Design Question

[Chas]

I am modifying an existing power distribution box. There is a main fused disconnect (for the box) that sub-feeds two fused disconnects. If the sub-fed fused disconnects are rated at 20A and 25A, what should the main fused disconnect rating be. Also, what should the wire size between the main fused disconnect and the sub-fed fused disconnects be as well.

 

[Nemrod]

It depends on your loads. But if you don't know them, or if you want to play safe, just make the sum and put a 40 or 45 fused disconnect.
Be sure your main feeding cable is at least 10 or 16 sq.mm XLPE. If not, you must put a suitable main protection.
To protect the subs, use minimum 5*6 sq.mm XLPE for both.

 

[gordonl]

You'll want to be sure you have co-ordination between the fuses as well. The manufacturer of the fuses will usually have a chart for the minimum size of upstream fuse.

 

[patpat]

I think it is a safe move to have a load audit and protection audit. This will give you a scure picture of the distribution.