95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fuses

[Rockingit]

Chr!s - wrong sort of maths! Try ignoring the tables and use some trigonometry. The tables aren't designed for this scenario, in fact for certain types of installation methods they are woefully inadequate.

 

[Rockingit]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Seems to me that the first set of touching cables would always be warmer (all other things being equal), so I can't understand why it shouldn't be derated slightly relative to the spaced out cables.

See my jugs of water analogy.

 

[Spoon]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

What's your point?? What sort of heat increases are you actually talking about here, temperature increases that will affect current ratings, that's just nonsense!!

Are we now saying that the temperature increase of a cable doesnt affect the current rating of the cable??? This is just getting a farce....
Just look at the regs EG table 4E4A. lets take 35mm 4 core cable. How this is installed affects the current rating doesnt it.. The max being 162A (in free air...) the min being 115A (direct in ground or in ducting in the ground...) This is due to the heat dissipation of the cable or temperature.
That is why in parallel cables, if touching (and a couple of other factors) you have to de-rate the cable... Single source supply or multiple supplies, they both need de-rating..

 

[i=p/u]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

If equally loaded is key word here.

 

[Rockingit]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Are we now saying that the temperature increase of a cable doesnt affect the current rating of the cable??? This is just getting a farce....
Just look at the regs EG table 4E4A. lets take 35mm 4 core cable. How this is installed affects the current rating doesnt it.. The max being 162A (in free air...) the min being 115A (direct in ground or in ducting in the ground...) This is due to the heat dissipation of the cable or temperature.
That is why in parallel cables, if touching (and a couple of other factors) you have to de-rate the cable... Single source supply or multiple supplies, they both need de-rating..

You're talking about installation method, NOT grouping!

 

[Spoon]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

You're talking about installation method, NOT grouping!

Please note the "This is due to the heat dissipation of the cable or temperature.
That is why in parallel cables, if touching (and a couple of other factors) you have to de-rate the cable... Single source supply or multiple supplies, they both need de-rating.."

This relates to installation methods and grouping

 

[Chr!s]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Chr!s - wrong sort of maths! Try ignoring the tables and use some trigonometry. The tables aren't designed for this scenario, in fact for certain types of installation methods they are woefully inadequate.

This ive got too see, so we have the "Real Heat" and the "Imaginary Heat" to find the total Heat dissipated we now need trigonometry lol?

I think you digginga hole here, you see in the tables we have "r", "x" and "z", what do you think z is?

The tables correlate to the BS for that cable type, what is it you think you know thay dont?

I tend not to use the tables in BS7671, i use the Manufacturers, who in turn use ERA guidance, theuy have differing ccc, but this is inly down to factors applied, if i apply the same factors to BS7671 then i get he same values.

Please explain your Trigonometry though thanks.

 

[Silly Sausage]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

This ive got too see, so we have the "Real Heat" and the "Imaginary Heat"

Haha....j heat, you think you're warm, but in reality you're not!

 

[Rockingit]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

I'm loosing the will to live, now. FORGET THE SODDING TABLES!! (to a point).

Basic principles -
1) the ability of a cable to radiate heat is proportional to its surface area
2) i) The surface area of a round cable is given by pi D (or 2 pi R if you will).
ii) The surface area of a cable is a resultant of pi R2.

So, let's assume we have a solid conductor that is 10mm csa.
area = 10
dia = 3.56824823
circ = 11.209982432795858

therefore, 2 x 10mm csa = 2 x circ = 22.42 mm

If now do the same maths for a conductor of csa 20mm -
dia = 5.046265044040321
circ = 15.853309190424046

Now, as covered in earlier posts, J amount of energy will produce H amount of heat.

H / 22.42 for two conductors is a lower figure to
H / 15.85 for one conductor.

So, the heat generated is exactly the same (H is a constant), but by using two identical conductors you are sharing (thanks to Mssrs Ohm & Kirchoff) the load over a greater surface area of conductor which creates a fundamentally more efficient environment. Because the energy source is the same, there is no potential difference (to use a phrase in a different way) in temperature between them therefore there is no affect on one to the other, they both heat and cool at the same rate.

The radiator effect will, of course, be affected by environmental principles, thermal conductivities etc, so other de-rates may apply, but NOT those for grouping, as they are NOT grouped cables.

 

[Silly Sausage]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

I'm loosing the will to live, now. FORGET THE SODDING TABLES!! (to a point).

Basic principles -
1) the ability of a cable to radiate heat is proportional to its surface area

.............

and also the temperature gradient of the cable to its surroundings.
So two touching objects, at the same temperature, won't be able to dissipate any energy from the contact area, which will lead to a temperature increase over the rest of the surface area of the objects and arguably a temperature rise in the objects themselves.