Electric boiler wiring

Certainly the power for the boiler and controls should be on different circuits (just make sure there is the correct separation between the two - e.g. the relay/contactor on the so called "volt free contacts" on the boiler). Introducing an MCB or fuse, by definition does create a new circuit though, where ever it's fed from.

Running heating controls off a fused connection unit (with a 3A or 5A fuse) is pretty normal in the wider world (and often provides a convenient isolation point) - I can't see that using a 6A MCB makes any practical difference (other than easier to reset if there's a fault, but if there is a fault, there would be a lot of investigating to do first so the replacement of a fuse would be the least of your worries). Tapping off the 63A circuit is a bit odd, while there's nothing particularly wrong in principle, 16mm² wires aren't going to be exactly a nice fit into fused connection unit terminals and using smaller cables for that bit will need care and some detailed calculations to prove it's OK (a bit of 2.5mm² T&E almost certainly won't be). A Fused connection unit off a local socket circuit might be a more common arrangement. Or a dedicated 6A circuit from the consumer unit - all seem valid options to me.

Manufacturer's instructions are often a bit, let us say, simplified - perhaps targeting one particular situation and ignoring other valid ones (these seem to presume the "split load" consumer units that were popular in the past, but are falling out of favour these days). We used to be constrained to "follow manufacturer's instructions" but after a lot of complaints about instructions not making an awful lot of sense (or often being originally written for non-UK markets so not considering UK practice and regulations) the regs these days only require that we "take account of" manufacturer's instructions - which gives a bit more leeway - at least as far as the wiring regs are concerned (manufacturer's guarantee dept might take a different view of course).  (The reference to "IEE Wiring Regulations" is a classic indicator of instructions not being well written, copied from something else or not kept up to date - it's been the IET rather than the IEE for several decades now). 

  - Andy.

Thanks Andy - appreciate your reply

Would you mind clarifying a couple of things for me? Apologies if they’re stupid questions…

For the avoidance of doubt, you think the boiler and controls should definitely be on separate circuits? Was just a bit thrown off by your subsequent comment that it’s odd to spur off the 63A circuit but nothing wrong with it in principle.

When you said running heating controls off a fused spur is pretty normal in the wider world - are you referring to, say, on a system with a gas boiler with negligible electrical power requirements?

KMA1987 said:

This raises another question - the manufacturer’s warranty engineer said 12kw seems oversized for the property size (and heating only), so suggested deactivating some of the elements to rate the boiler down to 6kw and see how we get on with that. In hindsight I suspect the installer didn’t do a heat loss calculation and just went for a like for like replacement.

A quick flick through Screwfix's catalogue shows that their least powerful gas boiler is 11 kW, so 6 kW implies a very well insulated, or very small dwelling (or a combination of the two).

AJJewsbury said:

I think it's meant to be L-N loop impedance

I see. So, if it is PME, it will be much the same.

0.1939 is clearly not to an accuracy of 4 s.f., but I cannot quite work out what fraction gives that value. (19/98 ≈ 0.1939 (4 s.f.))

Incidentally, is the 230 V +10%/-6% in R. 27 of ESQCR on or off load?

Incidentally, is the 230 V +10%/-6% in R. 27 of ESQCR on or off load?

The local load is not supposed to matter, as this is at the point of metering, and dominated, probably, by everyone else who shares the transformer secondary, not the load of the installation. Of course it does a bit...  especially with a 50A boiler. 
Drops within the installation are of greater concern, especially if, as likely it is soon, we are allowed down to 207V at the origin , as as that could mean as low as 190 or so at the far point in a large building on a fully loaded final circuit - which may be fun for some devices, as the product standards are also usually based on 230+/-10%.

You were not expecting joined up technical thinking were you ?
Mike.

Chris Pearson said:

Incidentally, is the 230 V +10%/-6% in R. 27 of ESQCR on or off load?

I thought both - it not being 100 miles away from 0.35Ω * 100A gives 35V ... whereas 16% of 230V gives 36.8V.

Or to put it another way if you had simple system feeding just one consumer and a constant 253V at the transformer and the DNO's lines had a resistance of 0.35Ω - the consumer would see 253V (230V+10%) at their supply terminals when drawing no load, and 218V (i.e. just a tad over 230V-6%) when drawing 100A.

    - Andy. 

Apologies if they’re stupid questions…

Not at all - the only stupid questions around here are the ones that aren't asked!

For the avoidance of doubt, you think the boiler and controls should definitely be on separate circuits? Was just a bit thrown off by your subsequent comment that it’s odd to spur off the 63A circuit but nothing wrong with it in principle.

Sorry, yes that probably wasn't explained well. In wiring regs terms a circuit is an arrangement of things that are protected by the same overcurrent protective device(s) (i.e. fuses or MCB/RCBOs) - so as long as the control circuit has its own 5A fuse or 6A MCB for instance, then it counts as its own distinct circuit. If you had just a 63A MCB that fed both the boiler and the controls without any further fusing down, then they wouldn't be separate circuits.

