main tails


Postby livewire » Sat Jul 25, 2009 8:44 am

hi

I know in a domestic environment we're used to a 100A main fuse generally with upto 25mm^2 tails. But what if you're design current Ib is > 100A, say 140A or even 200A? What then? How do we size the tails? Is this upto the supplier?
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Postby kbrownie » Sat Jul 25, 2009 10:00 am

Design current or maximum demand?
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Postby livewire » Sat Jul 25, 2009 5:53 pm

[quote="kbrownie"]Design current or maximum demand?[/quote]

hi KB

that leads me to another question:

I know: Ib < In < Iz

But in relation to which 'current' are the size of the main fuse and tails measured?

In my previous post, I referred to design current (but am not sure to be honest).

So, now I have two questions:

1. Which current do we refer to?

and

2. Do we use a table in the regs for a comparison: table 4D2A p276,
Reference method C, column 6, (nearest value) say?
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Postby sparx » Sat Jul 25, 2009 9:12 pm

Hi Guys,
since 100A is most available for domestic we are talking commercial/industrial.
Mains supplies for 3 Ph. are either 100/200/300A per phase, tails need to carry equal to or greater than fuse rating so since will almost always be surface tails I would use table 4E1A column 9, page 284,
giving 25/50/95mm2 respectively,
regards Sparx
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Postby kbrownie » Sat Jul 25, 2009 9:35 pm

The tails via your cut out will be rated to carry the supply current of the mains fuse this fuse is likely to be rated at either 80 or 100A so tails are likely to be 16 or 25 CSA.

When we design an installation we calculated the design current for each circuit first and then the protective devise size suitable, then calculate any other factors that could effect the current carrying capicity of the cable, this is how we get the cable size of a circuit.

Some circuits ie your socket outlet circuit are of three types only and we need to keep to the permitted area covered, fuse rating and cable size of these types.

Generally I use OSG for my calcs. The table used and reference method would depend on the circumstance, maybe I have to run single cables through conduit in a wall that has thermal insulation installed. So I would look at table 4D1A (page 274) big red book or table 6D1(page 128) in OSG.
This would tell me using Ref Method A if I needed to supply a circuit with say 32A that 4mm CSA could not carry that safely but 6mm CSA can, so that's my choice of cable.
Other factors may need to be taken in to account using the other 3 Cs , ambient temps, grouping, and BS3036 fuses.
So it's not the nearest value to the current we are looking for it is the cable that WILL carry the current, so same current or above.

Your CU may have a 80A main fuse and your board could be loaded up with breakers adding up to 120A but we except that all these circuits are not likely to be fully loaded at any one time. So an amount of diversity can be assumed.
Hope this answers your question well enough.
any querries happy to help.
KB
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Postby livewire » Mon Jul 27, 2009 6:06 pm

[quote="kbrownie"]The tails via your cut out will be rated to carry the supply current of the mains fuse this fuse is likely to be rated at either 80 or 100A so tails are likely to be 16 or 25 CSA.

When we design an installation we calculated the design current for each circuit first and then the protective devise size suitable, then calculate any other factors that could effect the current carrying capicity of the cable, this is how we get the cable size of a circuit.

Some circuits ie your socket outlet circuit are of three types only and we need to keep to the permitted area covered, fuse rating and cable size of these types.

Generally I use OSG for my calcs. The table used and reference method would depend on the circumstance, maybe I have to run single cables through conduit in a wall that has thermal insulation installed. So I would look at table 4D1A (page 274) big red book or table 6D1(page 128) in OSG.
This would tell me using Ref Method A if I needed to supply a circuit with say 32A that 4mm CSA could not carry that safely but 6mm CSA can, so that's my choice of cable.
Other factors may need to be taken in to account using the other 3 Cs , ambient temps, grouping, and BS3036 fuses.
So it's not the nearest value to the current we are looking for it is the cable that WILL carry the current, so same current or above.

