UK CU in Meter Box & Henleys vs REC4

[SaajanB]

Hi everyone,
I'm still training so some advice would be appreciated

We have a shed about 60m from the house where would like a supply. I've already ran some 2 core 4mm SWA and wired up a CU with a 30mA incomer, it will be protected upstream by a 16A MCB. I went for 2 core because I want the shed on TT becuase of the length of cable run, we're on PME.

Our main CU in the house is fed by SWA from an 100A switchfuse located externally in seperate enclosure to the meter. I don't want to run the shed feed from the main CU because it would be too disruptive to run internally.

We're having some building work done at the moment so I was having a chat with the builder's sparky and he was saying he could put a small 4-way CU in the main meter box, since there isn't enough space in the other one with the switchfuse. He's going for a 4-way just incase we want an EV charger in the future.

Is this permissible? I was under the impression that only the DNO and metering equipment should be in the meter box. His plan was to just use a set of henley's and split the tails which I'm fine with, but shouldn't the whole installation have a single point of isolation?

I suggested using a Wylex REC4 and linking out the incoming terminals with busbar to allow two sets of tails to connect on the outgoing side but he wasn't too keen on that. Getting a REC2 and henley to fit along with the CU would be a bit of a squeeze.

Any advice would be appreciated, cheers
Saajan

 

[pc1966]

Can you post a photo of the meter box to show us the layout?

What about the switch-fuse box, what sort of space is in there?

 

[Spoon]

Is there room to fit a 100A isolator just after the meter, instead of Henley blocks, and wire both CU from this?

 

[davesparks]

. I went for 2 core because I want the shed on TT becuase of the length of cable run, we're on PME.

Please do explain this one, I've heard it so many times and nobody can ever give a sensible answer as to what difference the distance makes, how you work out what distance is/is not acceptable to use a earth from a PME supply.

I mean some actual technical reasons for this, not just random statements like 'it's dangerous' 'you can't export PME' 'in case of broken neutrals'

 

[telectrix]

the only reason to TT the shed , IMO, is if there are extraneous parts there. a 2 core 4mm SWA will not suffice for bonding.

 

[SaajanB]

Please do explain this one, I've heard it so many times and nobody can ever give a sensible answer as to what difference the distance makes, how you work out what distance is/is not acceptable to use a earth from a PME supply.

I mean some actual technical reasons for this, not just random statements like 'it's dangerous' 'you can't export PME' 'in case of broken neutrals'

Hi, we have a 15mm copper water pipe popping up inside the shed for a neighbouring greenhouse, which was the main reason for TT.
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Can you post a photo of the meter box to show us the layout?

What about the switch-fuse box, what sort of space is in there?

Here is the meter box and switchfuse box. Plan was to move meter and comms hub to the left, then CU would be next to it. Isolator/henleys below it next to the cut out.

The FCU next to the fuseswitch is just for outdoor socket spurred off the downstairs ringmain.


An 100A isolator would fit fine minus the henleys, but the REC2 only has two poles, and the sparky wasn't keen on linking out a REC4 with busbar to give it two holes for each pole. I suppose I could do it myself, nothing better than learning on the job! Just wondering in that case if there is a reason to not link out a REC4 with busbar? Obviously Wylex haven't manufactured with that in mind, but I can't see how it would be unsafe...

 

[davesparks]

Hi, we have a 15mm copper water pipe popping up inside the shed for a neighbouring greenhouse, which was the main reason for TT.

That doesn't explain it, having a copper water pipe doesn't necessarily require the creation of a TT system.
In fact if that copper pipe is continuous copper from the house to the shed then the shed is going to be connected to the TNCS earth via the bonding regardless of what you do.

 

[pc1966]

In fact if that copper pipe is continuous copper from the house to the shed then the shed is going to be connected to the TNCS earth via the bonding regardless of what you do.

True, but it might not stay continuous so you would still need the 10mm bonding conductor size if the source is TN-C-S.
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Here is the meter box and switchfuse box. Plan was to move meter and comms hub to the left, then CU would be next to it. Isolator/henleys below it next to the cut out.

Being perfectly honest you don't have any usable space there for a mini-CU to be added.

The best I would try given that space would be to look at replacing the fused-switch with a mini-CU that had something like a 63A C-curve MCB to feed the house and something a bit smaller for your 4mm cable (or whatever you select).

