TT system scenario - Thoughts please ?

[AA410]

Thanks Chris you lads are great i have not had much to do with sub mains a few weeks ago when he told me the cable he had i calculated the cable at 5% and gave the lighting very little thought

 

[Richard Burns]

I have considered some of the options available here and made some assumptions on operating temperature and design current.
I think (but please do not take this as totally accurate) that there is a situation where you can have a 32A supply, 20A sockets and 6A lights.
Assuming a 20m run for lighting wired in 1.5mm and a 20M run for sockets wired in a 2.5mm ring
Using a design current of 32A for the supply (though it could be lower).
If you have evenly spaced the lights and split at the first light two left and three right (for example) and each light is 100W this gives a design current of 2.6A for lights.
Calculating volt drop individually for each section.
The sockets evenly distributed for current use. I have assumed volt drop for 2x2.5mm cables each carrying half the total current. (20m*13A*0.018 =4.68) or corrected (20m*13A*0.0178=4.63 this gives 10.70VD)
Assuming that at the low current demand for the cables that they are running at 35C and applying the correction factor for temperature to the volt drop then you can manage to get the lighting volt drop under 6.9V and the power volt drop under 11.5V.
Please remember I may not have got the calculations correct.

I have drawn a diagram of the way the values have been calculated, the first values are without temperature correction and the second values are with temperature correction. (first / second)

If once you have your design currents and cable sizes you calculate this more accurately then you may find that you are compliant. (though there is still a consideration of whether discrimination is required).

This may then allow you the desired capabilities of the system. You could also install it and then physically measure the volt drop and see if it does comply, but could be agonising if it does not!

 

[AA410]

WOW thats great Richard thanks i am thinking or wiring the lighting in 2.5 and the power in 4mm the lighting will be on 2 bank of light from the light switch and somthing else that comes to mind is the 16mm supply my R1+R2 reading was 0.12 that tells me my cable might be a bit shorter ? not worked that out yet in need of sleep

 

[AA410]

Not much sleep lol one other thing that come to mind is 3% of supply voltage 242v= 7.2v ?

 

[spark 68]

Hi mate,

I will likely be be shot at for this post, but here goes,

In this situation, where you are constrained as you are,

I would, feed your 16mm armoured with a 32mA MCB, your lighting circuit distribution cable in 2.5mm, and maybe split that into two 10M runs, the two cables can be 'junctioned' at the 6A MCB/RCBO, ie. terminated at the shed DB, and then stick your socket circuits (length ?) on either a 2.5mm ring, or 4mm radial fed from a 16A MCB/RCBO, or even two seperate 4mm radials, again both runs can be terminated at the MCB, this would give you the discrimination required, ie. if the sockets tripped, you would still have lighting.

Provided your disconnection times and Zs is within bounds this would then be the safest option.

If the VD is a little on the High side for the lights calculated, you could apply diversity, and maybe reduce the calculated maximum current demand, bearing in mind you will not be using either the full 6A for the lights, and you may not be using the full 32A for the sub-main either, these protective devices are providing the required protection for the cables to the circuits, and the RCD protecting the sockets, the CCC is not in doubt here either, this installation will be the best you can achieve in the circumstances.

I know in an ideal world all of this would be calculated before you started, but in reality we often have to do what is expedient, the main thing is, is it safe ?

If the answer to the above question is yes, then you are good to go IMO, so what if the VD maybe is a little high on the lights if the the circuits were used to the full capacity, this is ultimately a design decision, and at the very worst you may have exceeded the VD on the lighting circuit by a small amount, but to me this is the least onerous rule you could possibly break here, then there is rule 525.4 which will give you a little leeway.

At the end of the day apply some common sense.

Not much sleep lol one other thing that come to mind is 3% of supply voltage 242v= 7.2v ?​

sorry mate, all calcs at 230V

What Iam getting at here is, don't try and 'fudge' your calcs, only you know the design requirements, and how the installation is to be used.

Make your design decisions based on the available info., and run with that, what you cannot do, is to get someone else to say this is okay, neither me nor anyone else can tell you to break a rule, you need to justify the design decisions you take.

 

[Chr!s]

Still not sure how you come to a 32 amp maximum demand when there's only a 20 + 6 amp MCB?

Regards Chris

 

[spark 68]

Still not sure how you come to a 32 amp maximum demand when there's only a 20 + 6 amp MCB?

Regards Chris​

Exactly Chris, I think because some earlier (other) posts on here with regards to VD calcs suggest using the OCPD's to calculate the maximum current that can be used from the sub-main, for future additions etc., and this in itself is generally good advice for some degree of future proofing, and is generally good design practice.

Each install is different, and the person signing it off has to make the decisions, that's what I was getting at when I said apply some common sense, The regs are there to guide us at the end of the day, but cannot be used to design an install, as other factors come into play.

It is how you apply the regs, tailored to suit your requirements that can be the daunting part.

Cheers

 

[Richard Burns]

spark68 some good advice there, I should have emphasised that the installer (designer) must be able to make the justified decisions on the installation, based on the design calculations. Not from what someone says on a forum (although this may well give good guidance).

I have realised in my calculation that I have used 32A demand on the supply and 26A on the sockets, I did say they may not be accurate, but at least as Spark68 says these conditions are more onerous than the likely design values.

I think post65 is pretty good and is likely to comply once designed. Certainly there is a good chance of getting a usable compliant system, but do remember to consider all the requirements not just voltage drop, although CCC is not an issue.

Good luck with it AA410!

 

[Chr!s]

If we assumed 20 amp + 40% of the others then we could say 22.4 amp. Using correction factors then by my calculations, the distribution circuit should be fine assuming Iz is no worse than 70 amp. It would pretty much give you 3 volts for the lighting so there should be no issue.

So fuse/mcb 63/60 amp for distribution , 20 amp Radial, 6 amp lighting.

Regards Chris

 

[AA410]

Hi lad great advice again many thanks for all your help i hope to check the V/D on this instalation to see just what it is