16mm tails with 100A fuse ???

[Wilko]

Job today to change out old fusebox and I'd noted the 16mm tails but on pulling the fuse this morning it was 100A which I wasn't expecting. Fairly recent new smart meter installed by XYZ who are also the DNO ... Shouldn't the fuse be limited to 60A in this case? Thoughts and advice appreciated

 

[Octopus]

Pop a 60A fuse in .............. nobody will ever know

Tin hat still on!

 

[Ian1981]

Well, I did try to move the tails but only got the 20cm I needed for the new CU and they won't budge any more, although there is slack at the meter. So changing the tails will be mission messy. Tails leave the external meterbox in a cavity brick wall, go up and over the front door and drop down from the ceiling. CU mounted high up next to ceiling behind front door. New CU is in the location of the original fuse unit, at Customer's request. But somewhere in that lot the tails go through a beam I reckon and are jamming up. Nothing I can't sort if I rip into the plaster ceiling she's just had done .

In terms of load, I am sure a 60A fuse would be fine. 2BR mid terrace, newish combi for CH and HW. Biggest drain will be an induction hob she's got on order.

There's no insulation anywhere near the tails that I can see looking from both ends. The tails will be less than 3m long.

As long as regulation 434.5.2 is met for protection of the tails for fault current and your total load profile taking into account diversity is not going to overload the CCC of the tails you will be fine.

 

[Ian1981]

Of course the DNO may stipulate a maximum CSA of bigger than 16mm tails if connecting directly into their equipment.
Of course we all ring them all the time to enquire don't we?

 

[Wilko]

Of course the DNO may stipulate a maximum CSA of bigger than 16mm tails if connecting directly into their equipment.
Of course we all ring them all the time to enquire don't we?

Yes, what was the DNO thinking putting 100A fuse in and saying nothing to owner (she's lived there forever) about the 16mm tails... and the DNOs smart meter guy ...

 

[Wilko]

As long as regulation 434.5.2 is met for protection of the tails for fault current and your total load profile taking into account diversity is not going to overload the CCC of the tails you will be fine.

Got the books out, calculator valves just warming up.

 

[Toneyz]

Pop a 60A fuse in .............. nobody will ever know

Tin hat still on!

Take the tin hat off if you've got the fuse out de-rate the fuse to 60A there are millions of houses with 60A fuses that have had additional loads installed .

 

[Octopus]

Take the tin hat off if you've got the fuse out de-rate the fuse to 60A there are millions of houses with 60A fuses that have had additional loads installed .

Precisely.

Got a client near me, 5 bed house, separate annex, loads of external lighting, pool, tennis court, ..... 60A fuse

People get so hung up about needing a 100A fuse ................... unless you are doing space heating, cooking and water heating and live in a big house you won't get anywhere near 100A.

Just imagine the lecky bill if you were running these loads on a regular basis.............

 

[Wilko]

Ok - with k=115, S=16 and Ze 0.19 measured today, I get ... wait for it ...

Take the tin hat off if you've got the fuse out de-rate the fuse to 60A there are millions of houses with 60A fuses that have had additional loads installed .

Mine included ... that's why I'm pretty comfortable with 60A fuse .
Question is, do it myself (at least it's done) or ask DNO who may do it, or may not?

 

[telectrix]

hmmm. tails inside the CU are basically in free air, but with a slight derating due to be in the enclosure, although with plenty of airspace around. IMHO methed C can be applied without losing any sleep over it. if i doubt, replace the cut-out fuse with 80A. 16mm only gets lukewarm @ 110A. BS7671 figures are more pessimistic than me giving up beer.

 

[Octopus]

Ok - with k=115, S=16 and Ze 0.19 measured today, I get ... wait for it ...

Mine included ... that's why I'm pretty comfortable with 60A fuse .
Question is, do it myself (at least it's done) or ask DNO who will never do it

Edited that for you.............

 

[Wilko]

Using 230 x 0.95, I get 1,150A pssc and plugging that into 434.5.2, I get 2.56 seconds. The melting time for BS88-3 100A at 1050A is 0.4sec.

So for short circuit situation this 100A fuse will not damage the 16mm tails, if I've done that correctly

 

[telectrix]

my old man's house, built 1926 has a 30A rewireable DNO fuse. it blew in 1988, but only when we;d added a 12kW cooker, 8kW shower,washing machine, and tumble dryer. all were going when the fuse let go.

 

[westward10]

Maybe we shouldn't be IP rating the CUs then - let the wind flow free! Or fit a permanently running CPU fan!

Just fit correctly sized conductors.

 

[Ian1981]

Just fit correctly sized conductors.

I guess you must C2 every domestic installation that has 16mm tails and a 100 amp fuse arrangement without consideration other than 'it's not the right size'

 

[Ian1981]

Ok - with k=115, S=16 and Ze 0.19 measured today, I get ... wait for it ...

Mine included ... that's why I'm pretty comfortable with 60A fuse .
Question is, do it myself (at least it's done) or ask DNO who may do it, or may not?

K would be 143 as the earthing conductor is not incorporated as part of a cable
S= I2t square root /k
So I is 1210 amps
t I'd have to look up a bs 1361 or is it bs 88?
K is 143

Edit for some reason I'm working out the earthing conductor size so ignore me