That is not a great suggestion by any means, as will have quite poor selectivity between cascaded MCB, but it would allow something to fit and some garage CU are tolerably protected for non-domestic environments.

Maybe something like this with the supplied MCBs replaced:

Metal Clad Consumer Unit Garage And Shower | CEF

But obviously there are more things to check before proceeding, like if that has a 100A busbar, if the house demand is going to fit a 63A feed limit, etc.

 

[SaajanB]

That doesn't explain it, having a copper water pipe doesn't necessarily require the creation of a TT system.
In fact if that copper pipe is continuous copper from the house to the shed then the shed is going to be connected to the TNCS earth via the bonding regardless of what you do.

It is plastic in a couple sections due to a run in with the spade. Ideally the whole run should be replaced but I am not too keen on digging the rest of it up!

True, but it might not stay continuous so you would still need the 10mm bonding conductor size if the source is TN-C-S.
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Being perfectly honest you don't have any usable space there for a mini-CU to be added.

The best I would try given that space would be to look at replacing the fused-switch with a mini-CU that had something like a 63A C-curve MCB to feed the house and something a bit smaller for your 4mm cable (or whatever you select).

That is not a great suggestion by any means, as will have quite poor selectivity between cascaded MCB, but it would allow something to fit and some garage CU are tolerably protected for non-domestic environments.

Maybe something like this with the supplied MCBs replaced:

Metal Clad Consumer Unit Garage And Shower | CEF

But obviously there are more things to check before proceeding, like if that has a 100A busbar, if the house demand is going to fit a 63A feed limit, etc.

That is a good point actually. The type B MCBs in the house CU would trip well before the C type, well at least in theory. I could get away with a 63A but would prefer an 80A, although it looks like you can only get 80A MCBs in a 27mm width, which hopefully would not foul the busbar.
The only issue I would have with replacing the switchfuse with a CU is the neutral for the 25mm swa wouldn't fit nicely in the neutral bar, meaning I'd have to split it between two terminals which would look a bit naff, but functionally wouldn't affect it I suppose...

 

[davesparks]

That is a good point actually. The type B MCBs in the house CU would trip well before the C type, well at least in theory. I could get away with a 63A but would prefer an 80A, although it looks like you can only get 80A MCBs in a 27mm width, which hopefully would not foul the busbar.
The only issue I would have with replacing the switchfuse with a CU is the neutral for the 25mm swa wouldn't fit nicely in the neutral bar, meaning I'd have to split it between two terminals which would look a bit naff, but functionally wouldn't affect it I suppose...

Type B will not necessarily trip before a type C, MCB's often done discriminate at all.

You won't get an 80A MCB to fit a domestic CU.

Don't split the neutral between multiple terminals, fit a reducing lug.

 

[pc1966]

That is a good point actually. The type B MCBs in the house CU would trip well before the C type, well at least in theory. I could get away with a 63A but would prefer an 80A,

You get selectivity only to the instantaneous trip point of the upstream MCB, in the case of 63A C that is about 500A RMS fault current, for a 63A D it is about 760A (using Schneider's on-line tool) so not very good by any means.

As davesparks says you won't get a matching 80A breaker for a garage CU.

I have not looked at the sizes for a three-phase switch-fuse, as you might be able to parallel two of the feed fuses for L (more reliably than with a 4-pole isolator) and fit a link on the 3rd pole to switch N, thus have both a fuse for the house CU and a different one for the garage CU and so better selectivity than cascaded MCBs.

Gives you fuses and DP switching, just no independance of house and garage CU should you need to work on one feed cable.
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A quick check shows most TPN fused-switches are significantly larger than I suspect you have space for, of the order of (H)400 mm x (W)300 mm.
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Seems MCG do a 63A D-curve MCB, that might be your best bet for any tolerable selectivity given the space constraint:

63A Single Pole 10kA D Curve MCB

Insulated DIN rail clips

www.cef.co.uk

 

[SaajanB]

Type B will not necessarily trip before a type C, MCB's often done discriminate at all.

You won't get an 80A MCB to fit a domestic CU.

Don't split the neutral between multiple terminals, fit a reducing lug.

Good point, I’ll just need to get an indent crimper.

You get selectivity only to the instantaneous trip point of the upstream MCB, in the case of 63A C that is about 500A RMS fault current, for a 63A D it is about 760A (using Schneider's on-line tool) so not very good by any means.