Circuits may be arranged in many different ways - they might be taken directly off the only consumer unit in the house, or from a secondary consumer unit which itself is fed from the main consumer unit, or from a fused connection unit from some other circuit - the possibilities are almost endless. You could for instance have a 63A circuit supply a consumer unit for the boiler, which then has a second 63A MCB (or RCBO) for the boiler itself and another MCB/RCBO for the controls - the regs even allow the 2nd 63A MCB to be omitted since it's not providing anything over what the 1st one does - which pretty much boils down to having a single 63A circuit with a 5A FCU teed off it. Circuits are hardly ever entirely independent of each other - there's usually some common point upstream somewhere - so a single fault can still disconnect both. There are many ways of skinning a cat as the saying goes. My main reservation of a fused connection unit connected to a 63A circuit is simply the practicality of getting sufficiently large conductors into the FCU terminals. (There are rules that would allow thinner conductors to be used for just the branch that serves the connection unit - but there are limits - I'd have to do the calculations to be sure, but my gut feel is that such an arrangement would be "interesting".)

When you said running heating controls off a fused spur is pretty normal in the wider world - are you referring to, say, on a system with a gas boiler with negligible electrical power requirements?

Exactly so. Maybe not negligible - often a pump or two and a few motorized valves as well in addition to the thermostats/time-switches/programmable stats - so maybe a few hundred Watts. I'm presuming that the controls for your system wouldn't be that different - if anything power demand might be less as they only have to power a relay/contactor to control the boiler, whereas on a gas system they'd typically have to power the boiler's circulation pump, gas valve and flue fan as well.

Hope that helps. If not do, ask again!

   - Andy.

Just to add - it may well be thought useful to be able to isolate the boiler while leaving the controls energised (for testing/commissioning, maintenance or fault-finding for instance) - and having two circuits that could be isolated independently of each other might be an easy way of achieving that. That said, in some situations single pole MCBs (and some RCBOs) aren't suitable for isolation by themselves (typically where the building has its own Earth rod and doesn't depend on the supplier's earth connection) and in any event isolation can be achieved by other devices (e.g. FCUs or rotary isolator switches) so again the precise circuit arrangement needn't be that critical.

   - Andy.

AJJewsbury said:

My main reservation of a fused connection unit connected to a 63A circuit is simply the practicality of getting sufficiently large conductors into the FCU terminals.

It is not just a matter of getting one 16 mm² conductor into the FCU, a second one would be needed to take the supply on to the boiler's heating elements. I am not sure that any FCUs can cope with even one 16 mm² conductor.

So perhaps a Henley block?

Thanks again Andy

So the first electrician who inspected the installation wrote - amongst other issues requiring remedy - that the “Boiler control supply cable undersized for current carry capacity. Also not on its own circuit”. When I asked what he meant by the latter sentence he said this was referring to the manufacturer requiring a separate 6A MCB for the controls as both were currently on the 63A RCBO. I assumed this meant he would need to add a new circuit from the consumer unit for the controls with its own MCB.

The second guy said he would just replace the existing 3A fused spur for the controls with a 6A MCB in the same spot (i.e. not at the consumer unit) to satisfy the manufacturer’s requirements.

Does it sound like I should run a mile from either of these guys?! Or are they just different ways of skinning a cat, as you say?

Chris Pearson said:

So perhaps a Henley block?

I had in mind the 16mm² looping into the boiler and then out again, perhaps in something smaller to the FCU (as only fault protection would be needed, overload covered downstream by the fuse). A suitable joint box of some kind is of course a valid option too (an adaptable box with line taps in it might also work).

I've looked up the generic energy let-though for a 6kA B63 and seemingly it's 65 000 A²s - so k²S² would suggest S of at least 2.22mm² (with k=115) - so my gut feel that 2.5mm² T&E would be too small would be correct, if only due to the 1.5mm² c.p.c.. In conduit with 2.5mm² for all conductors might work, or a 2.5mm² cable with a full sized c.p.c.. If T&E then even 4.0mm² would be out as that still has a 1.5mm² c.p.c. so 6mm² would be the min. Possibly the RCD could reduced energy let-through for earth faults that would be needed to justify a smaller c.p.c. - but to justify a 1.5mm² you'd have to keep the let-through below 29,756 A²s so at the guaranteed max 40ms opening time the fault current would need to be under 862.5A (i.e. Zs would have to be reliably over 0.27Ω at the tap-off point). Not impossible, but not the largest of margins to play with.

  - Andy.

Chris Pearson said:

So perhaps a Henley block?

I had in mind the 16mm² looping into the boiler and then out again, perhaps in something smaller to the FCU (as only fault protection would be needed, overload covered downstream by the fuse). A suitable joint box of some kind is of course a valid option too (an adaptable box with line taps in it might also work).

I've looked up the generic energy let-though for a 6kA B63 and seemingly it's 65 000 A²s - so k²S² would suggest S of at least 2.22mm² (with k=115) - so my gut feel that 2.5mm² T&E would be too small would be correct, if only due to the 1.5mm² c.p.c.. In conduit with 2.5mm² for all conductors might work, or a 2.5mm² cable with a full sized c.p.c.. If T&E then even 4.0mm² would be out as that still has a 1.5mm² c.p.c. so 6mm² would be the min. Possibly the RCD could reduced energy let-through for earth faults that would be needed to justify a smaller c.p.c. - but to justify a 1.5mm² you'd have to keep the let-through below 29,756 A²s so at the guaranteed max 40ms opening time the fault current would need to be under 862.5A (i.e. Zs would have to be reliably over 0.27Ω at the tap-off point). Not impossible, but not the largest of margins to play with.

  - Andy.

Oddly I'd done a similar sum but assumed 2,5mm2 in singles  and decided it was OK .....   I guess you could use 1.5mm2 flex, or any other cable where the earth core is full size rather than ordering a reel of Irish T and E to be imported. But even so I'd try and keep that link below 3m long and inside something like conduit and not worry further. 

Mike.