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Correct wiring for twin 3Kw Immersion heaters

Rubia30

Morning folks,

I have a simple question presumably for an electrician that I could do with the answer to having just witnessed some poor installation practise. As a plumber I recently changed a 3.0Kw immersion for a 3.6Kw immersion. It was then left for the electrician to wire in whilst doing other work in the house. 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. Very luckily the customer was in the room at the time and caught it in time to go and isolate the power and put out the smoldering wire.

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 so 37amp or 47amp clipped direct with a junction box with two separate again 4 or 6mm cables and not anything less from the junction box? I'm asking since this is work that is taking place on an Island off the west coast of Scotland where building control or any CPS is totally absent. 

Thanks

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  • 0

    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.

  • 0 in reply to Sparkingchip

    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

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  • 0 in reply to Sparkingchip

    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

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