Underground cable size needed?

[GNServices]

Hello,
A friend asked what size cable he would need for the following and I was not sure so said I would ask on behalf of him,

3-phase and neutral supply (so 5 core) (415v)
Will be ran underground from the supply post, 100m to the building,
Cable current carrying capacity of 100amps

Any input would be appreciated, I had a look at a few cable size calculators but wasnt very good haha

Thanks,

 

[securespark]

What kind of cable?

 

[GNServices]

What kind of cable?

Whatever would be suitable? (SWA I guess?)

In terms of the core material, it shouldnt really matter so maybe sizes for both copper and aluminum so he could look at prices for either

 

[securespark]

Al. would be massive.

SWA....XLPE, I presume?

 

[RF Lighting]

You'll need 70mm², and at £40 per meter it's going to really hurt

 

[Iggifer]

Ouch.

 

[ban-all-sheds]

A friend asked what size cable he would need for the following

Why has he taken it upon himself to design "the following" when he isn't able to work out cable sizes?

 

[Lectrician]

Al. would be massive.

SWA....XLPE, I presume?

Not massive. Larger yes. Certainly cheaper and lighter. Needs some special care and attention to install and terminate. A national customer we contract for specs Alu for anything 70mm and above.

Where does this "100amp" come from? A guess? Most supplies from my DNO will be 80amp max on a standard headed supply, and if this is the case, there is a cost saving on cable size. What is at the load end? Do you really need the full 100amp?

 

[mfarrow]

Cable current carrying capacity of 100amps

That would give you a cable size of at least 25mm², depending on the load. What is the load, per phase?

 

[ricicle]

35mm²

 

[GNServices]

Hey, thanks for the responses,
I was thinking 25-35mm would be the approximate range, 70mm seems very excessive.

I dont know much details to be honest with you, i just got told to ask,
It will be for agricultural use (workshop), I don't know why it is 100amps he is after, giving that for the need of 100amp capacity then you'd be needing to run 3 x 32amp sockets on full load at the same time for that (which I highly doubt he would require),

Prehaps I could advise him to reconsider his needs?
He just wants three phase in his workshop I think and I suppose the absolute most he would need would be 1 x 32amp socket at full load, which 16mm should easily cater for?

 

[mfarrow]

giving that for the need of 100amp capacity then you'd be needing to run 3 x 32amp sockets on full load at the same time

You mean 9 sockets I assume? That would be one hell of a lot of heat, and at 70kVA, would cost a lot to run.

Perhaps I could advise him to reconsider his needs?

Yes, that's right. At this sort of distance, you need to size your submain for volt drop based on load, not the size of the fuse needed for discrimination.

 

[Lectrician]

Cable current carrying capacity of 100amps

That would give you a cable size of at least 25mm², depending on the load. What is the load, per phase?

It can carry the current, but what about the voltage drop over that distance? 25mm is taking the ****.

 

[JohnW2]

giving that for the need of 100amp capacity then you'd be needing to run 3 x 32amp sockets on full load at the same time

You mean 9 sockets I assume?

I had assumed that he meant what he wrote - i.e. 3 x 32A 3-phase (i.e. 'red') sockets - hence 96A per phase when fully loaded.

Kind Regards, John

 

[JohnW2]

Cable current carrying capacity of 100amps

That would give you a cable size of at least 25mm², depending on the load. What is the load, per phase?

It can carry the current, but what about the voltage drop over that distance? 25mm is taking the p**s.

It's actually not quite as bad as one might imagine. If I've done my sums right, 100A through 100m of 3/4-core 25mm² only results in a VD of 15V. with 35mm² it would be 11V. However, as being discussed, I seriously doubt whether anything like 100A would be required.

Kind Regards, John