Tails from meter to CU - pretty long...

[ed-ectrician]

Caveat: I am at college doing 2330 and shortly to do 2356, not a Electrical Trainee 'pretending to look at my own CU to get an answer off you lot" - seriously!

So... I look at my CU in rented flat (very nice landlord - happens to be my dad!) after putting in an unfused spur and doing the ring polarity and R1+R2 etc testing. I was a bit surprised to see that the cable going from the DNO isolator (after the meter of course!) to my main intake on the CU looks like 6 or 10mm T&E! hmmm, it's lasted bloody ages but that must have to be 25mm cable surely?! Also my main earthing conductor going from earth bar in CU to MET should surely be 16mm too?!

Am I missing something or is having vastly too small T&E cable carrying all my current just outside my daughter's room as dodgy as it looks to me?!

So anyway, the main service head is on the ground floor (block of three flats converted from Victorian house) and my CU is in the flat on 1st floor. That's gotta be about 8 meters. Is there anything wrong with using 25mm cable to do this upgrade. I calculated voltage drop as ... 1.8 mV/A/m (from table 4D1B) multiplied by 'pessimistically large to be on safe side' 100 amps multiplied by 'pessimistically large to be on safe side' 10meters divide by a thousand to get back to volts from millivolts = 1.8 amps volt drop - which is well within 5 percent parameter?! Pls correct me if I am wrong.

Do I need to stick any double pole isolator or protective device between the isolator already installed and CU? Didnt think so but not sure if there's a reg about that kind of length needing another isolator or fuse?!

I will be running it in trunking up surface of a wall and then thru a couple of joists under floor (>50mm from top and bottom!) then surface trunking up to the board



Ed

 

[Strima]

Before replacing the tails have you confirmed the DNO's fuse size? Have you done any calculations on your maximum demand? How old is the installation?

You may well find that the cable would suffice for the job unless there have been any additions such as large showers/cookers etc.

Also before starting any work you should be looking at the suitability of the supply and bonding, not the other way round.

If you go down the 'pessimistically large' route then you may have some very unhappy customers in the future... :lol:

 

[ed-ectrician]

The DNO's fuse size is 100a so took this figure because I am assuming that the fuse will blow at around 150 amps so 100 amps is a huge load for a flat of this size but your fuse always has to be smaller than your cable rating surely?! Thought this was fairly standard - the rating of the fuse or circuit breaker must not be greater than the the cable current carrying capacity and all that

I'm FAIRLY sure the cable will suffice but regs state 25mm minimum for these cables dont they? I know regs can be a bit OTT sometimes but they are there for a reason

Agreed I should've looked at supply and bonding beforehand.

I wont go pessimistically large route in the future haha cheers mate I just thought for argument's sake in this instance even with pessimistic figures it still looks all good.

Cheers for reply though!

Ed

 

[sparks1973]

Before replacing the tails have you confirmed the DNO's fuse size? Have you done any calculations on your maximum demand? How old is the installation?

You may well find that the cable would suffice for the job unless there have been any additions such as large showers/cookers etc.

Also before starting any work you should be looking at the suitability of the supply and bonding, not the other way round.

If you go down the 'pessimistically large' route then you may have some very unhappy customers in the future... :lol:

agreed, this `if in doubt go for a bigger CSA` lark just demonstrated a lack of understanding of:
1) volt drop
2)design current
3)diversity (and when/when not to apply it)
4)Iz of cable/s

 

[ed-ectrician]

sparky1973:

I'm asking a question - Learning is all about asking questions surely, at no point did I say I knew everything. Come on mate, play nice

I actually have a bit of understanding of all four of those things you listed. OK I was a bit lazy and didn't go and work out the likely exact demand because I was asking a question only about volt drop.

Sparks1973 at one point in your life you were fallible too, it is good to remember that!

Anyone else got any helpful advice?

 

[Amp David]

First off, the volt drop is measured in volts and not amps as you state in your OP.

Next, is this so called supply to your flat run straight out of the service head or has it been fused down before feeding your flat?

Personally I hate seeing T&E used as a supply. How the hell are you supposed to terminate it properly, without any insulation showing outside of and isolator or henly.

 

[Jonny66]

Hi ed-ectrician

Agreed I dont like T&E sub-mains with their small cpcs...
Your volt drop calc should be 1.8volts not 1.8amps.....and should be calculated using your max demand.

IMO I would stick a 60amp metal clad switch fuse in next to the existing isolator and run your sub-main in 3core armored 16mm2. Needs switch fuse as tails > 3m.

 

[HappyHippyDad]

I'd like to give you the answer Ed but unfortunately I'm one of the Electrical Trainee you mentioned in your beginning paragraph, so I think I'll keep the answer to myself and let you work it out. Hopefully you wont be mistaking volts for amps again though.

Good luck,

Bye..

 

[ed-ectrician]

Crapsticks! Just reread my post and yes everyone i do know the difference between volt drop and current capacity of cable - writing 1.8 amps was a slip of the tongue - I did know it was volts!!

happyhippydad - I have nothing against anyone mate I just know what some people get like on this forum about so called '5 week wonders!' - in my eyes it doesnt matter what course you've done if you know what you're doing then that's what is important. Also important to remember we are all learning and some are at different stages which is why asking questions like mine is important!

 

[ed-ectrician]

First off, the volt drop is measured in volts and not amps as you state in your OP.

Next, is this so called supply to your flat run straight out of the service head or has it been fused down before feeding your flat?

Personally I hate seeing T&E used as a supply. How the hell are you supposed to terminate it properly, without any insulation showing outside of and isolator or henly.

Hi, as i said just a second ago saying amps was slip of the tongue i did know it was volts!

so it is..... 100a dno fuse, then meter, then isolator, then long run of cable, then my CU in flat.

- - - Updated - - -

Hi ed-ectrician

Agreed I dont like T&E sub-mains with their small cpcs...
Your volt drop calc should be 1.8volts not 1.8amps.....and should be calculated using your max demand.

IMO I would stick a 60amp metal clad switch fuse in next to the existing isolator and run your sub-main in 3core armored 16mm2. Needs switch fuse as tails > 3m.

cheers jonny, i thought 25mm was regs minimum for meter tails consumer's side, seen this in OSG am i wrong?