Using adiabatic equation for CPC on SWA

[Michaelwgroves]

I want to use the armour as my CPC but need to check it complies. The cable will be run from a SP Distribution panel to CU.
As per Selection and Erection Guidance Note 1 table D10a I can see a 10mm XLPE SWA requires a 31.1mm csa to comply, but only has 26mm on 2-core.
20171231_114229828_iOS.jpg - https://1drv.ms/u/s!ApQQCRGg1ka_gvxxrqQ0QslTR-xaHg
So the only way I might be able to use this is to calculate using adiabatic equation.

Is my calculation correct?

Voltage = 230v
Ze = 0.09 (to be verified, taken from old EIC)
I = 230/0.09 = 2,555A
I2 = 6,528,025
t = (Fig 3A1, 100A BS1361 Type 2a > 1,450A) = 0.1s
I2 x t (6,528,025 x 0.1) = 652,802
sq Root 652,802 = 807
k = 46 (TABLE 54.4 steel)
S = 807/46 = 17mm

So looking at table D10a above we can see 10mm XLPE SWA has csa of 26mm, but as I only need 17.5mm I will be fine ?

On a similar note, Table 4E4A advises 10mm 2-core SWA capacity in ducting is 71A
20171231_140220034_iOS.jpg - https://1drv.ms/i/s!ApQQCRGg1ka_gvxzxVXvCVCdScy83Q

But manufacturers notes say in duct I can run their cable up to 89A, I assume in this instance I could use manufacturers notes over Table 4E4A as Table 4E4A is only generic ?
SWA Capacity.pdf - https://1drv.ms/b/s!ApQQCRGg1ka_gvxyaNUN1GXpn1Ob8w

In reality I'll protect with either 50A or 63A MCB, but it's possible I might want to run at 80A in the future. Now I know the cable spec so I know it's safe; I can also leave cable spec with EIC, but is there anything else I should do to prevent another electrician in the future condemning the cable?

 

[Octopus]

I would run 3 core SWA .... never happy to rely on the armour as the CPC................

 

[Pete999]

I want to use the armour as my CPC but need to check it complies. The cable will be run from a SP Distribution panel to CU.
As per Selection and Erection Guidance Note 1 table D10a I can see a 10mm XLPE SWA requires a 31.1mm csa to comply, but only has 26mm on 2-core.
20171231_114229828_iOS.jpg - https://1drv.ms/u/s!ApQQCRGg1ka_gvxxrqQ0QslTR-xaHg
So the only way I might be able to use this is to calculate using adiabatic equation.

Is my calculation correct?

Voltage = 230v
Ze = 0.09 (to be verified, taken from old EIC)
I = 230/0.09 = 2,555A
I2 = 6,528,025
t = (Fig 3A1, 100A BS1361 Type 2a > 1,450A) = 0.1s
I2 x t (6,528,025 x 0.1) = 652,802
sq Root 652,802 = 807
k = 46 (TABLE 54.4 steel)
S = 807/46 = 17mm

So looking at table D10a above we can see 10mm XLPE SWA has csa of 26mm, but as I only need 17.5mm I will be fine ?

On a similar note, Table 4E4A advises 10mm 2-core SWA capacity in ducting is 71A
20171231_140220034_iOS.jpg - https://1drv.ms/i/s!ApQQCRGg1ka_gvxzxVXvCVCdScy83Q

But manufacturers notes say in duct I can run their cable up to 89A, I assume in this instance I could use manufacturers notes over Table 4E4A as Table 4E4A is only generic ?
SWA Capacity.pdf - https://1drv.ms/b/s!ApQQCRGg1ka_gvxyaNUN1GXpn1Ob8w

In reality I'll protect with either 50A or 63A MCB, but it's possible I might want to run at 80A in the future. Now I know the cable spec so I know it's safe; I can also leave cable spec with EIC, but is there anything else I should do to prevent another electrician in the future condemning the cable?

