Terminating 16mm SWA at 80a switch

[Jase3693]

Hi.

I’m renovating a project house and have had a new meter installed by western power with smart meter. They also installed some 100amp switches which is helpful.

I’ve done something similar to this before, but our main supply for the house comes into the garage, and from the garage, there is currently a 16mm old cable running to supply the house CU underground.

I want to replace this old cable with a new 16mm SWA from the house CU to the switch I have installed in the garage. How best would I terminate the SWA into the switch?

I have tidied up the board as per my photo, and just attached the old white cable that supplies the house to the switch temporarily.

Am I right in saying I could do away with the switch, terminate the SWA in a knockout box and run the tails directly into the black 100amp connector blocks?

 

[ericmark]

It depends on installation methods, but would have thought 25 mm² is required, I note only 80 amp, but you give no distance say SWA then show twin and earth. So would say you need professional advice from some one on site so they know all details.

 

[flameport]

Am I right in saying I could do away with the switch, terminate the SWA in a knockout box and run the tails directly into the black 100amp connector blocks?

Not really.

Overload and fault protection is required for the SWA cable, the white switch contains a fuse, by removing it you also remove the fuse.
The DNO fuse can't be used for lengths of cable over 2m (3m for some DNOs) regardless of what value it is.
The appropriate fuse value depends on several factors, none of which have been given.

The black connector blocks are redundant, any connection would be made into the top of the 2 pole isolator.
If by 'tails' you mean the SWA insulated cores, they are single insulated and therefore cannot be exposed, additional containment is required.
SWA bedding is not appropriate as insulation or containment either.
If this is an 80A supply, 16mm² is probably undersized. Some current ratings may be above that but they are typically for 90C conductor temperature which is highly unlikely to be appropriate.

 

[Jase3693]

So the distance is roughly 20m.

Yes, the current cable in there is T&E as that’s the old cable connected temporarily and I’m replacing that one with the new 3 core SWA.

I’ve said 80amp supply as that’s the rating on the main house fuse installed by western power. I don’t believe we’ll be pulling anywhere near that in our 3 bed house.

I done a cable calculator, and they were all saying 16mm 3 core SWA was suitable. Can I put an 80amp fuse into the white switch, or would I be better putting a 63amp in to be safe?

 

[Adam_151]

Easiest way would be to replace the switchfuse with one like this https://www.directtradesupplies.co.uk/product.php/202439/ and gland straight in, use a tails gland for the supply in. The switchfuse you have is not intended for SWA, primaraily its designed for tails in, tails out, but you can use it with T/E or splitcon with the optional spout adapter (which has not been fitted - therefore exposing inner cores), and while you could mess around with an adaptable box, its not worth the effort to result in a poor result.

I'd also echo what others have said about possibly needing 25mm, remember the following, some of which has been pointed out by others:
1) The calculator will default to 90Deg conductor temperatures
2) It will also probably default to clipped direct and no correction factors
3) It will also be designing the submain on its own, remember that maximum volt drops apply to the installation as a whole, so if you allow nearly the full permitted for the submain, you'll have none for the final cirucits, and if you go to 5% for the submain, acheieving 3% total on the lighting cirucits would be impossible

 

[Jase3693]

Okay. That’s exactly what I’ll do. I was planning on terminating the swa into an adaptable box and then running in conduit etc to the switch to keep it hidden, but your plan does look tidier and more simple.

25mm cable it is. Thanks for the help.

 

[Risteard]

Okay. That’s exactly what I’ll do. I was planning on terminating the swa into an adaptable box and then running in conduit etc to the switch to keep it hidden, but your plan does look tidier and more simple.

25mm cable it is. Thanks for the help.

You should have the installation designed (and installed, and verified) by someone competent.

 

[JohnW2]

3) It will also be designing the submain on its own, remember that maximum volt drops apply to the installation as a whole, ...

As you will be aware, what I presume you are referring to as "maximum voltage drop" is merely the recommended guidance in an 'informative' Appendix of BS7671, the only regulatory requirement being that VD should not be so great as to "impair the safe functioning" of connected equipment.

so if you allow nearly the full permitted for the submain, you'll have none for the final cirucits, and if you go to 5% for the submain, acheieving 3% total on the lighting cirucits would be impossible

True, but to get 5% (11.5V) VD in 20m of 16mm² SWA would require a current of about 195 A !

Perhaps slightly closer to a possible issue with long distribution circuits would be exceeding of the maximum Zs for the circuit - but, again, 20m of 16mm² cable would increase the Zs by no more than about 0.058 Ω - so, unless the Zs due to the final circuit was already very marginal, that would not be a problem, either.

Finally, even if one did want to stick to the guidance of 3% maximum VD for lighting circuits, it would surely only be necessary to calculate the VD in the face of the maximum credible load current - and if the circuit was supplying just LEDs distributed along its length, I doubt that the VD within the final circuit would ever be anything approaching 3%.

It's also worth remembering that thoughts about 'maximum VD' are really only relevant for installations with a very low supply voltage.

Kind Regards, John

 

[Jase3693]

You should have the installation designed (and installed, and verified) by someone competent.

Surely DIY forums are designed to help people who are… you know… learning to do it themselves? No need for comments like this.

Thanks to everyone else.

 

[Murdochcat]

Surely DIY forums are designed to help people who are… you know… learning to do it themselves? No need for comments like this.

Thanks to everyone else.

Sitting on the fence here but many people who do DIY seem completely ignorant about most of the regulations that apply to domestic premises.

That means gas, electric, part F, part m, planning permission, building control etc

Just saying

 

[Risteard]

Surely DIY forums are designed to help people who are… you know… learning to do it themselves? No need for comments like this.

Thanks to everyone else.

Not where this causes safety deficits. Remember the "not" in the site's name - as in never DIY electrical work. Whether you want to hear that or not is irrelevant.

 

[JohnW2]

You should have the installation designed (and installed, and verified) by someone competent.

That is,of course, essentially true of any work in any field.

The question as to whether a person's need to ask questions or ask for advice makes them a 'non-competent'person is very different. No-one is omniscient and I would suggest that asking for advice about uncertainties is far better than just 'pressing on' despite those uncertainties.

... or are you perhaps just 'anti' DIY work in general?

 

[Risteard]

That is,of course, essentially true of any work in any field.

The question as to whether a person's need to ask questions or ask for advice makes them a 'non-competent'person is very different. No-one is omniscient and I would suggest that asking for advice about uncertainties is far better than just 'pressing on' despite those uncertainties.

... or are you perhaps just 'anti' DIY work in general?

He clearly isn't competent for the proposed task. My views on DIY aren't relevant to this observation.

 

[Jase3693]

Nasty get off the thread fella and have a beer. Calm yourself down.

He clearly isn't competent for the proposed task. My views on DIY aren't relevant to this observation.

 

[plugwash]

Those little switchfuses, while neat give very little room for terminating cables. Fine for tails, problematic for T&E, totally unstable for SWA.

IMO you should get rid of the little switchfuse and the henly blocks and fit a switchfuse that is stuiable for SWA.