Correct wiring for twin 3Kw Immersion heaters

I presume  your choice of electrician is very limited as well.

Presumably it will be configured so only one immersion will operate a a time, so 2.5 mm should be more than sufficient. 

If they can both use the same circuit at the same time,  then yes it needs 6.0 mm.

Or two separate runs of 2,5mm one per heater, allows one or other to be isolated at the origin, may make it easier if perhaps one thermostat needs changing, as there is no need to power down both while waiting for spares to arrive. It will depend on the layout and where/how things are switched.

Mind you for 1mm2 to fail that badly at ~20A suggests it was not a well cooled route. Cables need to be uprated to the next size up in hot locations, or if lagged or bunched up so heat cannot escape, so the choice of cable route is also important

Mike

Now this fella is going to go and wire in dual immersions both 3Kw to a single economy 10 unit. How should this be done safely? I'm presuming a single cable radial from the consumer unit 32amp MCB with either a 4mm or 6mm cable

For E10 (or E7) it's traditional to have the "bottom" immersion connected to the off-peak supply (normally left switched on so to give a tank full of hot water as cheaply as possible) and the "top" immersion connected to the 24h supply (normally left switched off) to be able to heat a relatively small amount of water to a decent temperature if needed to 'top up' during the day (when prices are higher). So two different supplies hence two separate (usually 16A) circuits.

   - Andy.

It could be possible to use 1.5 mm for the circuit, but I would not use it for a new installation,  but it might be found on an existing installation. 

When I first started out in the industry I once used 1.5mm2 T&E cable to supply a new immersion heater in a house. I was trying to save money. A section of the cable that was clipped direct onto a brick wall ran warm. I would not now use 1.5mm2 cable for a 3kW immersion heater. It will overheat the cable and cause energy loss in the form of cable heat.

Z.

He used a 1.0mm conductor with that size immersion. 16amp Mcb on the consumer unit and the wire got a little warm, melted the insulation and caught fire.

I wonder if the cause of the fire might have been something other than just undersized conductor - 1.0mm² should be good for 15A or 16A if clipped direct at ambients up to 30 degrees - and even if conditions were less than ideal (e.g. in a building void or ambient a bit warmer) I'd only expect it to run a little over warm, soften a bit perhaps, and maybe eventually break down in a decade or two instead of closer to a century. Bursting into flames requires quite a substantial temperature - so perhaps more likely a loose connection?

   - Andy.

Thanks for the responses. Specifically Andy this is a 250L direct immersion only unvented tank, there is no other source of heat input. The idea is that both immersions will come on at night using the economy, heat the water and then there will be no method of boosting during the daytime.  Now I can see running two separate cables from 2 breakers would make sense and using 2.5mm but that would also require 2 separate timers as far as I am aware as these are required by Part G of the building regulations. 1 timer would make sense to me however I can see the pro's and cons for any method of wiring. Personnally I do not see why the customer wants this since its about 5-6 hours on one immersion to heat that quanitity of water, so the other immersion could be used as a day time back up, but again its not my choice. And yes Sparkingchip, the electricians are rare here, almost as rare as plumbers. Z, I always carry a coil of 2.5mm flexi in the van and for straight forward swaps use this doing the work myself, that is what caught my eye when this fella used 1.0 and we got the result we did.

1.0mm is almost certainly too small, some might judge it "just about OK but right on the edge." I would expect it to get warm but not to the extent of catching fire. Some other factor must be involved for it to catch fire.

For a 3 kw immersion heater I would have used a 16 amp MCB and 2.5mm cable, unless derating for grouping and/or thermal insulation required a larger size.

For a 3.6 kw immersion heater I would have enquired a bit more since that is not a common size. If it was an imported unit and rated at 220 volt, 3.6 kw then for use on our mains which is still about 240 volts most of the time in most places, then a 20 amp MCB might be better. If it was truly designed to use 3.6 kw at 240 volts, then 16 amps should suffice.

If the bottom immersion can be supplied from a dedicated E7 supply that is only live during E7 period it only really needs a switch. 

I would put the top immersion on the 24/7 peak supply with a countdown timer 

www.tlc-direct.co.uk/.../SMTGBT4N.html

I have no issue using 1.25 or 1.5mm2 heat resisting flex for the final short immersion heater connection at the copper cylinder or equivalent.

Z.