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Mugwuffin

Recommended contactor config for immersions

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Mugwuffin

Hi, I’m looking for some advice regarding contactors. I have a 300l unvented hot water tank in the loft that is horizontal. It has 2 x 3kw immersion heaters but because the tank is horizontal they are not top and bottom. Usually the hw is heated with an oil boiler but in summer to save oil I’d like to use the immersion heaters. The immersions are supplied by a single 32A mcb with 6mm2 t&e and each one has a 20A dp switch in series at the tank where the tails are wired to. I switch the immersions using the mcb currently for a boost but want a more permanent ‘smart’ option. I’m thinking of using a Shelly 2.5 twin relay smart switch to switch 2x 20A 2pole NO contactors one for each immersion load. The Shelly’s will be powered from the boiler/ch supply also in the loft. The question is it ok to use the single 32A mcb feed and split at the contactors for 2x 20A one for each heater? Or do the contactors have to be rated more than the mcb @ say 40A each or just 1 x 40A contactor with 4poles? I’d prefer 2x independent contactors because having the ability to switch on 1 immersion at a time has its benefits for boosting or if 1 fails. I can’t run another feed from the cu as 2x 16A because the route is now well covered after an extension.

 

cheers. 

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Phoenix

Not keen on the immersions (pessumably with a 2.5mm/1.5 Heat resistant flex tail) on a 32A cirucit without overcurrent protection, although some will argue that the element isn't capable of overload, so not ideal to start.

 

If you are putting contactors in, I'd just put a couple of B16 MCBs in the same enclosure to fuse down for each immersion

 

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Mugwuffin

Thanks @Pheonix I managed to source two 32A  contactors. But I guess with what your saying the 2.5mm tails are the weakest link on a 32A circuit. Is it ok to place the two 16A mcb’ in a 4 way enclosure next to the contactors? I hear they need ‘breathing’ space. They will only be in use for about 1.5hours a day on and then off so not constant use and not multiple switching. 

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