TT system scenario - Thoughts please ?

[spark 68]

Hi,
Just a quick point on VD.

Reg 525.4, states that a greater voltage drop than stated in table 12A (app12)
(neglecting the motor circuits etc. part of this reg for the purpose of this discussion) may be accepted, provided that it is verified that the voltage variations are within the limits specified in the relevant product standard for the equipment, or in the absence of a product standard, in accordance with the manufacturers instruction.

So maybe the VD may not be so rigid, depending on the manufactures of the lighting, ie. if the lamps are rated at slightly less than the more common 220V-240V, Iam not suggesting trying to bend the rules by any means, but my interpretation of this reg, was it was meant to stop incorrect functioning of equipment, as always common sense applies.

Just a thought

 

[Richard Burns]

I am not sure I would like to try and justify a lower rated fitting, but might be OK.
I think for the lighting circuit even if you have 100W fittings you are only running 2.6A and have an inside shed VD of 1.5V (if using 1.5 cable). The 16mm is running well below its CCC and so there should be a lot of leeway there, I think if installed it would probably meet the requirements by measurement.

 

[spark 68]

Hi Richard,

I was not suggesting anybody break the regs with regards the 3% VD where lamps are concerned, but the regs are just guidance, and there is provision in the regs for leeway within reason (Reg 525.4).

The main reason for that reg. is for correct functioning of the devices (Lamps), and the other parts of the VD regs err on the side of caution to guarantee that no matter what devices are fitted, they will function correctly, which is good design practice, especially where the designer has no say in what may be fitted (empty building).

Now I would always try to design to the regs, but I would not get to hung up about it if I was a Volt or two out, and just note it as a departure, with the proviso that the lamps etc. started correctly and there was no adverse effects on the reliability etc. and my current capacity was not exceeded

I would be concerned if they were quite away out, and were borderline starting etc..etc., again just common sense applied rather than slavishly following the guidelines with no actual thought as to why. (I'll put my tin hat on now, LOL) :cyclist:

 

[AA410]

Is this correct the v/d is at 3% so at 80m 16mm s/w/a supply and 20m 1.5 singles 6A mcb it comes in at 5.7v is this right ?

 

[spark 68]

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.


I make it 10.65V in total, but this is calculated using the In of your ocpds, as Richard has already said your design current will most likely be lower than this, so you may be able to get it within bounds.

 

[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

 

[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