Timer for immersion and appropriate fuse rating

Discussion in 'Electricians' Talk' started by HAmos, Sep 16, 2021.

  1. HAmos

    HAmos New Member

    Hi everyone,
    I want to fit a timer to an immersion heater. Just a mechanical one would do although it could be digital if necessary. All the timers that I can see are rated 16A. However, this heater is on a circuit with a 20A MCB. Does this mean I shouldn't fit the timer? Or if I do, I have to get a smaller breaker? But would that mean that it starts to trip annoyingly?
    Not sure of the right solution, advice appreciated.
  2. Alan sherriff

    Alan sherriff Active Member

    What size cable s it ,I would imagine if a dedicated circuit it would be o/k possible 2.5 m/m if so breaker and timer good to go
  3. rogerk101

    rogerk101 Screwfix Select

    Most immersion heater timers are rated for 3kW, which is more than enough for most immersion heaters.
  4. HAmos

    HAmos New Member

    Yes, it's in 2.5mm, with a dedicated circuit. Thanks for your answers.

    To further my understanding then, I can see why most immersion heaters, at 3kW, are on a 16A breaker. So why would this be on 20A? And does that not mean that the timer wouldn't have adequate protection?

    Thanks again
  5. MGW

    MGW Screwfix Select

    The 20 amp MCB protects the cable, often there is a 13 amp fuse in the timer or fused protection unit, the timer depends on the plumbing.

    With the Wilson system the immersion is in a separate tank, and the time switched on will give you how much hot water is stored, only the Irish seem to have worked out how to do this, but it means timed switched on is critical.

    With off peak again time is important as heated at right time it costs less.

    However 40 gallons of water takes a lot of heating and cooling, I have boiler switch on 4 days a week and 1/2 hour per day, in fact after 20 minutes the boiler return water is too hot so only really gets 20 minutes every other day.

    With electric there is a built in thermostat, so unless on cheap rate or Wilson system there is very little point having a timer.
  6. HAmos

    HAmos New Member

    yes the 20A MCB protects the cable, which can carry 27A, so the MCB must be less than that. Should it not also protect any accessories permanently connected? I'm thinking of a permanently connected timer, something like this (unless any of you tell me otherwise) https://www.toolstation.com/axiom-24-hour-mechanical-timer-16a-2a/p78960#reviewTab , so I just want to make sure that it has the necessary protection.

    I agree MGW. But my mum wants one because she says it's really hot. I reckon she's hot because it's been hot lately and, even if it's on a thermostat and therefore only heating for 20 minutes across the day, there's a tank full of hot water acting like a big radiator. What can I say, she wants one. I just want to do it right and know she's safe.
  7. Fusebox Wales

    Fusebox Wales Member

    It's not uncommon to see a 2.5mm immersion heater circuits on a 20A mcb. However, if the load is only 3Kw, they should ideally be on a 16A mcb.

    It's possible that whoever installed your fuse box only had a 20A and used that instead of 16A.

  8. HAmos

    HAmos New Member

    Thanks for your help everyone. I don't quite understand why I can put a 16A rated accessory on a circuit only has protection down to 20A but I you all seem to think that it won't be a problem which is reassuring. Thanks to you all for taking the time to reply.
  9. rogerk101

    rogerk101 Screwfix Select

    The overcurrent protection device (fuse, MCB, RCBO) is there to prevent overloading the CABLE ... not the device being powered.

    Forget the power rating of the device being powered ... it is NOT being protected by the overcurrent protection device at all.
  10. MGW

    MGW Screwfix Select

    Not quite true, the manufacturer can stipulate the protective device needed for a non portable appliance, not uncommon to request semi-conductor fuses, and clearly most motors will have a motor overload which will allow a start amp and run amp rating, this was a one time oil filled dash pots.

    I have not seen a requirement for immersion heaters.

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