Replacing CU: 16mm or 25mm SWA?

[wizbongre]

Hey knowledgeable folks. Currently exploring what's needed to replace a very (VERY!) old CU. Early stages, so only rough conversations with some local sparks to get their opinions and already hit an immediate conflict.

We have a 100amp fuse on the service cut-out, feeding into main isolator via meter, and then on to the old CU. As well as being v. old its in a ridiculous position, shin height on the return wall of a very small pantry room. Far from ideal. Plan is to run SWA from the main isolator ~3m and fit a new CU at chest height in a large cellar space. Should be much easier to work with then, as well as being more central to the house.

The question: I've had 2 suggestions for the SWA from cut-out to new CU - 16mm and 25mm. Before I let the sparks battle it out with pugil sticks, what's the forum hivemind think?

 

[davelx]

Well, the current carrying capacity of 16mm² 70C Thermoplastic SWA is between 78 and 97A (from table 4D4A), depending on how it is installed, which is less than the 100A fuse in the cut-out (if there really is a 100A fuse in there). So strictly speaking you can't use that cable with a 100A fuse.

If using 90C Thermosetting SWA then it's 91-110A (from table 4E4A), so that might not be big enough for the 100A fuse either and all the accessories connected to it also have to be rated at 90C,

At 100A, the resistive loss in 3m of 16mm² is 84W with a voltage drop (VD) of 0.84V, and with 25mm², 53W with a VD of 0.53V. That being said, you are highly unlikely to be drawing 23kW/100 A for any length of time, if at all.

 

[ericmark]

Where do you read 100 amp, I see no sticker on the fuse, may be a 100 amp carrier but if anyone fitted a 100 amp fuse that would be rather naughty since the fuse box has a 60 amp isolator

and that is a fuse box not a consumer unit, made before type testing came in. So with a 60 amp fuse yes could use 16 mm² SWA but since it may be upgraded latter it would make sense to use a 25 mm² SWA to allow for future upgrade.

 

[EFLImpudence]

The question: I've had 2 suggestions for the SWA from cut-out to new CU - 16mm and 25mm. Before I let the sparks battle it out with pugil sticks, what's the forum hivemind think?

If the fuse is 100A and assuming method C you will need 25mm² 70º SWA.

 

[Murdochcat]

How far is the CU from the meter?

 

[ericmark]

As said my guess is it is not a 100 amp supply, but a 60 amp fuse in a 100 amp fuse carrier. This is what I have, and when I got a smart meter fitted it was confirmed only a 60 amp supply, which is just as well as my solar panels and battery mean the total can be around 95 amp before 60 amp drawn from the DNO supply. I would need a second 100 amp fuse before the DNO fuse could be changed to 100 amp.

There are extra fuses in your supply

so maybe they limit the supply to the Wylex fuse box? But does it matter? I would not want to stop the fuse being uprate by fitting 16 mm² for the small extra cost better to use 25 mm² even if not required today.

 

[sparkwright]

25mm will be future proof.

As Murdoch asks, what will be the length of the cable from the meter position to the CU?

Presumably you will be fitting a metal switch fuse unit just after the meter, to supply the SWA?

 

[EFLImpudence]

It says ~3m. in the OP.

 

[plugwash]

My first thought would be is there any reason not to just use tails, possibly with some conduit or trunking for neatness and/or protection. For such a short run, using SWA just seems to be creating a bunch of extra work for little benefit.

If you do use SWA.

The isolator you have has no room to terminate SWA, so most likely using SWA would mean runing tails from the existing isolator to a switchfuse which would provide somewhere to terminate the SWA and to act as the point from which you take responsibility for overcurrent protection, if you use 16mm² the max fuse rating with most installation methods is likely to be 80A. 25mm² would potentially allow 100A (though if the DNO fuse is less, this is kind of pointless).

The slightly awkward thing with 25mm² is what to do about earthing. A 25mm² core is too big for the earth bar in a typical CU. Potentially you could just use the armour for earth, but most electrians nowadays preffer to have an explicit earth conductor. I've seen at least one electrician split the earth core between two holes, but that seems rather a hack. You could of course run two core SWA with a seperate earth conductor but that brings one back to the question of why not just use tails.

 

[sparkwright]

It says ~3m. in the OP.

What does that mean though?

 

[Murdochcat]

Why SWA ?

 

[sparkwright]

What does that mean though?

Looked it up. Approximately.

But still need a metal switch fuse or similar to terminate SWA.

SWA could be pointless and impractical.

 

[Murdochcat]

So repositioning the CU too ……

I would recommend you get 2 or 3 sparks on-site to look first hand rather than random advice off the net

 

[wizbongre]

Thanks for all of the answers so far folks, appreciate it.

Both sparks said SWA - I was surprised as the new run will be under floorboards, clipped to a length of wood acting as a tray. As mentioned, the meter and proposed new CU are approx. 3m allowing some slack.

In my head, knowing that EV, solar, battery storage, etc. etc. are all on the long term plan, I assumed it made more sense to just lay the biggest now (25mm) as its the most future-proof.

I assumed as I had a 100amp fuse on the service head that was what matters. No idea what size fuse is in the other boxes, as there's no markings on them at all. The whole thing is a sh1t-show - we have several other random CU's dotted around the house as its been extended over the years!

 

[wizbongre]

The saga continues (well, it hasn't really started yet as still in initial planning talks with different sparks.)

To build on the above, 1 spark has told me I must use SWA for the connection from meter to new CU due to the length. Another says long tails are fine.

I assume there is clear guidance in the reg's about how long tails can be?