Three 3kW handwashers
- Thread starter oo7
- Start date
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And FCUs that will take the cable size too.
I'd be happier running 3 x 20A radials in 2.5mm. Standard circuits too.
I'd be happier running 3 x 20A radials in 2.5mm. Standard circuits too.
K
kai
Have you tried using a single B40 amp breaker to feed a small consumer unit, containing three B16 breakers, one per heater, wired in appropriate cable for the lengths involved, ending in a 13amp fused spur box for each hand wash heater?
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You wouldn't need the FCU if you're using 20A radials, which would be my preferred way of wiring this.
If the building is three-phase, one 16A breaker on each phase wired in 2.5mm terminating in a 20A DP isolator to which the water heater is connected?????
That would be ideal!
Unless the MIs specify a 13A Fcu!
Does this 0.725 rule apply if overload protection is not needed?
It's 3 fixed loads that is complicated by a useless installation manual
That's a good question where the answer could be no but it does not arise in this case because the maximum load is greater than the CCC of the cable anyway.
Technically/theoretically speaking, no, it couldn't possibly apply, since that 'rule' only results from the current that could flow (for 1 hour) in the event of an overload before a BS3036 fuse operates. If it is deemed that "overload cannot happen" (always an iffy concept, IMO), then that situation obviously cannot arise.
However, it doesn't stop there. Again theoretically, if one were really happy to go down this "overload can't happen' path, far from having to 'de-rate' the tabulated CCC by multiplying it by 0.725, one should really be able to actually 'up-rate' it, by multiplying by a factor of 1.45 - but try convincing anyone about that!
Kind Regards, John
Sorry I've not replied for the last few days, I've been busy and not had a chance to have a closer look at the installation.
I've now found a CU that contains 3 spare 60898 MCB. Now the first thing I though was just run 3 radials from these spares but then I thought why not run the system above (although there would be no requirement to knock it down to a 13A FCU?) and save 2 spares for any future work. John, please explain why this looks to be a case for none discrimination. I'm guessing this is because you were thinking that the 45A upstream fuse was 3036 and the 16A was a 60898? Even so I think discrimination would have been satisfied (please feel free to check as this is the first time I've considered discrimination).
As for the MIs it only calls for the use of a FCU if we're taking a spur of the ring.
I've now found a CU that contains 3 spare 60898 MCB. Now the first thing I though was just run 3 radials from these spares but then I thought why not run the system above (although there would be no requirement to knock it down to a 13A FCU?) and save 2 spares for any future work. John, please explain why this looks to be a case for none discrimination. I'm guessing this is because you were thinking that the 45A upstream fuse was 3036 and the 16A was a 60898? Even so I think discrimination would have been satisfied (please feel free to check as this is the first time I've considered discrimination).
As for the MIs it only calls for the use of a FCU if we're taking a spur of the ring.
Ooooh!
That, I think, would be far too involved and require sub-clauses in just about every relevant regulation and table.
Also, wouldn't/doesn't/couldn't it apply to almost every circuit apart from those with sockets?
12kW showers or 34kW cookers on 4mm² with 50A mcb.
6mm² tails. Are we over-engineering?
Edit. Of course the 1.45 is only for an hour, not continuous.
You were talking about a B40 MCB feeding three B16 MCBs which, in turn, were feeding three 13A FCUs. You obviously would get no signicant discrimination between the 13A fuses and the 16A MCBs. As for discrimination between the B16s and the upstream B40, that's pretty marginal. With a 3x differential, you would hope to get discrimination, but 40/16 is only 2.5x, so I certainly wouldn't put my shirt on that discrimination.
Fine, and that makes sense. In that case you wouldn't need to have both B16s and FCUs, so that discrimination problem goes away. However, I would still not put money on a B16 and B40 discriminating. The ideal, of course, would be to have the three B16s in a CU fed from it's own switchfuse from the main supply, with, say, a 60A or 80A fuse in that switchfuse. Then you would certainly hope to get discrimination from the B16s.
Kind Regards, John
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