Sizing bonding conductors

[jamie-spark]

Hi guys, i have a question what size of bonding conductor is required for A shed that has a 63 amp 3 phase( 25mm2 swa) supply going to it. It is supplied from the main building that has a 400 amp TN-S 400mm supply . hust want to see if you have the same answer

 

[jamie-spark]

Well here is how i see it . The maximum size of bonding conductor for a tns system is 25mm. The bonding conductors chould be sized on the earthing arrangments at origin. The select registered contracter has selected it on his supply condutors the the outbuilding 25mm live conductors with a 16mm earth wire and he has bonded the water and structural steel work in 10mm. correct me if im wrong

 

[NMc]

See Regulation 544.1.1

 

[jamie-spark]

not got my regs handy

 

[NMc]

short version supply TN-S half csa of the earthing conductor and not less than 6mm but upto maximum 25mm. assume shed is detached, 25mm supply and bond required in shed then main earth is 16mm so bond is 10mm

 

[jamie-spark]

NMc. the bond in the shed should be 25mm as its based on the supply not the consumers supply to the shed

If there was a 10mm supply to a shed that had water and steel it would still require to be 25mm as it is based on the distributors supply not the consumers

 

[needasparks]

Be suprized if it is TNS, Iv'e never come across a sheath earth above 160a.

 

[jamie-spark]

its supplied from a transformer. Its a single copper conductor

 

[NMc]

The main supply into the main building would require main bonding of 25mm to the services entering that building. I assume that all the shed supplies are all from the main building, a sub main circuit then the earth is base upon the circuit and not the main supply.

 

[needasparks]

I see, just read the OP again, missed it.

 

[jamie-spark]

The main supply into the main building would require main bonding of 25mm to the services entering that building. I assume that all the shed supplies are all from the main building, a sub main circuit then the earth is base upon the circuit and not the main supply.

I agree the earth is based on the the supply. But the bonding conductor still requires to be 25 as in Guidence note 8

 

[NMc]

what part od GN8?

 

[jamie-spark]

not got it just now . I will get it tomorrow find the page number and post back.

 

[jamie-spark]

NmC

check out page 49 on GN8. Installaition serving more than one building. It states that "the application of the regulation where it serves a number of buildings it can result in the csa of the main equipotential bonding conductors being required to be grater than the associated live conductors of the distribution circuit suppling that building, Particuarlly where PME conditions apply" . It also shows diagramms of buldings being supplied from the main building Of 16mm live conductors with a 25mm combined cpc and bonding conductor for tns and similar diagramm for pme , 16mm supply conductors and a 35mm combined cpc and bonding conductor.

We find most places we go to, bonding conductors to be sized on the consumers distribution circuit, which is wrong.

 

[NMc]

Jamie-spark

Not sure the direction you are going here. It must be the man flu I have

I have looked at GN8 although mine is to the 16th Edition dont think it has been amended can see what you are saying.

But think about it the original wiring should have been designed accordingly!!!!! If the sub supply earth is less then the bonding conductor then this would be wrong. The conductors have to be capable of carrying the maximum fault current.

You are going to need to perform calculations and not rely on trying to find a written answer

 

[jamie-spark]

Jamie-spark

Not sure the direction you are going here. It must be the man flu I have

I have looked at GN8 although mine is to the 16th Edition dont think it has been amended can see what you are saying.

But think about it the original wiring should have been designed accordingly!!!!! If the sub supply earth is less then the bonding conductor then this would be wrong. The conductors have to be capable of carrying the maximum fault current.

You are going to need to perform calculations and not rely on trying to find a written answer

Just my point it never has been designed to meet the regulations. the sizing of bonding conductors is the same in the 17th as the 16th so your guidance note 8 is still relevant. There is NO CALCULATION for bonding conductors. the only way to size bonding conductors is by use of table 54.8 and by looking at the supply characteristics at the origin of the installation. I think you may be getting confused by calculating earthing conductors by the use of the adiabatic. if you read over GN8 you will see .