Incoming supply cable and 16mm/25mm tails questions

There may well be 16mm2 tails on a 100A fuse. BS7671 says no, but in practice a problem is unlikely. street cables are not installed and rated like in BS7671 and again,  problem is unikely.  DNO in the UK will not worry unless you call an tell them it's on fire. I presume it isn't, nor is the cable outer getting too hot to touch.

Mike

Is it definitely a 100A fuse? The label on the carrier usually refers to the carrier rating - and it's very common practice to install a lower rated fuse inside (my local DNO has standardised on 80A and takes a lot of persuading to go higher these days).

Also the DNO might consider that their and the meter's terminals are good to say 90 degrees rather than the 70 degrees that BS 7671 usually assumes -  in which case 16mm2 can be good to over 100A.

   - Andy.

A long time ago I lived in a house where the supply cable was probably from the 1920s, and was less than 10mm2. One day it caught fire underground and the DNO turned up. Upon digging they found that it supplied 4 houses, and that the main street cable was well undersized. Oh well, never mind! A few new bits were added and a joint was replaced and everything was good to go. Each house still has a 100A fuse, and nothing else happened whilst I owned the house. At the time very few people had many high consumption appliances, but I do wonder what will happen if they get a few car chargers in the street.

but I do wonder what will happen if they get a few car chargers in the street.

?

I think that apply to much of the UK David 

If in doubt, simply ring the DNO, they will be happy to replace the fuse with a smaller one, and probably wont even charge for this.

Then if the next owner of the property wants 100 amps they can charge hundreds of pounds for the improvements.

AJJewsbury: 
 

Is it definitely a 100A fuse? The label on the carrier usually refers to the carrier rating - and it's very common practice to install a lower rated fuse inside (my local DNO has standardised on 80A and takes a lot of persuading to go higher these days).

Also the DNO might consider that their and the meter's terminals are good to say 90 degrees rather than the 70 degrees that BS 7671 usually assumes -  in which case 16mm2 can be good to over 100A.

   - Andy.

I'm just going on what the carrier says. How do you find out the fuse rating? 

broadgage: 
 

If in doubt, simply ring the DNO, they will be happy to replace the fuse with a smaller one, and probably wont even charge for this.

Then if the next owner of the property wants 100 amps they can charge hundreds of pounds for the improvements.

The maximum demand is a fair amount over 100A if I use table A1 and A2 of the OSG. Never a clear answer with electrics is there. I remember some of the guidance I've read if ‘for installations up to 100A’ and I don't know what needs to change when over 100A anyway. Maybe I'll choose jobs <100A maximum demand for now.

I'm just going on what the carrier says. How do you find out the fuse rating? 

Well as you have just rewired, presumably the main fuse came out to make the CU safe to work on, so by looking at it while i t is out. At that point out comes the sharpie pen and writes the value on the board beside it, and maybe the zs result.

 If the DNO came and took it out for you, or if you did, and what happened to the seals is a matter of conscience, and the policy of the DNO, some will issue paper seals to folk they trust, others do not. 

Do not get too worried by max demand calculated as per the OSG - it over-estimates, substantially, and in practice 16mm tails on a 100A load will not suddenly burst into flames. They may last 25 years instead of 50 if the house is fully loaded, but the electricity bills means it probably isn't going to be. 

Mike.

Supplier's tails outside the scope of BS7671 and do not belong to the consumer - or your customer in this case. I wouldn't worry about it because it is beyond your control. Quite why the IET have insisted upon including a requirement  on it's various forms for the electrician to inspect and sign off something over which he has no control - and hopefully no liability -  mystifys me.

You wish to find out the actual fuse size? Cut the seals and pull the fuse. The info you want is written on the side of the fuse link. Make sure you switch off any loads first, and don't tell anybody! If you have fitted a main isolator, then you would have had to have pulled the fuse in any case, unless you did it live, in which case the Electricity At Work Act might have something to say about that.

‘Thou shalt not work live if it can be reasonably avoided’.

OK thanks everyone. Yeah I pulled the fuse. I'll look inside from now on!