Meter Tails for new consumer unit

[john gross]

Hi everyone

This is my first thread on the forum. Iv'e been reading others and there's some good info out there. I've been installing solar panels for last six months and on many an occasion there is either no spare way in the existing consumer unit or I've been unable to match the existing MCB. I've therefore had to install a new 2 way consumer unit,breaking into the existing meter tails and using service connection blocks.My question is "is it really neccessary to use 25mm or even 16mm tails when really all their feeding is a 16amp MCB" ? I would be grateful for any feed back on this.

Thanks. John Gross

 

[malcolmsanford]

No ....................... as the maximum load demand for a domestic PV installation is 16amps then you can quite easily use 4/6mm tails.

As the load is constant often you will find the omission of an overload device if RCD is incorporated, personally I don't like this arrangement but it is within reg 433.3.1 (ii)

Have a look at sections 433.2 and 434.2.1 as well

 

[ncpitts]

Sorry to but in ,but you should only fit a second consumer unit if there is
a)An isolator that will isolate the whole installation or
b) A multiple isolation sticker
to comply with reg 514.11.01

 

[malcolmsanford]

Sorry to but in ,but you should only fit a second consumer unit if there is
a)An isolator that will isolate the whole installation or
b) A multiple isolation sticker
to comply with reg 514.11.01

Apart from the fact that a sticker would only need to be fitted if an installation or an item of equipment or enclosure is connected to more than one supply, so unless the house contained to cut outs that fed the above then it is not required.

Though I see what your saying in part A, the definition of the term "electrical installation" could be that PV system is an installation in it's own right and so an enclosure with a double pole main switch and a protection device would adhere to the regulations

 

[ncpitts]

Yes to part B but I also meant that there should be one isolator that de-energises the whole installation - I think

 

[malcolmsanford]

As I said have a read of the definitions of "electrical installation" and IMO the PV system could be classed as an "electrical installation" in it's own right.

I agree that like all our regs they are open to interpretation in a domestic situation where you have a single cut out then yes the entire installation could be classed as just one, no matter how many CUs are fitted.

But in a commercial or a large industrial complex would you class the LV transformer and LV switch gear as that form of isolation for 20 buildings or would class each building as it's own installation? Most likely you would class each building as an installation in it's own right.

Interpretation is a wonderful tool

 

[john gross]

Thanks for replies.
There would be a sticker on the new consumer unit stating that there are two supplies.
When switched off this would isolate both supplies to and from the inverter. Wouldn't interfere with any other installation.

 

[grandfortune]

25mm is a must on the feeding of new board of say henley link blocks, the tails have to carry the fault current of whatever the main fuse is. I havent got the new green on site guid yet, but im sure it now states min tails 25mm ??.

 

[Markc]

Couldn't agree more.....however!

If the PV system is connected via a standalone CU (in a domestic situation) then a single means of isolation is required according to our inspector on October 21st. We had a N/C for not having this in place on an installation where we installed a small 2 way CU from a 100A DP Block off the meter tails. Wylex now produce a purpose enclosed isolator for this exact reason.

 

[BruceB]

If you are saying what I think you are saying then your inspector is wrong I believe.

From Electrical Safety Council site:

"Q13. If a dwelling has an existing consumer unit with an integral main switch, and an electric shower is connected by means of a second consumer unit with an integral main switch, is another ‘main switch’ required to isolate both consumer units simultaneously?
Regulation 537.1.4 requires a main linked switch or linked circuit-breaker to be provided as near as practicable to the origin of every installation as a means of switching the supply on load and as a means of isolation.
However, it is permitted for a dwelling to have more than one electrical installation. Therefore a further ‘main switch’ is not required to isolate both consumer units.
Regulation 537.2.2.6 requires each device used for isolation to be clearly identified by position or durable marking to indicate the installation it isolates.
Regulation number(s)

• 537.1.4"

Substitute PV for shower and it describes a common arrangement.

Regards
Bruce

 

[kevp]

The tails can be 4 or 6 mm if feeding a PV CU only, as you can "back fuse" up to 3m.

As an example, you could take a spur from a 32a ring circuit in 1.00mm, & have it fused down up to 3m away. Perhaps not a good idea in this example though. So running .5m of tails is not an issue.

You are now supposed to only have 1 main switch though. Unless there is a dual supply entering the building (which does require labelling up). A simple solution to overcome adding an additional switch is to tap off the existing main switch (consumer unit) with a split load kit (busbar take offs). Easier than fitting a DP block as well.

I guess i should mark down dual tariff installations (night storage) if they dont have 1 master switch.

 

[Markc]

If you are saying what I think you are saying then your inspector is wrong I believe.

From Electrical Safety Council site:

"Q13. If a dwelling has an existing consumer unit with an integral main switch, and an electric shower is connected by means of a second consumer unit with an integral main switch, is another ‘main switch’ required to isolate both consumer units simultaneously?
Regulation 537.1.4 requires a main linked switch or linked circuit-breaker to be provided as near as practicable to the origin of every installation as a means of switching the supply on load and as a means of isolation.
However, it is permitted for a dwelling to have more than one electrical installation. Therefore a further ‘main switch’ is not required to isolate both consumer units.
Regulation 537.2.2.6 requires each device used for isolation to be clearly identified by position or durable marking to indicate the installation it isolates.
Regulation number(s)

• 537.1.4"

Substitute PV for shower and it describes a common arrangement.

Regards
Bruce

Arh but the Electrical Saftey Council website is not up to date. The new regs, The Big Green Book was waved under my nose on my inspection in October. My argument was the BGB is NOT enforceable currently and I'm working to the BRB! I've yet to get the new regs but I have lots to check out and this is one point.

 

[BruceB]

The regs in my quote have dots not dashes so are BGB not BRB

 

[TedM]

Err, BRB uses dots.

 

[moggy1968]

25mm is a must on the feeding of new board of say henley link blocks, the tails have to carry the fault current of whatever the main fuse is. I havent got the new green on site guid yet, but im sure it now states min tails 25mm ??.

Not correct according to my assessor. Your fuse will melt long before your 16mm tails (what we always use for a sub board) so the fault current is not an issue. It is the maximum demand (as others have mentioned here) that is the issue.
It is standard practice to wire shower boards or garage sub boards in 16mm tails.