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IET » Wiring and the regulations » Sizing of tails taken from busbar to feed DB for overload

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
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21 January 2019 05:59 PM
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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21 January 2019 06:00 PM
asmtech Posts: 42 Joined: 11 August 2016	Diagram here: https://imgur.com/a/MZ5Ky28 (Can't make it a link without errors. Copy and paste unfortunately.)
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21 January 2019 06:40 PM
chrispearson Posts: 1095 Joined: 15 February 2018	Originally posted by: asmtech Am I being over cautious here? Yes. 433.2.2 permits overload protection to be installed anywhere along the circuit before any branches.
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21 January 2019 06:44 PM
Legh Posts: 4427 Joined: 17 December 2004	I would have preferred the use of a larger 2nd DB with 25mm2 tails allowing for a larger number of additions. We know that many boards have under rated loading. I suppose that if the energy let through of a faulted circuit is greater than the energy characteristics of the cable then there maybe a problem. Is there a likelihood of adding so many lights that the circuit runs in a overloaded condition? Legh ------------------------- http://www.leghrichardson.co.uk de-avatared
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21 January 2019 06:50 PM
asmtech Posts: 42 Joined: 11 August 2016	Hi Legh. It's very unlikely that more lights could be added to overload the circuit to be honest. I haven't measured the existing loading but it's all LED lighting so unlikely to be a problem. I was just thinking about the fault protection of these cables. A fault is also unlikely as they are singles in earthed metal containment and not susceptible to damage. There's lots of space around the cable in containment too so heat build up is unlikely as well. The cable lengths are less than 3m in length too (I can never find that particular regulation but I'm pretty sure one exists!)
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21 January 2019 06:51 PM
asmtech Posts: 42 Joined: 11 August 2016	Originally posted by: chrispearson Originally posted by: asmtech Am I being over cautious here? Yes. 433.2.2 permits overload protection to be installed anywhere along the circuit before any branches. Thanks. Is this precluded when the length of said circuit exceeds 3m when protection is provided by a distributor's fuse?
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21 January 2019 06:54 PM
asmtech Posts: 42 Joined: 11 August 2016	Originally posted by: asmtech Originally posted by: chrispearson Originally posted by: asmtech Am I being over cautious here? Yes. 433.2.2 permits overload protection to be installed anywhere along the circuit before any branches. Thanks. Is this precluded when the length of said circuit exceeds 3m when protection is provided by a distributor's fuse? Just got BS7671 out and it is explicitly stated in that regulation! Thanks.
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21 January 2019 07:04 PM
Legh Posts: 4427 Joined: 17 December 2004	sections 433.2 and 433.3 maybe of interest to you. Legh ------------------------- http://www.leghrichardson.co.uk de-avatared
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22 January 2019 12:18 PM
AJJewsbury Posts: 17795 Joined: 13 August 2003	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. Ignoring the possibility of omitting fault protection for up to 3m for the moment, in general to be OK for fault protection the conductor (including its insulation) needs to be able to survive the actual fault current for a long as it takes the fuse to open. That ability isn't really a feature of it's normal current carrying capacity but is calculated using the adiabatic (S=sqrt(I² t) / k) using the worst-case conditions. To do that properly you'll need to know the characteristics of the cable (to give k), likely fault currents (I) and the time taken for the fuse to open for those currents (t). But as a short cut, the OSG gives max Zs values for various fuse & conductor sizes. Looking at the table for 100A BS 1361 (BS 88-3) fuses - 6.0mm² has a max loop impedance of 0.15 Ohms (measured) and all smaller sizes are "NP" for not permitted (i.e. they fail the adiabatic or exceed 5s disconnection time even in the most favourable conditions). (Although the table is set out for earth faults, the same physics & maths applies for L-N faults, although obviously using L-N loop impedance rather than earth loop impedance). So my initial conclusion is that at least a 6mm² is probably needed to withstand faults when protected by a 100A fuse. In this situation though, BS 7671 does permit fault protection to be omitted where certain conditions apply as specified in 434.2.1 - i.e. conductors no longer than 3m, installed in such a manner to reduce the risks of fault to a minimum and to minimise the risk of fire or danger to persons - if all those conditions are met then your 2.5mm² could be acceptable. - Andy.
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