Discussion Of The Week!

[securespark]

Tails.

We all see tails of all sizes connected up to meters or henley blocks.

Should all tails have their CCC matched to the fuse protecting them?

Your starter for 10...

 

[Lectrician]

No.

 

[Adam_151]

Agree with lec... as long as its suitable for the connected load and meets the adibatic equation (and as a rule of thumb, on a standard domestic cutout, anything 10mm or above should meet it)

who suggested that one simon?...I'd like to see one of their busbar chambers... must be packed

 

[holmslaw]

..

 

[Electrifying]

I thought the Adiabatic equation was for selection of Protective Conductor size.

Surely meter tails would be selected using the same method as any other cable selection

i.e. Calculation of design current (Ib), selection of protective device (In).... then calculate tabulated current carrying capacity of cable required, taking into account rating factors for installattion/ref method etc, then selection of suitable conductor CSA... .

....or am I missing something here?

 

[Adam_151]

No you don't you're contradicting him. He said 'No' you said 'Yes, as long as...'

Ah, its holmslaw being awkward again, maybe there were gaps in the angles we answered from, but surely even you can understand the sentimet espressed.

Have you ever seen meter tails sized on current rating that were shown to be inadequate by the adiabatic equation.

No, though if I came across 6mm I might have to do a fag packet calculation, but i know off top of my head that 10mm or above meets adiabatic even for a 100A 1361

So you telling us that 10mm will also carry the normal current?

It depends what the norminal current is... see above..." as long as its suitable for the connected load and meets the adibatic equation" .. possibly sloppy phrasing but clear enough

Its saturday, save the contractors gobblydegook for weekdays.
.

I'm not sure what that means, care to elaborate?

 

[ricicle]

holmslaw

I suggest you read Adam's post again and not waste your time trouble making !

 

[holmslaw]

..

 

[ricicle]

but it's pointless, because the line cable has been sized to carry the normal running currect it will also carry the fault current.

Not neccessarily - I can think of instances where a cable sized for the load would not carry the fault current.......

 

[Adam_151]

I thought the Adiabatic equation was for selection of Protective Conductor size.

The adibatic* is simply a way of selecting a conductor as to withstand the heating effect of faults, often employed if you wish to utilise an undersized cpc, but equally usful if you want to place overload protection somewhere other than the start of a cable (fault protection can normally be assumed to be met if the overload protection is at the start of the cable), such is the situation with :

- a spur off a ring final (this is a standard arrangement from the OSG and may therefore be used without resorting to calaculations, the principle is the same though

- A one way enclosure containing RCD and MCB supplying a 45A shower circuit connected to henley blocks by 10mm tails

*Adiabatic means quite simply, without energy transfer, ie. we are assuming the conductor does not loose heat into the air around it, which isn't strictly true, but is a reasonble way of modeling it simply when dealing with short duration timeframes

 

[holmslaw]

..

 

[ColJack]

in a standard domestic install the tails should always be sized to take the FLC of the main fuse since with a fuseboard / CU installed it's perfectly possible to draw the full current of the fuse. (ie it's not a single fixed load, it's variable )

in a busbar chamber connected to a switchfuse, you can use smaller than the fuse protecting the busbar because the fuse in the switchfuse will provide OL protection of the cable.. as per 433.2.2 ( and providing that the cable is sized for the flc of the fuse in the switchfuse and protected against faults ).

 

[holmslaw]

..

 

[Adam_151]

in a standard domestic install the tails should always be sized to take the FLC of the main fuse since with a fuseboard / CU installed it's perfectly possible to draw the full current of the fuse. (ie it's not a single fixed load, it's variable )

How about my shower supply unit proposed above, containing 1no RCD and 1no B45 MCB (no room for anything else)...I see no reason why it should not be connected to the henley blocks via 10mm tails

 

[Lectrician]

And when you see an old school 4 way wylex with 2 x 30, 1 x 15 and 1 x 5......total load of 80amp possible, 16mm tails are fine.

A fully loaded 10way split would be a different kettle of fish.