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Discuss Sizing DB and cable in the Industrial Electrician Talk area at ElectriciansForums.net

L

LOST319

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Hi

Im new to the forum so thanks in advance.
I have been out of the trade for many years and I am retraining at the moment. I would be grateful if you can shed some light on circuit diversity and cable sizing.

If a new single phase DB is to be installed. The new DB will have a max demand load of 40A over 8 ways with diversity accounted. This might be a stupid question but what is the best way to size the DB incoming cable and DB size taking into account future capacity.


When you see a DB main switch rated at 100A I take it the submains cable will have to be able to handle minimum 100A or is diversity a factor yet again. Does the 100A mean max load of circuits with diversity applied.

Also is it ever required or possible to have to upgrade cable size to DB without having to upgrade the DB size if the demand on that DB has increased.

Hope that makes sense

Thanks
 
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S

Shakey

  • Thread Starter Thread Starter
  • #2
Hi

Im new to the forum so thanks in advance.
I have been out of the trade for many years and I am retraining at the moment. I would be grateful if you can shed some light on circuit diversity and cable sizing.

If a new single phase DB is to be installed. The new DB will have a max demand load of 40A over 8 ways with diversity accounted. This might be a stupid question but what is the best way to size the DB incoming cable and DB size taking into account future capacity.


When you see a DB main switch rated at 100A I take it the submains cable will have to be able to handle minimum 100A or is diversity a factor yet again. Does the 100A mean max load of circuits with diversity applied.

Also is it ever required or possible to have to upgrade cable size to DB without having to upgrade the DB size if the demand on that DB has increased.

Hope that makes sense

Thanks
Yes, the incoming tails should be able to handle the rated isolator current, 25mm is 114A clipped direct, which is why this is pretty much the standard tails size for a domestic CU, with 16mm earth

As a guide I add up the propsed MCB sizes (actually breaker sizes, so you would already have applied diversity where neccessary to achieve this value), then as long as 66% of this total value does not exceed the isolator capacity, then you will be fine - this is a form of diversity application

so roughly:

40A shower
32A ring
32A ring
32A cooker
6A lights
6A lights

= 148A x 66% = 97.68A which is less than 100A isolator so is fine;)
 
W

wayne

  • Thread Starter Thread Starter
  • #3
shakey,i always understood it that if an isolator or a rating of any other switch was given then the fuse supplying shouldnt exceed that
 
S

Shakey

  • Thread Starter Thread Starter
  • #4
shakey,i always understood it that if an isolator or a rating of any other switch was given then the fuse supplying shouldnt exceed that
quite right Wayne, and thats how we can get away with applying diversity to the CU MCB's because the incoming fuse is the current limiting device in the circuit
 

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