New mains spurs

[paulski]

I want to add 2 new mains spurs (not rings). My plan is to add a new CU which will have a 100A incomer and 2 32A RCBOs. My question is how do I feed the new CU? Obviously I can't hook up to the meter but I would like to use 25mm tails to the new CU. Can I legitamately take a feed from the existing CU and thereby ensure that the MCB in that unit will still disconnect all circuits. The purpose of these new spurs is solely to supply a hi-fi system.

Also the existing earth connection to the meter looks to be fairly insubstantial compared to the tails, is there any reason why I shouldn't uprate this?

Finally (!) I'd like to uprate the supply fuse to 100A (it's currently 60A / 80A) - any idea what it will cost to get the local rec to do this?[/b]

 

[andy]

I want to add 2 new mains spurs (not rings). My plan is to add a new CU which will have a 100A incomer and 2 32A RCBOs. My question is how do I feed the new CU? Obviously I can't hook up to the meter but I would like to use 25mm tails to the new CU. Can I legitamately take a feed from the existing CU and thereby ensure that the MCB in that unit will still disconnect all circuits. The purpose of these new spurs is solely to supply a hi-fi system.

Also the existing earth connection to the meter looks to be fairly insubstantial compared to the tails, is there any reason why I shouldn't uprate this?

Finally (!) I'd like to uprate the supply fuse to 100A (it's currently 60A / 80A) - any idea what it will cost to get the local rec to do this?[/b]

1: your installing a RADIAL. a spur is a socket taken form a ring main.

2: why 25mm²? you could use 10mm² from existing CU with a 45/50A MCB

3: are you aware that youll have to use 4mm², possibly 6, to wire the radial?

4: dont bother tryin to upgrade the fuse. if it blows then chances are theyll change it for a 100A if the existing mains cable can take it

5: for the earth, it depends what earthing arrangments you have. do you have TT,TNS, TNCS? (see for reference)what size is the existing earth?

 

[paulski]

1: your installing a RADIAL. a spur is a socket taken form a ring main.

2: why 25mm²? you could use 10mm² from existing CU with a 45/50A MCB

3: are you aware that youll have to use 4mm², possibly 6, to wire the radial?

4: dont bother tryin to upgrade the fuse. if it blows then chances are theyll change it for a 100A if the existing mains cable can take it

5: for the earth, it depends what earthing arrangments you have. do you have TT,TNS, TNCS? (see for reference)what size is the existing earth?

1. My mistake

2. I'm keen to use the largest practically possible feed cable to reduce limitations on the 'quality' of the incoming mains. I realise I'm at risk of sparking a debate about how the mains does or doesn't (believe me it does!) affect sound quality. I suppose that if I'm taking this from the existing CU the run will be very short and unlikely to have a negative impact.

3. I'm going to use 10mm² t&e for the radials - I know it'll be a pain to work with but it's worth the effort for the end result. On this point am I right in assuming using an overrated cable won't contravene BS 7671?

4. Noted

5. Earth is TNC-S (PME), the earth looks to be no bigger than 6mm²

 

[JohnD]

My question is how do I feed the new CU? Obviously I can't hook up to the meter

Yes you can but you'll need the electricity supplier to connect it as the tails will be hot. You will need a new block to split the tails from the meter into a set of tails to the old CU and a set going to the new.

You also need to have a 2pole 100A switch disconnector immediately after the meter and before the block. This will become the Main Switch for the installation. It can be very convenient to have a main switch as this means your CUs and their tails will not be hot (if you remember to switch it off).

Also get an 8-way earthing block and run the main earths to the CUs in 16mm. And buy the on-site guide or borrow it from the library.


Edited to ask, what size theatre are you working in that needs 64A for the sound system? I work in a 160-seat regional theatre that uses less than a third of that. The 200w speakers are the size of refrigerators.

 

[BJS_Spark]

I'm keen to use the largest practically possible feed cable to reduce limitations on the 'quality' of the incoming mains. I realise I'm at risk of sparking a debate about how the mains does or doesn't (believe me it does!) affect sound quality

Debate started! The quality of the incoming mains may well have an impact on sound quality of your Hi-fi setup as you say but that quality can not be affected by using 25mm² tails instead of 10mm² T+E as a supply. This cable choice will affect the current handling and volt drop but not a problem unless you are running the kind of rig to power a small gig in your living room!!!
It might be worth thinking more about how you can really affect 'or improve' the quality of this mains supply with the use of mains filtering/conditioning. Routing of cables and shielding may also be worth considering.

 

[JohnD]

So that'll be shielded cables then (SWA is self-shielded); power cleaner on the fridge and the CH pump; no thermostatically-controlled electric heaters allowed in the house; a steel cabinet or cage by the front door to put all mobile phones in; brick up the windows in the listening room to prevent noise from singing birds or traffic... what else can we think of?

 

[andy]

if there is an earth terminal you can get to on the service head, you can upgrade the earth to 16mm to CU and 10mm to water and other services

10mm for sockets seems way OTT. youll probably have difficulty getting it into the socket. just make sure you use a 47mm backbox. using over-rated cable isnt a problem. its under-rated cable that is

 

[kai]

if there is an earth terminal you can get to on the service head, you can upgrade the earth to 16mm to CU and 10mm to water and other services

10mm for sockets seems way OTT. youll probably have difficulty getting it into the socket. just make sure you use a 47mm backbox. using over-rated cable isnt a problem. its under-rated cable that is

Over rated cable has a BIG adavntage, if you earth is connected to the main Service Fuse Block, the earth fault impedance will be really low indeed - making sure the fuse operates really quickly in the event of an earth fault, well below the 0.4.second threshold ...

 

[pdcelec]

But ensure that the earth fault loop impedance is not too low as to cause the prospective short circuit current to be too high for the protective device

 

[plugwash]

in the unlikely event of a mcb failing to open due to high short cuircuit current the service fuse should provide disconnection.

 

[HDRW]

But ensure that the earth fault loop impedance is not too low as to cause the prospective short circuit current to be too high for the protective device

How likely is this? MCBs these days all seem to have a breaking capacity of 6,000A, which at 253V (worst case, 230V+10%) means an EFLI of 42milliohms (0.042ohms) which is really tiny (in my house it's five times this at the CU incomer, let alone after the MCBs).

Is this even possible in a domestic installation?

Cheers,

Howard

 

[JohnD]

I was thinking that even if you had enormous copper busbars running from your CU, of negligible resistance, then the prospective short-circuit current could be no worse than if you poked a screwdriver into the CU and shorted the connections. Presumably the MCBs are designed for such a possibility (though they need not be in re-usable condition after breaking it).