Cable calc??

[paul11]

Hi guys just looking for a bit of help / advice

i have a 100amp 3 phase isolator tap off connected to a rising bus bar.

im installing a new 16 way 3 phase DB approximately 55 metres away from the isolator.

it will be wired in 4 core armoured on tray, and on its own located with in a ceiling void.

the electrical consultant has indicated to use a 50mm 4 core armoured, I think a 35mm would be fine.

Can any body point me in the right direction for the correct calc?

 

[trev]

Try your normal calcs mate, although I'm not sure why you're querying it. It'll be the consultant signing off the design.
You could always give the gadgie/wifey a ring and talk through the calcs.

 

[paul11]

Hi trev

the run is a nightmare and would make it a lot easier with a 25mm/35mm armoured.

not really up to date with my calcs, once I get the bread and butter of it i suddenly remember. But then usually forget it a few months down the line

 

[trev]

So how did you come up with 25/35mm swa mate?
Think logically and it'll all come back to you.
In no particular order=
CCC
VD
Temp
Grouping
Etc
Etc

 

[Sintra]

I would go with the consultants sizing without a query. He will have allowed for the adequate VD from source to final circuit rather than just this sub.

 

[peterinreading]

Looks primarily a voltage drop issue. A rough calc suggests 8v@25 6v@35 and 4V@50 so the issue will be what are the design criteria? Could you ask the consultant to divulge them?

 

[Sintra]

It could also be possible to use 2 x 25mm 4 cores in parallel.

 

[davesparks]

it'll work out at less than 25mm if you calculate it for 2 x cables in parallel, 2 x 16mm is more likely but I wouldn't be too surprised if it came out at 2 x 10mm

 

[bigdavs83]

Can't be arguing with consultants now! They are the most experienced and helpful guys ever!!

 

[wozz]

Am I being stupid but how can you really comment on this without knowing the load?

 

[peterinreading]

Thats assuming the max 100A of the isolator. If the load were significantly less then it would be difficult to justify the 50mm cable without knowing more about the requirements.

 

[peterinreading]

deleted

 

[Besoeker]

Am I being stupid but how can you really comment on this without knowing the load?

Well, the 100A isolator determines the maximum it can be.

 

[shanky887614]

Surely 35mm would do it, 50 does seen quite a bit overkill but I guess the main factor affecting it will be from where its fed from.

like many others have said unless you go for a bigger cable it might not comply with tripping times

 

[Engineer54]

If this is a specified job/project then you, or your company have signed a contract to provide the electrical installation based on the project drawings and specifications. Any deviation from either of those contract documents without prior permission from the ''Engineer'' and you'll be in a S**tload of trouble!! As already suggested, you could seek permission to use a parallel supply of a suitable size if the routeing of this sub main is is going to be a real hassle with a 50mm 5 core SWA.