Moving C.u. Away From Meter

[Switch Snow]

Hi to all those willing to comment and advise. (Said with caution)

Though I'm gradually gaining experience having completed a number of installations, I've often found info on this forum useful but until now haven't needed to ask for specific advice.

Along with a full re-wire, a customer wants their C.U. moving to a point under the hall stairs ( approx 10 meters from meter)

I appreciate that the first thing required will be a fused switch close to the meter.  (Though I can only find these rated to 80a, wouldn’t this make the 100amp switch in the C.U. irrelevant? can you get a 100amp fused main switch?)

My main question though is, is there a preferred option for running from the fused switch to the C.U.?

(Bulk of the run will be visible in the cellar with approx 1.5 meters needing to hide / be buried as it comes up the hallway wall to the C.U.

Would you advise 25mm SWA ? or 25mm tails (16mm earth) with some steel plate (earthed) protection as they become concealed? or any other options??

The house is a fair size ( 4 Bed ) with  TN-S earthing.The current installation already has a similar set-up with an old fused switch next to Meter & 16mm tails run approx 15 meters to the old fuse board in the kitchen. However this was possibly done by the old fella that used to live there, and anyway the regs have moved on since this was installed.

I appreciate all advice and comments.

Thanks Switch Snow

 

[steptoe]

first thoughts would be 25mm SWA

have you worked out the actual loading? my house runs off a B63 quite happily,

size isnt so much of an issue as loading,

10 bed mansion with 2 pensioners Vs 3bed semi with 4 teenagers,,,,,

 

[matt hunt]

Make sure discrimination is applied here, you mentioned that the Cu has a 100amp, this is only an isolator so discrimination does not apply, if 60amp incoming fuses then you cannot fit an 80amp switched fuse, if 100amp incoming fuse then an 80amp switched fuse is fine, I agree with Steptoe over the 25mm cable to be used

 

[apprentice87]

Hello,

You do not have to worry about discrimination at all. The OCPD [does not have to be a switchfuse, several ways of doing it] is there for one reason, and one reason only, to protect the tails/submain [as they are going to be more than three metres in length], So, you can have any size OCPD you like, provided it is of a suitable value, 1, to match the cable selected for the tails/submain, and 2, suitable, [bearing in mind the Ze of the incoming supply] having regards to the  disconnection time of 5 seconds allowed for this circuit.

If this does not discrimate with the fuse in the cutout, so what, you are not allowed to rely on this as an OCPD anyway, not allowed to tinker with it, and besides, the only reason it is there, is to act as a "back to front" means of protecting the service cable. As far as we are concerned, it might as well not exist.

I know i will now have all kind of replies saying that you cannot load the system up regardless of board fuse size, well..... not really, for one thing, in a domestic setting, how often have YOU seen one blow. Secondly, leaving aside supply impedance and volt drop, the PRIMARY reason you have to make sure loading is not excessive, [remember, you will not be able to damage the service cable anyway] is that you have contracted with the DNO for a supply of a certain size. If they think that you are exceeding the agreed supply capacity, replacing the board fuse will be the last of your worries, they will simply disconnect your supply and that, will be that. [unless you give them large sums of money that is] in which case they may do a range of things, from putting a bigger fuse in, to giving you your own substation, and everything in between!!.

Oh, by the way, if you do fit a switchfuse, it is not making the incomer in the CU redundant, that is there to enable you to isolate, [possibly in an emergency] the CU. It would be 1, a breach of the regs, and 2, plain silly, if the means of isolation were to be 18 metres away in another section of the building..

john...

 

[Switch Snow]

Hi Guys, thanks for your replies.

If the job comes, then S.W.A. is probably the way.

The Incoming fuse is 100A. so would probable go the 80a switch.

John, appreciate your input. Though I didn't mean to suggest that the incomer in the CU should be removed.

My thought was that as the switches are also OCPD's, then in the case of an overload situation surely the 80A fused switch besides the meter would trip before the 100a mains incomer in the C.U. ?

if this was to happen then the inconvenience of having to go down into a dark cellar with no power to reset the fuse switch would be required. 

(Unlikely as this may be)

 

[NozSpark]

The 100A main switch in any CU/DB is just a switch.... It is not a protective device

 

[Blue Duck]

its just how much amps it will take until it pop's 

(I'm not watching photonicinduction right now) :innocent

 

[apprentice87]

Hi Switch Snow,

As Noz said the incomer in the CU is a switch is all, nothing else.. Regardless as to whether your submain has a CU, 20000 christmas tree lights, or a welding set on the end, you will HAVE to have some form of OCPD and an isolator for it. So, as you say, a switchfuse, or better, an MCCB, [cos it would be easier to reset] 

As to which "blew first" I do not think that you would have reliable discrimination between an 80 amp fuse and a 100amp one, things are not as simple as that, you would have to look at the characteristic charts for the fuses concerned to know for sure, but i am pretty sure if you wanted to have discrimination, you would have to use a 60 amp one.

ALL THIS IS ACADEMIC ANYWAY as, you WILL have to have an OCPD within the first 3 meter run of your submain, you CANNOT [usually] have it at the "other" end, so i am afraid you are stuck with the "groping about in the celler in the dark" scenario, there is no way round this as far as i am aware.

Another BIG problem you have not thought of is this...

The "main switch" in a domestic installation, is, REGARDLESS of earthing type, required to isolate ALL live conductors, this means "live" [to put it simply] AND neutral too. Whether this applies to your submain i do not know, [without going to have a read of the regs] but i rather think it does, which is why such things as a Wylex REC2 isolate BOTH conductors......

Having thought of this, this means scrap the switchfuse, and get yourself a DP MCCB. This will be cheaper than the switchfuse you were thinking about anyway...

john...