Supply to Portable buildings and portacabin/container

[Thebonse]

Hi Fellas

Sure i had another login but seems my account has been recycled......anyway.

Im running a supply to multiple portacabins and shipping containers.

The Supply is a TNS and is approximately 95m away.
I have decided on a 63A 3phase circuit and calculated the cable size @ that length to be 25mm (total volt drop at this distance is 11.5v (Calculated)).

It will be supplying a new DB which will be enclosed in an Ip rated enclosure and separate circuits to be supplied to each of the units/containers.

Ok so now my question, i have been reading the regs and guidance notes regarding this and it clearly says no PME which i completely understand, But what about a TNS supply?

I get that there can be a difference in potential between the earth supplied and the true earth but if theres an Earth rod for each unit.

I personally cannot see why this is not acceptable but please let me know what you guys think in this situation.

cheers

Darren

 

[Thebonse]

I have also considered going from TNS to TT at the new DB location but as i stated above.....Do i need to?

 

[Strima]

You might want to recheck with regards to volt drop as well. 11.5 just on the distribution circuit means all the circuits off the connect DBs will be below the required minimum especially the lighting.

 

[Thebonse]

The new circuits are within a few meters of the new DB. my mistake its more like 8.4 total volt drop. 11.5 is 5%

 

[Strima]

What about lighting at 3% (6.9 volts)?

 

[telectrix]

TN-S is fine. the reason PME is frowned on is that a loss of the supply N ( and hence the site earth) would lead to the metal structures possibly becoming "live".

 

[telectrix]

What about lighting at 3% (6.9 volts)?

install LED lighting.

 

[Thebonse]

Hmm...

How can this be done then? The Regs don't specify a figure for 4 core cables over 25mm

 

[Strima]

install LED lighting.

Stop thinking, it makes you tired...

 

[Thebonse]

?

 

[Thebonse]

TN-S is fine. the reason PME is frowned on is that a loss of the supply N ( and hence the site earth) would lead to the metal structures possibly becoming "live".

Yeah i know mate i was looking for clarity as the regs make no mention of TNS (for this kind of installation)

 

[Strima]

Hmm...

How can this be done then? The Regs don't specify a figure for 4 core cables over 25mm

What cable are you calculating for?

 

[Thebonse]

well a bigger one than 25mm.... my original calc was for 25mm

 

[Strima]

70c SWA?

 

[jackhammerJIM]

TNS is acceptable . in swa 4 core cables are specified up to 400mm

 

[Strima]

Using 50mm 70c SWA will bring you volt drop down to 5.56, this gives you something to play with. This isn't taking into account of VD from source of supply to your DB.

That was using R for a resistive load.

Do you know what loads will be applied to the circuits as this will have a large impact on your VD calculations. You could get cheeky with the diversity to get the CSA down a little.

 

[Thebonse]

Literally a few ringmains, heaters and lighting ccts.

Using Z or R are the same 2.5 Mv/63A/90M thats worse case....

50mm cable is a big jump and @90m is 7.75 VD which is already more than 3%

If i drop the current down to 40A its still 6.4VD

Seems ridiculour that it would end up being 70mm+ for a few circuits?

 

[Strima]

It is a pain to work out, also adding correction factors for burial etc. Just as you get it sussed you finish off with the adiabatic and that sends you back to square one!!!

Oh, and ignore my calc, I was looking at the wrong column which doesn't help!!!

 

[Thebonse]

lol i did wonder....

lets assume the VD was all good the answer to my original question is yes to TNS yes? and do i need to add earth rods or not? i feel like i should

 

[Thebonse]

I like the wording of this paragraph.....(the bottom one)

I dont envisiage any of the load per phase to exceed 20A and therefore the results are going to differ tot he tabulated values.

what you think? i know i'm clutching.....

Dropbox - IMG_20170925_194949.jpg - https://www.dropbox.com/s/46ym3q6xiiauyob/IMG_20170925_194949.jpg?dl=0