Your CU may have a 80A main fuse and your board could be loaded up with breakers adding up to 120A but we except that all these circuits are not likely to be fully loaded at any one time. So an amount of diversity can be assumed.
Hope this answers your question well enough.
any querries happy to help.
KB[/quote]

thanks, KB

So, with a CU with eight circuits say, with a maximum demand (after diversity) of 140A say; then a 100A main fuse with 25mm2 tails will suffice??
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Postby kbrownie » Tue Jul 28, 2009 7:42 am

yep
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Postby livewire » Tue Jul 28, 2009 4:43 pm

[quote="kbrownie"]yep[/quote]

So, if ive got this correct, just to repeat what you said in your first post, for a 80A or 100A supplier fuse we will only EVER need either 16mm or 25mm2 csa respectively. So, it doesn't matter if we had 20 circuits with a max demand of whatever current (140A, 300A say) - the tails would still be the same csa either 16mm or 25mm2 resp.
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Postby kbrownie » Tue Jul 28, 2009 10:54 pm

[quote="livewire"][quote="kbrownie"]yep[/quote]

So, if ive got this correct, just to repeat what you said in your first post, for a 80A or 100A supplier fuse we will only EVER need either 16mm or 25mm2 csa respectively. So, it doesn't matter if we had 20 circuits with a max demand of whatever current (140A, 300A say) - the tails would still be the same csa either 16mm or 25mm2 resp.[/quote]

They could be bigger but 25m can carry 100Amps as can 35mm or 50mm but likely not find them in a domestic, most likely to be 25mm.
Your breaker is going to offer you under 100Amps discrimination.
I'd would never rule out the possibility of an overload and the HRC going but it'll take some doing. Have a look in the big red book and the graphs for BS88 and disconnection times.(appendix 3)
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Postby livewire » Wed Jul 29, 2009 2:00 pm

[quote="kbrownie"][quote="livewire"][quote="kbrownie"]yep[/quote]

So, if ive got this correct, just to repeat what you said in your first post, for a 80A or 100A supplier fuse we will only EVER need either 16mm or 25mm2 csa respectively. So, it doesn't matter if we had 20 circuits with a max demand of whatever current (140A, 300A say) - the tails would still be the same csa either 16mm or 25mm2 resp.[/quote]

They could be bigger but 25m can carry 100Amps as can 35mm or 50mm but likely not find them in a domestic, most likely to be 25mm.
Your breaker is going to offer you under 100Amps discrimination.
I'd would never rule out the possibility of an overload and the HRC going but it'll take some doing. Have a look in the big red book and the graphs for BS88 and disconnection times.(appendix 3)[/quote]

Interesting, thanks KB

I just had it in my head that somehow the number of circuits and maximum demand of the whole installation would have an effect on the size of the tails for some reason. But if a supplier is only providing a customer with a max 100A service, then we will only ever need a 25mm2 csa; since 25mm2 is adequate for 100A of current. From the regs a 100A BS 3036 fuse will cut out in 5s with a current of 430A going through it. So, I might conceivably be able to squeeze 250A through it without breaking it?

What I'm getting to is; say I have designed an installation from scratch. Ive calculated my max demand for the whole installation and arrived at a figure of 180A say. (Its a large exotic domestic) Is it then down to the supplier to provide me with a larger HRC and bigger tails?

Looking at the regs I would need a 50mm csa to cope with 180A.
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Postby kbrownie » Thu Jul 30, 2009 11:25 am

Rarely a problem 180A should be fine on 100Amp main fuse, if the demand of the installation ever did become so high that the main fuse could not cope it's a three phase upgrade.
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Postby livewire » Thu Jul 30, 2009 7:33 pm

[quote="kbrownie"]Rarely a problem 180A should be fine on 100Amp main fuse, if the demand of the installation ever did become so high that the main fuse could not cope it's a three phase upgrade.[/quote]


thanks for all your help!
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