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Help with cable calcs

Discuss Help with cable calcs in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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elementps

Regular EF Member
Hi all, just after some help with the following, i have use the cable cals template as seen inthe sticky at top of forum.

Have been asked to price for a sub-main to a new shed/office being installed in garden.
TNCS system at moment, ze unknown, but have used max of 0.35 for calcs
cable run to outbuilding is 30m

Planning to install a 20a radial and 6a lighting circuit so design current max 26a, 32a breaker
Correction factor of 0.9 for burried cable, comes out at 35.55a
so i use a 40a breaker???

from table 4e4a, 4mm burried will carry 43a

vd = (30x26x12)/1000 = 9.36v within limit of 11.5v

max zs for 40a EN60898 is 1.15*0.8 = 0.92
zs = 0.35 + (9.22x30x1.2)/1000 = 0.33 + 0.35 = 0.68

Now here's where I am unsure...

fault current = 230/0.68 = 338.24a

min cpc size = square root of (I2xt)/k

do i use 0.4 or 5 for my t value?

telectrix

Scouser and Proud of It
Respected Member
agreed^^^^ 0.1sec.

elementps

Regular EF Member
Perfect thanks for that, all makes sense now.
So my 3 core 4mm SWA protected by 32a mcb will be fine as a supply.

Now when thinking about the 2 circuits in the out building, will the zs of the supply become the ze. Hopefully the circuit resistance will be low enough to use the earth in the supply to out building, otherwise install a rod for the out building circuits?

Does this sound like the right plan of action?

telectrix

Scouser and Proud of It
Respected Member
use the 3rd core of the SWA as cpc. no problem unless you have extraneous metalwork in the outbuilding.

G

Guest123

Hi all, just after some help with the following, i have use the cable cals template as seen inthe sticky at top of forum.

Have been asked to price for a sub-main to a new shed/office being installed in garden.
TNCS system at moment, ze unknown, but have used max of 0.35 for calcs
cable run to outbuilding is 30m So far, so good.

Planning to install a 20a radial and 6a lighting circuit so design current max 26a, 32a breaker
Correction factor of 0.9 for burried cable, comes out at 35.55a
so i use a 40a breaker??? Falls down a bit here, max demand (Ib), without diversity will theoretically be 26A. A 32A MCB (In), will more than deal with this as, as I have said, no diversity has been applied.

You apply your factors to your (In) of 32A so in your case 32/0.9 = 35.55A. You now select a cable from appendix 4 with a ccc of greater than 35.55A, not increasing the breaker size to deal with it.

from table 4e4a, 4mm burried will carry 43a

vd = (30x26x12)/1000 = 9.36v within limit of 11.5v Again, with your un-diversified (Ib), this is correct

max zs for 40a EN60898 is 1.15*0.8 = 0.92 Alter this for a 32A MCB now
zs = 0.35 + (9.22x30x1.2)/1000 = 0.33 + 0.35 = 0.68 Correct.

Now here's where I am unsure...

fault current = 230/0.68 = 338.24a

min cpc size = square root of (I2xt)/k

do i use 0.4 or 5 for my t value?

Look at the time/current graphs in appendix 4 to see what time the device will operate at with your calculated fault current (338A).

Looks ok to me, apart from the first bit I highlighted regarding your MCB & cable selection.

elementps

Regular EF Member
Thanks Lenny, I think i got myself conused when applying correction factors and thats where i came up with 40a mcb. All makes sense now.

Telectrix, i am using the 3rd core of swa as cpc.

Just trying to establish now the if zs at circuits inside the outbuilding will be acceptble without its own rod?!?!?

Dannyspark

Active EF Member
Thanks Lenny, I think i got myself conused when applying correction factors and thats where i came up with 40a mcb. All makes sense now.

Telectrix, i am using the 3rd core of swa as cpc.

Just trying to establish now the if zs at circuits inside the outbuilding will be acceptble without its own rod?!?!?
You do realise it is completely acceptable to use the sheath of an SWA cable as the CPC? Even if you don't use it as the CPC then it must be earthed. Sorry if stating the obvious.

elementps

Regular EF Member
Yes i understand this, dont worry about it, I know what you meant. thanks anyways

J

Jonny66

Hi elementps,

Its been a while since I’ve done cable calcs, but maybe able to help a little and perhaps revise at same time. Just some thoughts..

Firstly, as Lenny stated your Ib has no diversity. It’s always a good idea to find out what will be plugged into the final circuits. You state shed/office which would suggest to me some form of heating will be used along with IT equipment. The lighting IMO would be nowhere near 6amps more likely around 1 to 2amps in this case. I would use the figure of 26amps for Ib<In<Iz, but use a diversified Ib figure for volt drop calcs.

“from table 4e4a, 4mm buried will carry 43a”
You need to read the notes for this Table. Are your accessories connected to the conductor, suitable for the operating temp of 90[SUP]0[/SUP]C. If not, as in most cases, the Table for 70[SUP]0[/SUP] C armored should be used - giving ccc of 37amps for 4mm2 direct in ground and volt drop of 11mV/A/m.

The volt drop of 9.36v you calculated, although within the 5% for the submain, leaves no headroom for the lighting final circuit (3% = 6.9v). As volt drop is usually the limiting factor I would increase csa to say 6mm2. This would give the submain a vd of 5.7v (using Ib=26amps) obviously this would be a lot smaller using diversity on Ib.

Thermal constraints of cpc as already stated, it is the time taken for the device to operate, at the fault current calculated, usually 0.1sec for MCBs.

As for Zs and earth stake you’ve lost me……….you wont need earth stake if no extraneous parts in shed. The Zs calculated for submain becomes Zdb of ccu in shed. Add to this the R1+R2 final circuit value from Table 9A OSG , with temperature correction and there you go…ie, using Zdb + ((R1+R2)1.2 x L) to calculate each circuits Zs and compare to values in Regs book not OSG.

Hope this helps a little, Im sure any mistakes I've made will be corrected

Jon

elementps

Regular EF Member
Hi jon, thanks, some great points there and very clear.

Just to clarify regarding swa, i used the table for 90degree, as this is what supplier lists as selling, not 70 degree.
So are most accessories rated for 70degree operation?
Would using 90degree cable make an impact?

J

Jonny66

I believe most accessories, con units and switch fuses etc are rated at 70 degrees and as such the 70 degree table should be used. Read the notes next to the 90 degree table.
70 degree swa is also cheaper i think

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