Reference Method ?

[ian932]

What reference method would you apply for derating cables fitted in the following constructions.

Starting from the outside inwards -concrete block, 100mm cavity, 60mm kingspan fixed to inner concrete block leaving a 40mm air gap in the cavity, cable fixed to inner block with metal capping, 42.5mm kingspan/plasterboard on dabs.

Next one is the roof starting from the outside again - tile, paper, 50mm air gap, 160 kingspan between rafters, 15mm void with cable in that, 42.5mm kingspan/plasterboard (rafters are 225mm)

I reckon first one could be classed A I think 103 would be OTT as the cable will discharge heat through the block. The Second one looks like it may be 103.

What do you guys think?

 

[green-hornet]

The simple way of recording reference method is to take the longest run only.

If 80% of the cable is clipped direct, then go with that.

Other derating values can be added.

Cables could be run in almost all reference methods in one run! but it would be hopeless to assume one is better than the other.

I always use a rule of thumb, ther longest part of the cable fixed is my reference method.I then add all derating values as per the BOOK and bobs your uncle.

 

[Andy™]

wouldnt using the method that requires the most de-rating make more sense - at least any future work, you can check the (AWOL!) certificate and know if it goes through insulation etc and something that could effect i more than ma have been expected

 

[ian932]

The simple way of recording reference method is to take the longest run only.If 80% of the cable is clipped direct, then go with that.

Other derating values can be added.

Cables could be run in almost all reference methods in one run! but it would be hopeless to assume one is better than the other.

I always use a rule of thumb, ther longest part of the cable fixed is my reference method.I then add all derating values as per the BOOK and bobs your uncle.

I get what your saying, but I'm not trying to figure out what I need to put on the form, I'm trying to establish whether I need increase my cable sizes, so what reference method would you class these 2 as?

 

[green-hornet]

To calculate cable size you have to have a design current ib

you have to have supply voltage and other factors let me give you an example.

supply voltage = 230

Power factor =1

ib (design current) = 25.7 amps

Protective devise rating =MCb type B

In =32 Amps

Installation method single core or multy core cable direct in masonory having a thermal resisivity not greater than 2K.m/W- without mechanical protection - method c

Then you apply the correction factors

Iz = current carrying capacity of the cable

Cg grouping

Ca ambient temp

Ci insulation

Can you see where I am going?

Protective devise factor

Given all the corrections I end up needing a 4mm cable but I am far too drunk to explain fully, other than to say the reference method is 80% or the greatest run of cable.

Correction factors have little to do with reference methods of installation, other than to allow you to calculate the size of cable required for a given purpose.

 

[ian932]

To calculate cable size you have to have a design current ibyou have to have supply voltage and other factors let me give you an example.

supply voltage = 230

Power factor =1

ib (design current) = 25.7 amps

Protective devise rating =MCb type B

In =32 Amps

Installation method single core or multy core cable direct in masonory having a thermal resisivity not greater than 2K.m/W- without mechanical protection - method c

Then you apply the correction factors

Iz = current carrying capacity of the cable

Cg grouping

Ca ambient temp

Ci insulation

Can you see where I am going?

Protective devise factor

Given all the corrections I end up needing a 4mm cable but I am far too drunk to explain fully, other than to say the reference method is 80% or the greatest run of cable.

Correction factors have little to do with reference methods of installation, other than to allow you to calculate the size of cable required for a given purpose.

Cheers Green Hornet, but I was not after a lesson in how to calculate cable sizes. Please read the original post - I'm simply trying to verify what method these 2 forms of construction are classed as, I never mentioned correction factors either all I refered to was derating which is what we need to do if any part of a run is anything other than Method C.

Neither of these construction methods are shown in Table 4A2, which means it is open to some interpretation which is why I was after some fellow sparky opinions.

I think your right about one thing you've had to many of these Guiness DrinkGuiness DrinkGuiness DrinkGuiness DrinkGuiness Drink

 

[green-hornet]

LOL I have your right.

Its always difficult to asses a particular reference method when multiple methods of installation are involved.

If its all in trunking or conduit its easy, but if some of it is in trunking and some of it is clipped direct then which one do you choose?

I personally choose the method that represents the greatest run of cable.

Drunk as I am I would say your first description would be clipped direct, plenty of free air and is not liable to be effected by the insulation.

Second one would be influenced by insulation

So I will say

A = c

B = 103

C = you will tell me off in the morning when I sober up.