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

 

[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

 

[Strima]

Your not clutching. Take a real world look at diversity, not what's written in the OSG.

If you applied the OSG theory to a standard domestic property they could have you drawing around 50 amps, but in reality it would be closer to 20 for example.

I haven't had much to do with supplying locations such as this, once all is connected I would probably carry out a few tests to see if there is a potential difference between containers. No doubt someone will be along soon to correct me.

 

[Wilko]

If there is anything to be bonded you might TT the remote container perhaps?

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

Unless the installation draws full load for significant periods the conductor won't be at 70C . In my humble opinion if average load is 20A on 25mm cable the temp won't rise and resistance at ambient temp may be more relevant. But you are doing a design with 63A fuse .

 

[Thebonse]

I'm down rating it to 20a/phase

Absolutely certain it'll never come close.

The containers are the same as the portacabins. Proper metal.

Only real difference is the cabins have a consumer unit.

To be safe they will all be rodded and bonded to each other.

 

[telectrix]

Stop thinking, it makes you tired...

i been tired for years. . usually soon after 11.00a.m.