Discuss Sizing SWA Armoured for CPC in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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claret73

Something I haven't really done, so am I correct with the Maths? Just looking to reduce costs & use 2 core against 3.

Found the Table from the Sticky. Anyhow 10mm SWA is labelled Red & the footnote states the Armoured would not satisfy as the CPC (but maybe by Calculation). So...

2 Core SWA 90deg XLPE, 55m Run Approx

Using the K Values in the above 'Sticky Table' to find Copper equivalent:
K2/K1xS
46/143x10mm=3.2mmsq (K2 being SWA, K1 Factor for 10mm Line in 2 Core SWA x 10mm Line conductor)

Is this right? Regs is K1/K2xS

Ze TNS=0.8 (Not measured using default for calcs)
R1= 1.83
R2= 6.0 (Found from a Link to a Table for resistance of SWA)

Zs= Ze+ (R1+R2)
Zs= 0.8 =(1.83/1000x55mx1.28 + 6.0/1000x55m)
ZS= 0.8 + (0.12+0.33)
ZS= 1.25ohms

Fault Current = 230/1.25=184A
Log Log disconnects 0.1s

Adiabatic:
S=Sqrt (184sqx0.1)/46 = 1.26mmsq

So as this is less than the K Factor calc for the armoured I can use the Armoured as CPC??? Think I need a Vorderman...& is there an less longwinded way to find this out?
 
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Engineer54

You obviously didn't read the whole thread or you would have found this table, and saved you a lot of head scratching...lol!!!

In essence, your 10mm Does comply, in fact all 2 core cables comply up to 95mm...


Here is an updated table that i have been using for several years now on SWA compliance, for using the armour as your CPC

Figures in brackets, ... DO NOT COMPLY.



Table for 70[SUP]0[/SUP]C Thermoplastic PVC SWA cables.


Conductor CSA
Minimum CSA of SWA to meet 54G
CSA of armour 2 core
CSA of armour 3 core
CSA of armour 4 core
1.5
3.4
15
16
17
2.5
5.7
17
19
20
4
9.0
21
23
35
6
13.6
24
36
40
10
22.6
41
44
49
16
36.1
46
50
72
25
36.1
60
66
76
35
36.1
66
74
84
50
56.4
74
84
122
70
79.0
84
119
138
95
107.2
122
138
160
120
135.3
(131)
150
220
150
169.2
(144)
211
240
185
208.6
(201)
230
265
240
270.6
(225)
(260)
299
300
338.3
(250)
(289)
(333)
400
403.9
(279)
(319)
467


Table for 90[SUP]0[/SUP]C Thermosetting SWA cables operating at 70[SUP]0[/SUP]C.


Conductor CSA
Minimum CSA of SWA to meet 54G
CSA of armour 2 core
CSA of armour 3 core
CSA of armour 4 core
1.5
3.4
16
17
18
2.5
5.7
17
19
20
4
9.0
19
21
23
6
13.6
22
23
36
10
22.6
26
39
43
16
36.1
41
44
49
25
36.1
42
62
70
35
36.1
62
70
80
50
56.4
68
78
90
70
79.0
80
90
131
95
107.2
113
128
147
120
135.3
(125)
141
206
150
169.2
(138)
201
230
185
208.6
(191)
220
255
240
270.6
(215)
(250)
289
300
338.3
(235)
(269)
(319)
400
451.0
(265)
(304)
452
 

Murdoch

Regular EF Member
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For the difference in price, plus you can almost guarantee a better earth using the 3rd core, its 3 core every time for me.
 

johnboy6083

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Can i point out that the table that E54 has kindly posted, uses table 54G as its reference. Just because a certain size SWA doesnt comply with the table, it MAY comply by calculation.if the cost difference is high, or you are required to confirm a size in an EICR, then calcualtion is king.
 
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claret73

You obviously didn't read the whole thread or you would have found this table, and saved you a lot of head scratching...lol!!!
I didn't!!! Ahem! Thanks for putting that up! I'm Printing it off...I did find something like that though elsewhere...
 
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claret73

For the difference in price, plus you can almost guarantee a better earth using the 3rd core, its 3 core every time for me.
It's about £40-50 difference, so at £1 P/m it should justify itself...Thanks!
 