I want to use the armour as my CPC but need to check it complies. The cable will be run from a SP Distribution panel to CU.
As per Selection and Erection Guidance Note 1 table D10a I can see a 10mm XLPE SWA requires a 31.1mm csa to comply, but only has 26mm on 2-core.
20171231_114229828_iOS.jpg - https://1drv.ms/u/s!ApQQCRGg1ka_gvxxrqQ0QslTR-xaHg
So the only way I might be able to use this is to calculate using adiabatic equation.

Is my calculation correct?

Voltage = 230v
Ze = 0.09 (to be verified, taken from old EIC)
I = 230/0.09 = 2,555A
I2 = 6,528,025
t = (Fig 3A1, 100A BS1361 Type 2a > 1,450A) = 0.1s
I2 x t (6,528,025 x 0.1) = 652,802
sq Root 652,802 = 807
k = 46 (TABLE 54.4 steel)
S = 807/46 = 17mm

So looking at table D10a above we can see 10mm XLPE SWA has csa of 26mm, but as I only need 17.5mm I will be fine ?

On a similar note, Table 4E4A advises 10mm 2-core SWA capacity in ducting is 71A
20171231_140220034_iOS.jpg - https://1drv.ms/i/s!ApQQCRGg1ka_gvxzxVXvCVCdScy83Q

But manufacturers notes say in duct I can run their cable up to 89A, I assume in this instance I could use manufacturers notes over Table 4E4A as Table 4E4A is only generic ?
SWA Capacity.pdf - https://1drv.ms/b/s!ApQQCRGg1ka_gvxyaNUN1GXpn1Ob8w

In reality I'll protect with either 50A or 63A MCB, but it's possible I might want to run at 80A in the future. Now I know the cable spec so I know it's safe; I can also leave cable spec with EIC, but is there anything else I should do to prevent another electrician in the future condemning the cable?

 

[Toneyz]

Would it comply with volt drop at 80A? or whatever your design current is?

 

[Leesparkykent]

Adiabatic looks ok but think you should be usink the value of 51 as K. Even though the cable maybe XLPE and rated for 90~C the connections/switch gear will probably be rated for 70~C.

10mm 2core installed reference method D has a CCC of 60A. The manufacturers data will again probably be for XLPE at 90-C....where your connections etc will be rated for 70~C. You are using the wrong table, you should be using 4D4A not 4E4A.

 

[Richard Burns]

You are using the values for an XPLE cable run at 90°C, this is unlikely to be suitable for the connections into the CU as it is likely to be designed to run only up to 70°C.
If you look at table 10b that is for XPLE cable run at 70°C you will see that it complies.
However your calculation method is correct though taking into account that the time will be less than 0.1s and the I²t value for the fuse should be used in preference. Also if the cable is at 70°C then the K value would be 51.
So long as the cpc is not also a bonding conductor you will be OK.
But Lee got there first!

 

[Michaelwgroves]

This is really helpful, guys thanks for detailed explanations.
that makes sense using 70°C.
So I now know XLPE @ 70°C complies, and I should use table 4D4A. But purely for the sake of improving my understanding where can I find the I²t value for the fuse?

 

[Leesparkykent]

This is really helpful, guys thanks for detailed explanations.
that makes sense using 70°C.
So I now know XLPE @ 70°C complies, and I should use table 4D4A. But purely for the sake of improving my understanding where can I find the I²t value for the fuse?

Manufacturers data or the BYB.

 

[Michaelwgroves]

What's BYB ?

 

[Leesparkykent]

What's BYB ?

Big yellow book (BS 7671)

 

[Dave OCD]

Regs book - 'Big yellow book'.

Edit - Drat beaten to it by Lee.

 

[Michaelwgroves]

I can't find anything in BYB that would give me more accuracy than Fig 3A1, which only goes down to 0.1s ?

 

[Richard Burns]

You should be able find from the manufacturers something like the attached for the fuse you have.
This one would give an I²t of 100,000 for a 100A BS88 fuse.

 

[Spoon]

It's been ages since I've seen that PDF on here hate. A was going to mention it...

 

[Michaelwgroves]

Okay, found something similar for Bussmann, but I don't understand how to read chart. You get 100,000 (10 power 4) I follow bottom row until I get 100A, then up, but where to stop?
It looks more to me that it should be 10 power 5 as there is where thick black line is?