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Topic Title: Sizing of tails taken from busbar to feed DB for overload
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Created On: 21 January 2019 05:59 PM
Status: Read Only
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 Sizing of tails taken from busbar to feed DB for overload   - asmtech - 21 January 2019 05:59 PM  
 Sizing of tails taken from busbar to feed DB for overload   - asmtech - 21 January 2019 06:00 PM  
 Sizing of tails taken from busbar to feed DB for overload   - chrispearson - 21 January 2019 06:40 PM  
 Sizing of tails taken from busbar to feed DB for overload   - asmtech - 21 January 2019 06:51 PM  
 Sizing of tails taken from busbar to feed DB for overload   - asmtech - 21 January 2019 06:54 PM  
 Sizing of tails taken from busbar to feed DB for overload   - Legh - 21 January 2019 06:44 PM  
 Sizing of tails taken from busbar to feed DB for overload   - asmtech - 21 January 2019 06:50 PM  
 Sizing of tails taken from busbar to feed DB for overload   - Legh - 21 January 2019 07:04 PM  
 Sizing of tails taken from busbar to feed DB for overload   - AJJewsbury - 22 January 2019 12:18 PM  
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 21 January 2019 05:59 PM
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asmtech

Posts: 42
Joined: 11 August 2016

Hi.

Not posted for a while but a conversation with a colleague caused some confusion and wondered if anyone can clear it up.

The installation we were looking at had three separate DBs connected to a busbar. One DB had approximately 10 circuits and the other two had only one circuit each. The busbar is fused with a 100A BS88 fuse and is suitable to carry this current. Fault protection for all circuits was provided by an RCD.

The DB with 10 circuits was fed with 25mm2 insulated/sheathed tails which we felt was satisfactory (the final circuits fed from this board were unlikely to ever exceed 100A, the rating of the tails and the busbar).

The second DB served a single 16A load protected by a B16 MCB and was connected to the busbar using 10mm2 insulated/sheathed tails. As this load was fixed and therefore could not exceed 16A we agreed this was satisfactory, even though the upstream fuse rating exceeded the current carrying capacity of the 10mm2 tails.

The final DB served a lighting circuit protected by a B6 MCB and was connected to the busbar using 2.5mm2 cable. This is where the confusion came. As the lighting load could theoretically be increased and wasn't fixed, I felt that the conductors supplying this board should be able to carry the instantaneous tripping current of the circuit breaker (5*6=30A) which, in this installation would require 4mm2 cable. My colleague felt that 2.5mm2 was satisfactory and it could easily carry 6A continuously.

Am I being over cautious here?
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