Guitarist

Regular EF Member
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5,250
Location
Norfolk
Although we should always lay swa correctly, and not assume it can just be dumped in the ground, Just remember that if you are using the armour as the cpc you will need to make damn sure the cable is protected from anything sharp penetrating the outer sheath, thus causing corrosion. Either follow the guidelines regarding sand, depth etc., or use ducting at a suitable depth (my personal choice every time). Unless there is a huge difference in price, I always prefer using 3-core.
 
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claret73

Thanks Guitarist. This is that long winded Outbuilding Job...It's clipped direct on show, but will go with 3 core using the 3rd as a cpc. Ta!
 
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Engineer54

Can i point out that the table that E54 has kindly posted, uses table 54G as its reference. Just because a certain size SWA doesnt comply with the table, it MAY comply by calculation.if the cost difference is high, or you are required to confirm a size in an EICR, then calcualtion is king.

Those equations have already been conducted in this pair of tables, which is why it looks totally different to the table Lenny posted. In that table you need to do all the calculations yourself on most of the tabled cables....
 

johnboy6083

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Those equations have already been conducted in this pair of tables, which is why it looks totally different to the table Lenny posted. In that table you need to do all the calculations yourself on most of the tabled cables....
i dont think they have mate. ive just done a quick calc using 185 cable on a 400A fuse. The i2t of the fuse is 1414000. so the adiabatic comes out at at 25.85mm2 for the armour. the table you posted says that it will not comply, but the calc i have used says it will.

the info i got was from http://ep.mersen.com/en/pdf/GP025_BS88Catalog.pdf

as far as i can see, the table uses table 54G as its reference, which is used if calculation is not used
 
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Engineer54

Can't seem to get on to your link at the moment, i'll try again later....
 
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claret73

So, as per my calcs I did for the cable in my original post. Are my calcs correct?? Have I covered all I need to assume I could use the armoured?
 

johnboy6083

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You dont need to convert to copper equivalent. Just use the adiabatic with the K factor for steel armour, and the resulting size will be the size in mm squared of the SWA. You can get SWA data from the eland website.

Its only worth converting to copper equivelant of you are going to use the SWA as a bonding conductor.

Its worth mentioning that this is not reccomended in a TNC-S system as in the event of a failed supply neutral, this can cause overheating of the cores of the cable which the SWA protects.
 

ackbarthestar

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Ze TNS=0.8 (Not measured using default for calcs)
R1= 1.83
R2= 6.0 (Found from a Link to a Table for resistance of SWA)

Zs= Ze+ (R1+R2)
Zs= 0.8 =(1.83/1000x55mx1.28 + 6.0/1000x55m)
ZS= 0.8 + (0.12+0.33)
ZS= 1.25ohms

Fault Current = 230/1.25=184A
Log Log disconnects 0.1s
Just a couple of points
1/ I would use a measured value of Ze rather than using the worst case

2/ Zs = Ze + R1+R2 = Ze+ ((1.83+6)/1000) * 1.28 * 55 = 1.35 ohms

3/ I assume you're are using a B32A breaker to cover the cable and installation.
 
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claret73

Can I ask why you suggest a 32A? The install in the outbuilding will have 51A Total demand so MCB at Head end I've selected at 63A. The figures you recalculated were not my R1+R2 but the resistivity figures of the cable size & Armour & I did not apply the 1.28 factor to the SWA as this isn't grouped.

Might be overcomplicating my Maths?
 

ackbarthestar

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I applied your figures. I assumed that you had used a multiplier of 1.28 to make adjustments for ambient and operating temperature of the SWA CPC.

However, that's easily checked.

Now an earth fault at the intake of the new building, albeit slight, will mean that your cable is unprotected with a B63A breaker. since you will need a resistance of less than 0.73 ohms.

So if you use a Ze of 0.8 ohms plus 0.55 ohms for the resistance of your cable as your starting figure you will have 1.35 ohms.
From app4 fig 3A4 your 63A breaker will take roughly 40secs for a disconnection.

I think placing a B63A RCBO at the front end would be your only option using your method of design, but as others have stated its not what I would regard as slick.
 
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