As davesparks says you won't get a matching 80A breaker for a garage CU.

I have not looked at the sizes for a three-phase switch-fuse, as you might be able to parallel two of the feed fuses for L (more reliably than with a 4-pole isolator) and fit a link on the 3rd pole to switch N, thus have both a fuse for the house CU and a different one for the garage CU and so better selectivity than cascaded MCBs.

Gives you fuses and DP switching, just no independance of house and garage CU should you need to work on one feed cable.
[automerge]1590478564[/automerge]
A quick check shows most TPN fused-switches are significantly larger than I suspect you have space for, of the order of (H)400 mm x (W)300 mm.
[automerge]1590479269[/automerge]
Seems MCG do a 63A D-curve MCB, that might be your best bet for any tolerable selectivity given the space constraint:

63A Single Pole 10kA D Curve MCB

Insulated DIN rail clips

www.cef.co.uk

Looks like you’re right regarding the TPN fuse-switches, some would fit but would leave no space for the SWA to exit. I will go for the CU in that case.

63A would be adequate, considering we don’t have an induction hob, electric shower or an EV charger on the main CU. The only time we’d approach 63A is if our heating fails and we have fan heaters going. What I can do is fit a socket in the lounge (other side of the meter box) off a 16A MCB from this new CU which would could be used for heaters etc.

Thanks all for your advice, much appreciated

 

[pc1966]

What I can do is fit a socket in the lounge (other side of the meter box) off a 16A MCB from this new CU which would could be used for heaters etc.

If you can do that, why can't you run the new SWA in to the current CU?

It would need to be an RCD socket as this supply has no RCD protection. And no, do not put a 30mA RCD in there in place of the main switch of any new fuse-switch replacement as that is very likely to have spurious trips!

 

[richy3333]

I thought I read in another thread that you are not allowed to mix manufacturers breakers/RCBO's in consumer units, unless the consumer unit manufacturer says it's ok??

Please do explain this one, I've heard it so many times and nobody can ever give a sensible answer as to what difference the distance makes, how you work out what distance is/is not acceptable to use a earth from a PME supply.

I mean some actual technical reasons for this, not just random statements like 'it's dangerous' 'you can't export PME' 'in case of broken neutrals'

It's 42 isn't it? The answer to everything is 42; so in this case its 42m

 

[pc1966]

The only time we’d approach 63A is if our heating fails and we have fan heaters going.

I doubt you would hit that unless you had several electric heaters on and trying to shower & cook. Also it takes a while to trip even at 40% overload, so really don't bother with the extra socket.

 

[SaajanB]

If you can do that, why can't you run the new SWA in to the current CU?

It would need to be an RCD socket as this supply has no RCD protection. And no, do not put a 30mA RCD in there in place of the main switch of any new fuse-switch replacement as that is very likely to have spurious trips!

The SWA or 2.5mm t&e would have to run through the lounge and hallway which I am not keen on doing since black SWA on the skirting is an instant no and t&e in trunking doesn’t look that nice, especially since a lot of it will be on show.
re. the MCB I meant to say RCBO

I doubt you would hit that unless you had several electric heaters on and trying to shower & cook. Also it takes a while to trip even at 40% overload, so really don't bother with the extra socket.

Ok, if D type MCBs take that long to trip, I won’t bother with the socket then.

 

[davesparks]

Good point, I’ll just need to get an indent crimper.

It doesn't have to be indent, a hex crimper is also acceptable.
Most wholesalers have a crimper you can hire, and if you have a good working relationship with your wholesaler the hire is usually free.

 

[pc1966]

The SWA or 2.5mm t&e would have to run through the lounge and hallway which I am not keen on doing since black SWA on the skirting is an instant no and t&e in trunking doesn’t look that nice, especially since a lot of it will be on show.

If there is no way to hide it in walls/under floor then I guess that is your limit.

re. the MCB I meant to say RCBO

OK, that would be acceptable, but still probably not needed and a bit odd to be separately isolated from the rest of the house circuits!

Ok, if D type MCBs take that long to trip, I won’t bother with the socket then.

The B/C/D aspect only makes a difference once you are up to 3 x overload or more as it tells you when the "instantaneous" magnetic trip part fires.

At 300% overload the thermal trip is somewhere between about 5 and 25 seconds (independent of B/C/D type).

At 50% overload trip times are 1-10 minutes (independent of B/C/D type).