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?

 

[1shortcircuit]

For those onlookers coming from the Electrical Trainee courses, I would just like to say that I too came into the trade this way and took the batterings on the forum that came with it.

It does get better after time when people read your posts and realise that maybe you know what you're doing or when a newer victim comes onto the scene. :devilish:

If you don't have thick skin you will need to grow some pretty sharpish. Sift through the posts that are helping you and ignore the ones that do not, members soon move on once they realise that you're not biting

Good Luck:thumbsup

 

[Strima]

This thread is making me chuckle... :lol:

Ed, work out your maximum demand and go from there. No pint over engineering it as it will only hit you in the pocket. The OSG has a handy section but I'm sure us Electrical Trainee's will be happy to answer any questions...

I'm not a fan of T&E tails either but like it or love it they're out there and we'll see them for many years to come. If cost isn't too much of an issue I like to use SWA as it terminates tidily.

 

[Jonny66]

- - - Updated - - -




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

OSG is what it says, a guide. Better to calculate as it saves money and lets face it thats what its all about.

 

[ed-ectrician]

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.

Jonny66 - Are you referring to reg 434.2.1? the reason I ask is because if I am (HYPOTHETICALLY!) not reducing CSA of cable, (i.e doing it with 25mm tails - for argument's sake) then i wouldnt have to put a switch fuse in but if i assessed max demand and then found that i could get away with 16mm cable tails i would need a fuse as i am reducing the csa of the cable. Yes?!

 

[Jonny66]

No mate, not referring to BS7671, but ESQCR/ local DNO requirement of not using their cutout fuse for meter tails >3m.
Sorry fella, got the day off, had a few glasses of wine listening to a bit of Amy whinehouse and .......
Loads of threads on here to help you, just look up sub-main and switch fuse.

Also if you use a metal clad switch fuse it becomes nice and easy to terminate SWA cable and at load side you could use a metal clad box before CU....

 

[Jonny66]

Sorry fella, you do need to use a switch fuse due to the length of your sub-main again >3m. You can spend all day on the phone if you like to check with DNO, but its standard practice or should be. Nothing to do with reducing csa of cable.

 

[Paul.M]

@ Ed. The beginning is a very good to start. Have a look at all your loads in your flat and find out your demand (with or without diversity depending on cooker etc) and what are you pulling?

Then work out what size tails (and what type of cable) is suited to the installation method and what up front isolation switch/rcd is needed. What type of earthing arrangement is it? What size earth is needed to achieve disconnection times?


Some times its just a process to ask all the questions from the beginning to understand the full picture. Good luck ed, at least your putting an effort in to your education.

 

[telectrix]

best answer so far was jonny66 post. you need a switch fuse as your tails are > 3m, so fit a 60 A or 63A MCB then 16mm to CU.

 

[Jonny66]

Little Eddy Baby, or if you prefer Sir Edward or Eddy, or Ed, tis but as easy as putting in a switch fuse and some armored cable. If your rich beyond all comprehension, and have no intelligence at all then base your calculations on the OSGs maximum demand calcs. You say your talking about a flat so your max demand imho will probably be no more than around the 30/40amp mark on a bad day, if your lucky or unlucky depending on how you look at it, clamp the tails and see, or add all the breakers and times by 0.4 for a worse case ect. Bet your only pulling 30 amps, ok you might have a 10kw shower and Im a prat but hey ho. 25mm too big I bet, 16mm too big I bet, 10mm might be good enough...........do your calcs fella, its quite rewarding when you do it yourself and hey ho you've designed your submain.....good luck

 

[Jonny66]

Cant see a thanks button Telectrix, so thankyou