Size of meter tails

[Ray Tay]

I was reading that if a 100A main fuse is fitted, 25mm meter tails have to be used from the meter to the consumer unit, even though 16mm tails are suitable for the total house load. Is that so?

If that is the case then what would be suitable in this scenario. A plastic box is fitted with a RCD/incomer switch with 25mm tails to the box. Then 16mm cable from the RCD box to the consumer unit, which does not have an RCD. I see the consumer unit split into two sections, the main unit and the box with the RCD in it, some 25mm cable only to the RCD is fine. Am I right?

What do you all think?

EDIT:
The reason why I ask is that the existing consumer unit has 16mm tails and is on the other side of the wall to the electric meter at high level. The 16mm tails from the meter run through the wall and up. An electrician was called in the fit an RCD and gave a ridiculous price for a new consumer unit with a RCD inside and 25mm tails back to the meter ripping out part of the wall.

This was overkill. I saw that an RCD/mains incomer can be fitted near the meter with 25mm tails to the RCD box and then connect onto the existing 16mm tails in the wall to the existing consumer unit.

 

[EFLImpudence]

I was reading that if a 100A main fuse is fitted, 25mm meter tails have to be used from the meter to the consumer unit, even though 16mm tails are suitable for the total house load. Is that so?

Yes, the DNO are allowed to stuff more electric into their cables than are we.

If that is the case then what would be suitable in this scenario. A plastic box is fitted with a RCD/incomer switch with 25mm tails to the box. Then 16mm cable from the RCD box to the consumer unit, which does not have an RCD. I see the consumer unit split into two sections, the main unit and the box with the RCD in it, some 25mm cable only to the RCD is fine. Am I right?

Hard to tell from that.
Are you sure it is a 100A fuse?
It may have a label stating it is a 100A rating but may actually have a 60A fuse in it?

What do you all think?

 

[Ray Tay]

EFLImpudence, It says 100A, and the spark who quoted said it was 100A. Can the main fuse be reduced to 60A with 100A written on the case and all can be in 16mm? I though the main fuses only came from the DNO.

EDIT:
For info, the main fuses are available:
https://www.tlc-direct.co.uk/Produc...W5vmJa42PbSOWct_ZwhBpPGoX6RMvB1QCoBoCGyHw_wcB

 

[EFLImpudence]

It says 100A, and the spark who quoted said it was 100A.

May be.

Can the main fuse be reduced to 60A with 100A written on the case and all can be in 16mm?

Yes, 80A will be alright with 16mm² if that is enough for the house.

I though the main fuses only came from the DNO.

Yes, you will need to get them to do it.

 

[Ray Tay]

EFLImpudence, so best to drop to 80A main fuse and go all in 16mm?
https://www.tlc-direct.co.uk/Products/MM804R.html

I believe Part P Sparks have fuse seal machines so the fuse can be changed by him and then resealed, rather than the DNO? A fuse is well ....a fuse

Going 16mm all the way from the meter to the new RCD box and new fuse by a Part P spark will be far easier and a hell of a lot cheaper.

 

[Ray Tay]

Thanks. So the rule of thumb is 80A main fuse and 16mm tails are fine as long as the total load is less than 80A?
100A mains fuse and 25mm tails are needed.

But this still does not answer my question. Suppose it is a 100A fuse, can only 25mm be run to the RCD box inches away from the meter and then 16mm to the consumer unit from the RCD box?

 

[ban-all-sheds]

A plastic box is fitted with a RCD/incomer switch with 25mm tails to the box. Then 16mm cable from the RCD box to the consumer unit, which does not have an RCD. I see the consumer unit split into two sections, the main unit and the box with the RCD in it, some 25mm cable only to the RCD is fine. Am I right?

What do you all think?

I think that it's probably an academic question, since it is highly unlikely that your idea of an RCD like that would comply with the Wiring Regulations.

An electrician was called in the fit an RCD and gave a ridiculous price for a new consumer unit with a RCD inside and 25mm tails back to the meter ripping out part of the wall.

Maybe because what you think ought to be done ought not to be because it would not comply with the Wiring Regulations.

This was overkill. I saw that an RCD/mains incomer can be fitted near the meter with 25mm tails to the RCD box and then connect onto the existing 16mm tails in the wall to the existing consumer unit.

Have you actually read 314.1 and 314.2?

 

[ban-all-sheds]

Going 16mm all the way from the meter to the new RCD box and new fuse by a Part P spark will be far easier and a hell of a lot cheaper.

The other thing I think is that you must stop making "as-cheap-as-possible" your most important criterion when doing property renovations.

 

[Ray Tay]

ban-all-sheds, I know you can have an RCD in a separate box between the meter and the consumer unit. I have seen many retrofit RCD's like that.

Being ripped off is one thing, which appears to be the case in this RCD case. Economical engineering is another matter entirely.

So you cannot answer the questions I asked in the first post.

 

[ban-all-sheds]

Have you actually read 314.1 and 314.2?

 

[chivers67]

Yes but having it like that if it trips your entire power supply goes out on all circuits, the electrician quoting the new Consumer no doubt factored in more than one RCD I'd have thought. For instance lights on a separate RCD to sockets, so if a fuse trips on the socket feed you'll still have lights available

 

[Ray Tay]

chivers, the quoting Spark may have done that, but it was overkill and not the real point here. One RCD fine here. Can you comment on the 16mm cable from the RCD box to the Consumer unit?

 

[chivers67]

That is dependent on the rating of the RCD in question

 

[Ray Tay]

chivers, what would you rate the RCD at in this situation?

 

[EFLImpudence]

It's not dependent on the rating of the RCD; it's the other way round.

If the supply is 100A then the RCD will have to be 100A and all the tails 25mm².

Technically you could use lower rated RCD and tails if the loads are limited to whatever but they may not be.