sizing SWA armour as cpc using adiabatic......

[brman]

I'm doing a mod for a 3 phase circuit already installed. The only trouble is I can't work out why the existing cables are as they are so i am doubting my sums.

Existing final circuit.
3 pole memshield 2, D32 rated feeding 10mm[SUP]2[/SUP] 4 core swa with a seperate 16mm[SUP]2[/SUP] G/Y as cpc.
Measurements at the DB are max 5.6kA single phase. Zs 0.04

Cable run is about 20m so I don't think voltage drop is an issue. So why has someone fitted an additional 16mm[SUP]2[/SUP] cpc?

If I use table 54.7 I need a cpc of 10mm[SUP]2[/SUP].

If I use the adiabatic if find:
1) max Ipf across phases is 1.73x5.6=9.7kA
2) let through energy for the mem D32 MCB is 43000 at 10kA according to their specs.
3) 43000 A[SUP]2[/SUP]s and a k of 143 (assuming xlpe) gives me a minimum cpc csa of 1.45mm[SUP]2
[/SUP]
So if I wanted to use a seperate cpc I would have gone for either 10mm[SUP]2[/SUP] if using the table values or 4mm[SUP]2[/SUP] (min unprotected) if using the adiabatic.

In fact I would probably have used the pyranha nuts and the armour of the swa as 10mm2 4 core has a armour csa of 43mm[SUP]2[/SUP] which is much more than 8 times 1.45mm[SUP]2[/SUP]

Have I got my sums right as someone spent good money putting in 16mm2 so I am thinking there must have been a reason?

The only thing I can think is that using the adiabatic simply with a 5.6kA ipf and t=0.1 gives 15.4mm[SUP]2[/SUP]
but a) which select a size bigger than table 54.7 requires and b) the 5.6kA is the single phase fault current.

Plus, why 10mm[SUP]2 [/SUP]SWA? Voltage drop at 32A would be about 2.5% I think even if I used 4mm[SUP]2[/SUP]and Zs should not be a problem either, even with a type D breaker.

My job is to put a spur on this circuit but am not keep to match the cable sizes already used for no good reason.........

 

[telectrix]

you can't use the phase/phase fault (PSCC) current in the adiabatic, as you are looking at a fault of negligible impedance between phase and Earth. (PEFC).

 

[brman]

you can't use the phase/phase fault (PSCC) current in the adiabatic, as you are looking at a fault of negligible impedance between phase and Earth. (PEFC).

A good point, one which I should have realised given I am looking at sizing the cpc......:rolleyes4:

apart from that brain fart is there anything else wrong with my sums?

 

[Richard Burns]

Your calculations look right using the figures you have given.
Possibilities are that it is sized as a bonding conductor or that someone had some 10mm and decided to use it or that it was originally employed for a greater load.

 

[telectrix]

you have to use manufacturers data for the calculation as the trip time is out of range of the time/current graphs in BS7671. using that figure of 43000, your 1.45mm is correct. however, as richard said, if used as a bonding conductor, 10mm would be required. maybe they only had 16mm to hand.

 

[snowhead]

Have you allowed for the "We always fit 16mm earth" factor.?

 

[telectrix]

" to boldly go where other sausages have gone before"

 

[brman]

Thanks Guys, you have confirmed I am not going mad at least

I did think it might have been sized for bonding but it isn't used for it. I think the "what we had in the van" might have some truth.....

I've gone back and quoted for a new circuit on a new breaker as it is only a few quid more than messing with the old one when I take my time into account. Plus it has the bonus that I am not trying to guess what the original design intent was!

 

[Engineer54]

Rule of thumb. ....All SWA cables ((be they of PVC or XLPE insulation) from 1.5mm 2 Core, up to 95 mm 4 Core, the SWA is of a suitable CSA to be used as that cables/circuit CPC!!

 

[rmc86uk]

E54 one of the lads at work tried to tell me that depending on the length of run you cannot use the SWA as the cpc. I thought he was talking rubbish and from your reply above it sounds like he was! This was a 1.5 4 core.

 

[Bobster]

Length will come in as a factor with keeping your earth loop resistance down. However on a 1.5 4c it must be a bloody long run!

 

[rmc86uk]

About 90 metre I think.

 

[cobrauk]

Found this which may be of use to some http://www.earthingnuts.co.uk/pdf/pvc_xlpe.pdf

 

[Engineer54]

Found this which may be of use to some http://www.earthingnuts.co.uk/pdf/pvc_xlpe.pdf

Jesus, the times that old table rears it's ugly head!! At best this table is confusing, ...worst most of values also need calculating as many that state do not comply actually DO comply!!

Throw this table away is my advice, and use this table for SWA CPC compliance with no need for any futher calculations. This table is NOT intended for use to indicate Bonding compliance

Blue = min CSA of cables SWA to comply.

(Red) = Cable does 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​

​

​

 

[Engineer54]

E54 one of the lads at work tried to tell me that depending on the length of run you cannot use the SWA as the cpc. I thought he was talking rubbish and from your reply above it sounds like he was! This was a 1.5 4 core.

He is actually right, length is a depending factor on long runs, but for most installation cable runs, the SWA used as the CPC should be more than adequate...