UK Why Are Incoming Live/Neutral 16mm and Tails From Consumer Unit 25mm

[OnlQQker]

I just can't seem to understand this and can't find a clear reason behind it either. I asked a chap on the DNO once and he didn't know either.

Anyone?

 

[ipf]

If you're talking about DNO cable and consumer tails...Different cable types and installation methods..... plus regulations.

 

[davesparks]

Older installations had 16mm tails fitted because that all they need.
In recent years it has become the norm for people working in the domestic sector to fit 25mm tails regardless of what size is actually required.
This has been fuelled by the IETs picture book which shows diagrams of typical supply and CU setups where 25mm tails are fitted.
So now we have a lot of installations with shiny new CUs and 25mm tails, fed via 16mm tails from the cutout which in turn is fed via a 100 year old paper insulated lead sheathed cables of an imperial size a bit smaller than 16mm.

Why electricians these days seem to forget/ignore the theory, science and regulations and choose to follow the herd like a bunch of sheep I don't know.

 

[Wilko]

Hi -as per @davesparks the different sized conductors often reflect the practice at the time they were fitted rather than being the result of engineering thought. It’s common to see the fuse-to-meter tail in 16mm from when 60A fuses were used.

 

[Dustydazzler]

Still come across loads of 10mm tails from the cut-out to the meter

 

[R-fur]

As above. I used to be an installation inspector for the local DNO. We were told to use 16mm for a 60 or 80 amp fuse and 25mm for 100 amp fuse. I think a lot of sparks just use 25mm when 16mm would be plenty for most installations.

 

[telectrix]

i've been slated by others for fitting 16mm tails when the DNO fuse is 60A or 80A. even if the fuse is 100A, 16mm tails will cope with any domestic installation the average household will never draw more than 40A at any on time, averaging out at maybe 10 - 15A. our average consumtion over 24 hours is 4.5A. unless siwimming pools/12kW showers / hot tubs are in the equation.I'll stick with 16mm.

 

[freddo]

When we have NICEIC inspections they don't like it when 16mm tails are fitted! 25mm is overkill for a normal UK house, even with electric heating. I rewired a small 4 bed house a few years back, electric convector heaters throughout and electric cooker. With everything turned on to full still only got to 73 amps. That soon dropped as the thermostats/simmer stats started switching.
I did a very small install a while back where the entire installation consisted of 2 small LED sign lights. Feeling reckless and throwing caution to the wind, I used 10mm2 tails for that.

 

[davesparks]

When we have NICEIC inspection

That's weird, our NICEIC assessor this year was happy with 16mm tails straight from a 200A supply to a switchfuse. I can't see any real difference.

 

[EddieSparks]

It is so over kill with 25mm, just think of the normal length between the cutout and the meter, 300/400 average, hardly a long run. Also now a days, with LED lamps and down lights and how much more efficient appliances are, loads in peoples house are being reduced compared to 15 years ago,

 

[davesparks]

It is so over kill with 25mm, just think of the normal length between the cutout and the meter, 300/400 average, hardly a long run. Also now a days, with LED lamps and down lights and how much more efficient appliances are, loads in peoples house are being reduced compared to 15 years ago,

Loads are being reduced, but there always seems to be this obsession with increasing the size of cables.

The last time I installed a domestic CU remote from the incoming supply I ran a 63A submain in 10mm SWA. All the threads we see here on the subject seem hell bent on installing a 100A submain in 25mm.

 

[Ian1981]

25mm2 tails are always stipulated by the DNO for new build housing (northern power grid) along with a 16mm2 earthing conductor.
Local housing associations and the like also stipulate this in their electrical specifications.
In truth the sizes should reflect the expected loading with diversity taken into account.

I feel that people are more comfortable installing 25mm2 tails if the primary ocpd is known to be 100amp as a play it safe concept.

 

[davesparks]

I feel that people are more comfortable installing 25mm2 tails if the primary ocpd is known to be 100amp as a play it safe concept.

I feel that those people are lacking in the technical knowledge they are supposed to have.

 

[Ian1981]

I feel that those people are lacking in the technical knowledge they are supposed to have.

You’re probably right Dave.

 

[Lucien Nunes]

To be fair to the 25mm² people, there is a certain logic in using the largest reasonable cable size in a situation where it's slightly more of a pain to alter for future upgrades (due to being (theoretically) sealed in the meter) than if they were entirely within the customer's equipment. They are usually short, so the additional cost is minimal, and there's then no need to consider changing them for any reason in the future, as the service cable would the limiting factor. And 0.0225 sq. ins. PILC service cables, although only 14.5mm², will usually take whatever you throw at them.

Obviously with a long submain run where oversize cables would be reflected in the job price, it's important to spec the cable accurately to the demand.