sub main (individual XLPE conductors) which originated at the MSB and was protected by a 350 amp MCCB. It was a commercial site with 6 or 7 individual tenancies, each with there own DB.

He was unable to find where each individual tenancy was isolated from. However, he eventually discovered they were each in fact tapped off the 300mm2 sub main, which appeared to be a common sub main for all the tenants.

The method in which this was done, was by means of using line taps. So a 16mm2 4Core NS was line tapped directly onto the 300mm2 for each tenancy. The 16mm2 was then run down to the tenancy DBs, each which had a 40amp circuit breaker as a main switch.

The whole arrangement sounded quite odd, and certainly not something I\'ve ever heard of, or come across before. He showed me a photo, and it literally looked like the kind of thing you see on O/H lines, where a 25mm2 4 core taps off the main lines to feed a boundary box. Except it was in an installation, and concealed within a wall :/

I\'m thinking as far as compliance goes there may be some issues. There is no electrical protection between where the 16mm2 taps off the 300mm2 cable, it is only relying on the 350 amp MCCB at the MSB, which in a short circuit would end up isolating all tenancies. Is a 300 amp MCCB even acceptable as providing short circuit protection for a 16mm2 cable? Is this info in AS/NZS3008? Also the line tap joints are not done in any sort of enclosure, so there is access to basic insulation inside the wall cavity? Can line taps even be used for connections like this? I would\'ve thought line taps would need to be in an enclosure if they\'re going to be used in an arrangement like this?

The whole arrangement seems pretty irregular, or just plain wrong. I would\'ve thought if you were going to have an arrangement with a common sub main like this, for each tenancy, you would need to make the sub main go through some sort of accessible \'through box\' enclosure with insulated stand offs, which you could T off at to feed a DB via some break down fuses.
   

Nickg
Jun 06 2018 21:14

I\'ve noticed AS/NZS3000:3.10, seems to suggest insulated unsheathed conductors are permissible in wall cavities, but not roof spaces. I\'m thinking of the likes of a flush plate with a socket or light switch fitted to it, where the basic insulation is accessible in the wall cavity.

I would of thought a 300mm2 cable on a 350amp breaker would really be pushing it a bit though?
   

pluto
Jun 07 2018 09:18

Nickg Jun 06 2018 20:56
Your comment 1
An electrician friend recently came across a job he was telling me about, where a 300mm2 4 core + earth sub main (individual XLPE conductors) which originated at the MSB and was protected by a 350 amp MCCB. It was a commercial site with 6 or 7 individual tenancies, each with there own DB.

He was unable to find where each individual tenancy was isolated from. However, he eventually discovered they were each in fact tapped off the 300mm2 sub main, which appeared to be a common sub main for all the tenants.

My comment 1
It is using the rising main provisons used in high rise buildings, excepting that is in a horizontal plane.

Your comment 2
The method in which this was done, was by means of using line taps. So a 16mm2 4Core NS was line tapped directly onto the 300mm2 for each tenancy. The 16mm2 was then run down to the tenancy DBs, each which had a 40amp circuit breaker as a main switch.

The whole arrangement sounded quite odd, and certainly not something I\'ve ever heard of, or come across before. He showed me a photo, and it literally looked like the kind of thing you see on O/H lines, where a 25mm2 4 core taps off the main lines to feed a boundary box. Except it was in an installation, and concealed within a wall :/

My comment 2
As described there appears to be primary insulation not screened from touch and this would not comply with ESR 2010 or AS/NZS 3000 requirements.

Your comment 3
I\'m thinking as far as compliance goes there may be some issues. There is no electrical protection between where the 16mm2 taps off the 300mm2 cable, it is only relying on the 350 amp MCCB at the MSB, which in a short circuit would end up isolating all tenancies. Is a 300 amp MCCB even acceptable as providing short circuit protection for a 16mm2 cable?
My comments 3
On a table of the current rating of 300 mm2 CU single XPLE I looked at the current rating is 847 A but the actual approved rating will depend seperation between cores, thermal cooling available for the cable, thermal insulation. etc, The use of a 350 A MCCB will protect the cable from overcurrent.

Your comment 4
Is this info in AS/NZS3008?

My comment 4
AS/NZS 3008.1.2 has the tables required including the current de-rating for seperation between cores, thermal cooling available for the cable, thermal insulation. etc.

Your comment 5
Also the line tap joints are not done in any sort of enclosure, so there is access to basic insulation inside the wall cavity?

Can line taps even be used for connections like this? I would\'ve thought line taps would need to be in an enclosure if they\'re going to be used in an arrangement like this?

My comment 5
See my previous comment on exposed basic insulation. If live metal parts (line taps)the same provision applies.


Your comment 6
The whole arrangement seems pretty irregular, or just plain wrong. I would\'ve thought if you were going to have an arrangement with a common sub main like this, for each tenancy, you would need to make the sub main go through some sort of accessible \'through box\' enclosure with insulated stand offs, which you could T off at to feed a DB via some break down fuses.

My comment 6

The arrngement you describe is covered in AS/NZS 3000 clause 2.5.3.3. (b) and figure 2.4

The current edition of AS/NZS 3000 does not describe the rising main system, this will corrected when as/nzs 3000:2018 edition is cited for use in NZ.

Some other factors
4/16 mm2 + N/S current rating for 1 second is 1.76 kA and thus is able to be protected for Short circuit currents by the 350 A MCCB at the supply point to the 300 mm2 CU cable. The 40 A MCB in each switchboard would provide overcurrent protection for th 4/16 mm2 + N/S cable.

