6mm t&e circuit design max current?

[oldman2]

I have quite a long run of 6mm twin & earth that I need to work out the max current capacity for.

From CU its 7mtr clipped on open tray with other cables, then 3mtr in 40x40 plastic trunking (space factor observed) then 3mtr clipped direct to joists, the last 3mtr is under capping at plaster depth.

I would like the circuit to manage a max current of 32amps but my feeling is it may fall short in 6mm.

What do you make the max current capacity pls?

 

[big-all]

any insulation on or under it in the loft!!

ps i dont know the answer

 

[Lectrician]

How can you say 'space factors observed'?

You need to tell us what other circuits are grouped in the trunking/on the tray, as this derates the cable. You can't have already derated it.

Without looking at the regs, I think 6mm T+E clipped direct, on it's own will be 35 amps. With grouping factors applied to your 32amp load you will need a cable that can carry 40-50amps I would guess. So 6mm is too small.

I am going by judgment though - you need to check this.

 

[Aragorn84]

6mm on its own clipped direct will handle 47A

http://www.tlc-direct.co.uk/Technic...230&length=10&submit=Calculate+Min+Cable+Size

See there for the derating factors.

 

[ericmark]

Depending on type of cable you have not said if 70 deg or 90 deg cable likely 47A or 58A but de-rated by other cables and again you don't say what other cables and what current they carry.

But can't see a problem with 32A

However a 70deg cable running with 90 deg cables could in theory have a Zero current ratting when bunched with them so you do need to work it out for your installation.

 

[oldman2]

No insulation involved at all.
House wiring cable
Conductor temperature 70°C max

Space factor of 45% in mini trunking not exceeded.

Cables in 40x40 trunking are 2 x 6mm, 3 x 2.5mm and 2 x 1.5mm.
Area of cables 385, avalable area is 1300 so 45% would be 585 max.

The tray has many other circuits on it, perhaps 10 or more but some leave the tray well before the 6mm in question does.
The cables listed for the trunking above do continue alongside the 6mm all the way along the run.

I was hoping to use this as a 32amp radial but it looked to me like its going to get derated to a 20amp radial instead.

 

[Lectrician]

Space factor of 45% in mini trunking not exceeded.

This has no bearing on the cable selection on calculations.

The cables are still grouped.

The spacing factors are not there for circuit design in terms of current carrying capacity.

6mm will carry 47amp clipped direct (on your tray, spaced from other cables). It will carry less than this in trunking, and even less if in trunking and grouped with other circuits.

 

[oldman2]

Space factor of 45% in mini trunking not exceeded.

This has no bearing on the cable selection on calculations.

The cables are still grouped.

The spacing factors are not there for circuit design in terms of current carrying capacity.

6mm will carry 47amp clipped direct (on your tray, spaced from other cables). It will carry less than this in trunking, and even less if in trunking and grouped with other circuits.

Thanks, the cable is already selected and installed, so its a bit of a retrospective design.
I was taught many years ago not to over fill the trunking or it would derate the current capacity of the cables enclosed, thats why I gave the info that the spacing factor had not been exceeded.

So its less than 47amps for this run but how to work out the exact capacity?

I'm very old school and would in the distant past have looked at the cable run, checked the volt drop for bunched or clipped direct for the length and if less than 6v at the current rating I wanted to fuse it for that would be it providing roofing insulation didnt need to be taken into account.
In fact a lot of the time it could be done by experience without needing to consult tables, but times change and i am retired

 

[ban-all-sheds]

Voltage drop is not affected by grouping factors...

 

[oldman2]

Perhaps I didnt word that well, I meant it to mean we only usually really had 2 methods to use in the calc, either clipped direct or enclosed which would affect the current capacity of the cable.

 

[oldman2]

So is it not possible to give a current capacity for this cable run?

Its less than 47amps but is it less than 32amps.

How would you work it out?

 

[Aragorn84]

Try this:

http://www.tlc-direct.co.uk/Technic...230&length=10&submit=Calculate+Min+Cable+Size

From having a quick prod, 6mm in conduit can only handle 32A

Any grouping factors will derate it further and require a larger cable or less load.