cpc sizing

[oxocube]

Table 54.7 states that cables 16mm or smaller shall have the same size line and cpc.

So why does T&E have a reduced size cpc?

 

[telectrix]

i think that is more for distribution cables as opposed to circuit cabling.

 

[oxocube]

Don't think so, heres what it says.

"543.1.1 The csa of every protective conductor, other than a protective bonding conductor, shall be

(i) calculated ........543.1.3 (adiabatic)
or
(ii) selected ..........543.1.4 (table 54.7)

Very strange.

Edit - missed out 'or' as pointed out by telectrix

 

[IQ Electrical]

The reduced CSA of the CPC should be confirmed as adequate by the adiabatic equation.

It's normally far more cost effective to calculate a CPC CSA this way-imagine what happens when you start increasing line & neutral CSA for volt drop and grouping factors-the CPC CSA will have to rise in proportion if selected from 54.7

 

[i=p/u]

rule of thumb is cpc same size as line upto 16mm but the adiabiatic can save you few bob

 

[oxocube]

The reduced CSA of the CPC should be confirmed as adequate by the adiabatic equation.

So that would mean whenever T&E is used the adiabatic should be applied to each length of cable.

I'm sure there must be another reason, perhaps lost in the sands of time.

 

[IQ Electrical]

So that would mean whenever T&E is used the adiabatic should be applied to each length of cable.

I'm sure there must be another reason, perhaps lost in the sands of time.

Oh the reason will be cost.

From Guidance Note 5:

"Except for 1mm, twin & earth cables do not comply with Table 54.7 as the protective conductor is smaller than the live conductor. For circuits employing these cables, the adequacy of the protective conductor must be checked by using the adiabatic equation."

 

[telectrix]

Don't think so, heres what it says.

"543.1.1 The csa of every protective conductor, other than a protective bonding conductor, shall be

(i) calculated ........543.1.3 (adiabatic)
(ii) selected ..........543.1.4 (table 54.7)

Very strange.

OK. 543.1.1., csa shall be (i) calculated in accordance with 543.1.3 OR (ii)selected in accordance with 543.1.4

i understand this to mean that it can be calculated with the adiabatic, or use table 54.7

 

[IQ Electrical]

OK. 543.1.1., csa shall be (i) calculated in accordance with 543.1.3 OR (ii)selected in accordance with 543.1.4

i understand this to mean that it can be calculated with the adiabatic, or use table 54.7

That's right.

 

[oxocube]

From Guidance Note 5:

"Except for 1mm, twin & earth cables do not comply with Table 54.7 as the protective conductor is smaller than the live conductor. For circuits employing these cables, the adequacy of the protective conductor must be checked by using the adiabatic equation."

I bet there ain't many sparks that know that, or have ever done that or ever will do that.

 

[telectrix]

too right. you generally assume that it's adequate. however, the lower csa of the cpc in T/E will mean a higher resistance, thus a lower fault current, chicken and egg .

 

[Engineer54]

Well T&E in it's various forms has held it's own and hasn't done too bad over at least the 70+ years or so it's been around then!! ....lol!!!

 

[i=p/u]

cpc is only designed to take massive currents but very short time

 

[telectrix]

true. it's also designed to fit in that tight space between the line and neutral conductors in T/E cable. now that's the clever part, threading it through.

 

[ringer]

So how many cases do you know of where the adiabatic has been used to verify the cpc size in a length of T&E and it has come out as being inadequate? And what is the solution? Rip out your install and fit 4mm instead of 2.5mm then do the calculation again? Or run a length og G/Y single alongside the T&E?

How many people on here actually use the adiabatic on a day to day basis?