TT system scenario - Thoughts please ?

[spark 68]

I was just thinking of the length of the run you have, which is quite long.

25mm = 1.75mV/A/M

(1.75*32*80)/1000 = 4.48 V for 25mm

1.5mm = 29mV/A/M

(29*6*20)/1000 = 3.48 V for 1.5mm

Total VD = 7.96 which is 1 volt just over, which you may get away with, or make the 20M run in 2.5mm which will give you

2.5mm = 18mV/A/M

(18*6*20)/1000 = 2.16V

Total VD = 4.48 + 2.16 = 6.64 V this is much better and is just under the 6.9 Volt limit, so this will comply using 25mm armoured feeding 2.5mm for your 20M run.

This is quite a long run (80M + 20M) so I would go for this, unless anyone else has any better ideas.

 

[AA410]

So if i drop it to 20A supply 2.8*20*80/1000= 4.48v how do i add the lighting on to this ? do i work out the lighting V/D and then add it to the supply V/D

 

[AA410]

So if i drop it to 20A supply   2.8*20*80/1000= 4.48v how do i add the lighting on to this ? do i work out the lighting V/D and then add it to the supply V/D

 

[AA410]

SO will 2.5 save my bacon ????????????????

 

[spark 68]

So if i drop it to 20A supply 2.8*20*80/1000= 4.48v how do i add the lighting on to this ? do i work out the lighting V/D and then add it to the supply V/D​

That's about it, it is the two voltage drops combined to give you the total VD for that circuit.

There is an added complication, in that your lighting load is distributed along the cable, so in theory you may get away with 1.5mm for the 20M run, but the above using 2.5mm covers the worst case and will definitely comply.

And yes, reducing the OCPD from 32A to 20A gives you more headroom so to speak, if that is suitable for your needs.

Cheers

 

[AA410]

This drives me mad this job is for an old mate of mine who went out some time ago and bourght 100m of 16mm s/w/a then tells me got the cable when can you put it in for me lol if this is correct then a great big thanks for the help you have given me

 

[spark 68]

Ah! I see,

You were stuck with the 16mm cable, I thought it was the 32A MCB that you were tied to.

By reducing the MCB, you have reduced the design current, but usually this is not always an option

 

[AA410]

32A to a garden building is a great thing but 20A is ok just means you have to be carefull with what you use as far as the power i work that as 5% V/D is that correct

 

[spark 68]

You had 11.5 Volts to play (well 7.02 Volts remaining) with for 5% of 230V, so this will comply.

It was the lighting @ 3% which caused your main problem

Without trying to give you anymore grief, you may have a problem with discrimination for the sockets now, 10A MCB for the sockets ?

 

[AA410]

can i cry will 4mm save me

 

[spark 68]

Sorry mate I was editing my previous post while you were posting.

You had 11.5 Volts to play (well 7.02 Volts remaining) with for 5% of 230V, so this will comply.

It was the lighting @ 3% which caused your main problem

Without trying to give you anymore grief, you may have a problem with discrimination for the sockets now, 10A MCB for the sockets ?​

 

[AA410]

Can i get away with a 16 mcb on 4mm for the power?

 

[spark 68]

Can i get away with a 16 mcb on 4mm for the power?

Not really, because in a previous post you had to reduce the supply MCB down to 20A to comply with the voltage drop, as it was higher than 3% for the lighting circuit, even before the lighting circuit.

16mm = 2.8 mV/A/M

(2.8*32*80)/1000 = 7.168 V

3% of 230 = 6.9V

This already exceeds the 3% drop without the 1.5mm run.

A 16A MCB won't discriminate with the supply 20A MCB, which means if the sockets tripped on overcurrent, you would/could lose power to the whole shed.
4mm cable does not come into it.

One option would be to reduce the MCB supplying the sockets to 10A, this is not ideal, but would provide some discrimination, depending on the use for the sockets.

Another option may be to supply the shed from a fuse/switched fuse @ 20A, and hope the 16A MCB tripped first, but you would need to check the co-ordination tables in the BRB/BGB

In an Ideal world you would have selected everything beforehand, but now as you are stuck, you may just have to do the best you can within the regs, and within the constraints imposed on you, or tell your mate you can't/won't do it with the materials he has provided

I don't mean that in a nasty way, in any shape or form

 

[AA410]

Its the old saying a mate in need is a pain in the --- lol i will have to explain this to him this weekend and i think i will try the 16A for the power and if the 20A trips thats it one other thing is that he wants to put an other supply in for a green house and an other shed so we might change this for a 25mm and use the 16mm elsewear

 

[Chr!s]

First of all you need to decide what the maximum demand is, then you can work out your volt drop.

Second, is discrimination between the distribution circuit and the final circuits an issue?

Regards Chris