Cable CSA?

Crosswire · Jul 9, 2010

OK boffins, help me out ........

Called to job where there was no bonding to main incoming gas to a block of flats, and the client required bonding to be installed.

I knew it had to be large, so consulted table 54.8 in order to size it, based on the size of the neutral. Which led me to my problem.......

How do I size the neutral?

This is what I did : opened 3 phase busbar chamber and wrapped a piece of string around the neutral, then measured the string. It was 90mm, which I took to be the circumfrence of the cable (including insulation).

Thinking back to my school days (ok I looked it up) I found these formulas:

circumference = pi X diameter

area = pi r2

Therefore I could find the area of the circle, and presumably the csa of the neutral. So (transposing the formula) 90/ 3.14 = 28.66
The diameter is 28.66 , so the radius is 14.33

so the area is 3.14 x 14.33 squared

3.14 x 205. 3489 = 644.795 !!!!

Now I'm pretty sure the csa of the cable isn't 644mm !

( Looked more like 70mm) Where have I ballsed it up?

Anyone ?

philip962 · Jul 9, 2010

What system is it tt tns tncs main bonding off the top of my head is 16mm but the osg will tell you will check mine and get back to you

xraytek · Jul 9, 2010

Crosswire said:
Now I'm pretty sure the csa of the cable isn't 644mm ! ( Looked more like 70mm) Where have I ballsed it up?

The problem is that you have added the insulation as part of your calculations.

I prefer to use a micrometer to measure the diameter of a single strand then use 0.79 x D x D x number of strands.

Crosswire · Jul 9, 2010

1) system is TNS - but not sure how this affects bonding size as is it based on size of neutral cable

2) not sure that the insulation would make a massive difference as is only about 1mm thick, so calcs will be based on 89 mm, not 90 mm. Unfortunatley can't count strands as supply is in use and can't disconnect

Good idea tho........

pushrod · Jul 9, 2010

Crosswire said:
1) system is TNS - but not sure how this affects bonding size as is it based on size of neutral cable

2) not sure that the insulation would make a massive difference as is only about 1mm thick, so calcs will be based on 89 mm, not 90 mm. Unfortunatley can't count strands as supply is in use and can't disconnect Good idea tho........

no, if the insulation was 1mm thick then you would calculate the diameter(28mm) and take double that off, allowing for the insulation on both sides of the diameter. TBH i reckon that the insulation is probs in the region of 50% of the diameter and there by lies the discrepancy

allowing for error in measuring and the fact that you are dealing with a stranded conductor(tiny spaces between strands) not a solid one you could well be dealing with 70 mm² or 95mm²

timitds · Jul 9, 2010

mmm not sure about the insulation thing but even if you halve the the circumfrence stilll gives 160 ish
i was gonna say do you not square root the answer (cos i cant find my folder

) giving 25 ish but that seems way off if our thinking 75 (still at coll not got to 3 phase yet

)

would love someone to answer it though bugging me now

TPES · Jul 10, 2010

Ha ha ha, just reading that you totally lost me... And you call us boffins! lol

Sounds more like you d man.

GStueyXR · Jul 10, 2010

Crosswire said:
OK boffins, help me out ........

Called to job where there was no bonding to main incoming gas to a block of flats, and the client required bonding to be installed.

I knew it had to be large, so consulted table 54.8 in order to size it, based on the size of the neutral. Which led me to my problem.......

How do I size the neutral?

This is what I did : opened 3 phase busbar chamber and wrapped a piece of string around the neutral, then measured the string. It was 90mm, which I took to be the circumfrence of the cable (including insulation).

Thinking back to my school days (ok I looked it up) I found these formulas:

circumference = pi X diameter

area = pi r2

Therefore I could find the area of the circle, and presumably the csa of the neutral. So (transposing the formula) 90/ 3.14 = 28.66
The diameter is 28.66 , so the radius is 14.33

so the area is 3.14 x 14.33 squared

3.14 x 205. 3489 = 644.795 !!!!

Now I'm pretty sure the csa of the cable isn't 644mm ! ( Looked more like 70mm) Where have I ballsed it up?

Anyone ?

All your calcs are correct. The problem then must be your measurement.

GStueyXR · Jul 10, 2010

Not to forget though, that the conductor is stranded, and the inulation is massive part of that circumference measured.

pushrod · Jul 10, 2010

GStueyXR said:
Not to forget though, that the conductor is stranded, and the inulation is massive part of that circumference measured.

Or you could have just said that Pushrod got it right in post # 5

mcspoo · Jul 10, 2010

I'm bored sitting here on the nightshift!

Got the regs book out and confirmed what I thought. It doesnt really matter if its 70mm2 or 90mm2 or bigger. You only need to install a 25mm2 Max. (544.1.1)

andyb · Jul 10, 2010

mcspoo said:
I'm bored sitting here on the nightshift!

Got the regs book out and confirmed what I thought. It doesnt really matter if its 70mm2 or 90mm2 or bigger. You only need to install a 25mm2 Max. (544.1.1)

That's correct mcspoo, it's a tns supply so nothing to do with the neutral.

Also 544.1.1 talks about a main protective bonding conductor having a csa not less than half the csa required for the earthing conductor (not actual csa)

Therefore you could do an adiabatic equation to determine the size required, half that and then go up to the next csa. But not less than 6mm

Crosswire · Jul 12, 2010

Thanks for the input guys,
sorry not been on thread but am on holiday in remote part of Ireland only just got on a comp.

Pushrod : thanks for the post and I do accept that I under estimated the insulation - I should have subtracted from the diameter and not the circumfrence - Boy is my face red!

However, the cable in question is an swa, not a double insulated meter tail as per your pic. Also I measured inside the busbar chamber where only one layer of insulation was present, so I don't feel insulation can account for the discrepancy.

Granted, there are gaps between the strands, but can this really account for an answer being 10 times what it should be?

Gstuey XR : logic would seem to dictate that, but all I did was measure a piece of string. I'm pretty sure I didn't confuse 9mm with 90 mm !

Andy b : didn't bring my Regs book on holiday with me so can't look it up, but was pretty sure that the main bonding was based on neutral size (e.g. no less than half for low sizes, changes as you go get higher). Please correct me if I am wrong.

Anyway, I've sized the bonding at 25mm and will do the job when I return next week. Just bugging the hell out of me is all.........