Discuss Why Are Incoming Live/Neutral 16mm and Tails From Consumer Unit 25mm in the Electrical Forum area at ElectriciansForums.net

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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?
 
If you're talking about DNO cable and consumer tails...Different cable types and installation methods..... plus regulations.
 
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.
 
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.
 
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.
 
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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. :)
 
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,
 
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.
 
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.
 
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.
 
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.
It's not strange at all really Dave. It can be different assessor each time and they all have there little pet hates and ideas of what is right and wrong. My last assessor stated it needs to be 25mm tails whereas the Stroma tech line said use your knowledge as an electrician to determine the size.
 
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.

In a new installation where the service cable is 35mm and the tails 25mm yes.
It's the insistence on breaking that same seal you mention and/or damaging decorations etc to replace 16mm tails with 25mm that I object to.
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It's not strange at all really Dave. It can be different assessor each time and they all have there little pet hates and ideas of what is right and wrong. My last assessor stated it needs to be 25mm tails whereas the Stroma tech line said use your knowledge as an electrician to determine the size.

They should all be working to the same regulations,their personal opinions should be kept out of it.

I've made a formal complaint about one of their assessors for this in the past. We got in to a bit of a row over something, I proved him wrong using the regulations and asked him how I could make a formal complaint about him and he suddenly backed down and signed off the assessment.
He seemed to think that signing off the assessment would somehow change my mind about making a complaint, it didn't.
 
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Hi,
Fairly new to posting on the forum so be gentle with me! I agree entirely with the comments of previous posts that there seems to be this wish to upgrade to 25mm tails regardless.
The bit that gets me is that within the OSG under 2.2.3.1 Consumer Tails it states on para 2 "Polarity should be indicated by the colour of the insulation and the minimum cable size should be 25mm." The reg no it refers to is 514.3.1 - however this doesn't mention anything about cable size.

I am currently in the process of replacing a customers old re-wireable fuse board with RCBO unit. 3 circuits, 32A cooker, 32A ring and 6A lighting. The main cut-out fuse is 60A. New meter been fitted within the past year or two and the tails from the meter to fuse board were obviously replaced at the same time. They are 16mm tails. Personally I see no reason to replace them - Am I wrong?!
 
comments of previous posts that there seems to be this wish to upgrade to 25mm tails regardless.
a) This may be foresight for the future without Petrol/Diesel !
--Short high demand charging cycles--

b) (Surely a lead covered cable 14.5mmsq sheds heat
much better -- than similar sized pvc insulated-sleeved )
 

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