Nickg Jun 06 2018 21:14

Your comment 7

I\'ve noticed AS/NZS3000:3.10, seems to suggest insulated unsheathed conductors are permissible in wall cavities, but not roof spaces. I\'m thinking of the likes of a flush plate with a socket or light switch fitted to it, where the basic insulation is accessible in the wall cavity.

I would of thought a 300mm2 cable on a 350amp breaker would really be pushing it a bit though?

My comment 7
If the rear of switch or socket outlet is accessible thru the wall cavity the rear of the fitting live parts have to be screened from touch. A true enclosoed flush box may be able to do that, but it precludes the use of mounting ring type fixings for fittings.

As regards the use of 300mm2 XPLE cable on 350 A MCCB, see my above comments

   

Nickg
Jun 07 2018 14:53

Okay. With regards to insulated/unsheathed conductors within wall cavities. I\'m referring to AS/NZS3000 3.10.1.1 exception (b). I\'m finding it a bit difficult to fully understand this clause.

\'...in an enclosed wall cavity between an accessory and a wiring enclosure or sheathing terminated within 100mm of the hole or within which the accessory is mounted...\'

With this clause in mind, my thoughts are, conductors in wall cavities where connections to \'accessories\' are not involved, such as in the example mentioned where cables are just connected/joined via line taps. The presence of only primary insulation would not be acceptable.

Does this sound right? This clause in 3000 doesn\'t seem to be very well worded.
   

pluto
Jun 07 2018 16:10

Nickg Jun 07 2018 14:53
Your comment 1

1Okay. With regards to insulated/unsheathed conductors within wall cavities. I\'m referring to AS/NZS3000 3.10.1.1 exception (b). I\'m finding it a bit difficult to fully understand this clause.

\'...in an enclosed wall cavity between an accessory and a wiring enclosure or sheathing terminated within 100mm of the hole or within which the accessory is mounted...\'

My comments 1
The exception (b) is intended to cover that primary insulation near fittings is keep to a minimum. It is necessary to ensure that excessive TPS sheath removal does not take place near fittings.

You also should read clause 3.10.1.2

Your comment 2
With this clause in mind, my thoughts are, conductors in wall cavities where connections to \'accessories\' are not involved, such as in the example mentioned where cables are just connected/joined via line taps. The presence of only primary insulation would not be acceptable.

My comment 2
That is correct, if you read the first two lines of 3000 clause 3.10.1.1 it covers the position in your comment 1 above.

Your comment 3
Does this sound right? This clause in 3000 doesn\'t seem to be very well worded.

My comment 3
the new AS/NZS 3000:2018 when it is cited in NZ at sometime in the future is using the identical wording to the current edition of as/nzs 3000.

   

ShaneR
Jun 07 2018 18:57

\"........ each which had a 40amp circuit breaker as a main switch.......\"



\"....There is no electrical protection between where the 16mm2 taps off the 300mm2 cable,......\"


The 40amp circuit breaker will protect the 16mm cable upstream (and downstream) from over current.

2.5.1.1 (b)






   

Sarmajor
Jun 07 2018 19:06

I suspect that the cable will be XLPE/PVC construction as this was commonly used in rising main systems when I worked in construction.

As long as the joints are insulated and enclosed in something that requires a tool to gain access to it then there is no issue.

I suspect that the connections will not be line taps but rather specialised crimp connectors.
There were several types that were either a C type crimp where the tapped on connector came out parallelish with the main conductor.
There was also a T type crimp where the tapped on connector came out at 90 degrees to the main conductor.

   

ShaneR
Jun 08 2018 13:51

\"4/16 mm2 + N/S current rating for 1 second is 1.76 kA and thus is able to be protected for Short circuit currents by the 350 A MCCB\"


How does one find this information?

I couldn\'t find it from the cable supplier?
https://www.firstflex.co.nz/product/mcx0416/
   

pluto
Jun 08 2018 14:16

Olex Cables NZ (now Nexans NZ) in the back of their technical manual available on the website.
   

ShaneR
Jun 08 2018 14:59

This book?


Still can\'t find it?


https://goo.gl/xLi7fG
   

pluto
Jun 08 2018 16:28

I have an old copy of the Olex technical manual it has the table with 1 second current ratings in it. At a quick look, it does not appear to be in the newer editions of the Olex NZ technical handbook.


   

pluto
Jun 08 2018 16:34

Use the web address below for 2012 edition and in section 2 page 13 is the 1 second current rating table you require.
https://www.nexans.co.nz/NewZealand/2013/Power%20Cable%20Catalogue%20Full%20version%202012.pdf
   

mf51to1
Jun 08 2018 18:10

To the OP, was there no space in the MSB for induvidual MCBs? And what is the length of the “sub main”?
   

Nickg
Jun 08 2018 18:56

I\'m not sure about whether there was space at the MSB for additional submain circuits or not. I haven\'t visited the site, I\'ll hopefully get to go have a look at some stage, to get a clearer idea of the full arrangement.

Answering a previous comment. No enclosures were installed at the connection points, where the 16mm2 tapped onto the 300mm2. So primary insulation was exposed within wall cavities and roof spaces.

I understand the general arrangement of the installation complied, so far as to where the short circuit and overload protection were located. However, it doesn\'t seem to me like the best arrangement in this case. As it is supplying several commercial tenancies. In order to isolate an individual DB, where maintenance etc may be involved, all DBs would need to be isolated to work safely. I think break down fuses or separate submain for each DB would\'ve been a far better arrangement.

It sounds a bit cheap and nasty, and too much cost cutting.