Imagine this Christmas day scene...

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TTbangbang

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The 3kW dishwasher is on as someone forgot to put it on the night before after midnight mass. The 1.5kW water heater is on for hot warming coffee. The 2.7kW hob is cooking potatoes and other veg for roasting later. There is 500W from two undercounter fridges humming away and someone has put the 3kW 6-slice toaster on for breakfast!

This is applying a worst case diversity according to actual habits of the occupants. The resulting current load is 46.5A which suggests using 4mmsq and 50A RCBO.

A 32A RCBO would trip under such a load in about 2.7hours. Such a long breakfast time is?! unlikely, so choosing a 32A RCBO seems more sensible.

A 2.5mmsq ring can carry 37A so it will be overloaded on Christmas day.

Finally the question!, thanks for reading so far.

When designing circuits do you allow for rare short term overloading occurring (use a 50A RCBO) or do you ignore it (use a 32A RCBO)?

 

TTbangbang

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Why would Hob be on ring main. Should be on cooker circuit.Batty
Because it's only 2.7kW and comes with a 13A plug. It's not a heavy duty cooker and the main cooker will be running on gas (until that runs out!).

 

TTbangbang

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I always put cooker etc on there own circuit can be fused down behind appliance.Batty
If I put the cooker on its own circuit then I would have to think about putting the dishwasher and toaster on their own circuits too?

Am (slowly) beginning to realise the ring circuit options are:

4mmsq2 50A RCBO - covers everything, mostly OTT and v.expensive.

2.5mmsq2 32A RCBO - covers everything, some short term overloading will occur but hopefully not reach trip state, and cheapest.

4mmsq2 32A RCBO - covers everything but doesn't follow the I(design) < I(breaker) < I(cable rating) rule. I would have 46.5A > 32A < 52A ...unless I applied some additional creative diversity!

:_| :_| :_|

Which do you recommend ?:|

 
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But the hob won't draw 2.7kw unles every ring is on full blast - hence in diversity calcs. cookers are treated as 10A, 15A with socket outlet.

Sounds like you're suffering from the trend towards bigger / better /faster appliances.

If you split the hob from rest of circuit (provides a bit of future proffing aswell) my rough calc shows 8Kw = 34A which is unlikely to cause any problems on a 2.5mm ring on 32A RCBO

 

TTbangbang

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Thanks for that Binky and Batty. Sane voices in a mad bigger-is-better world!

Nice atavar Binky, where are you all getting your pictures from?!

 
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The 3kW dishwasher is on as someone forgot to put it on the night before after midnight mass. The 1.5kW water heater is on for hot warming coffee.
Try sticking a clamp meter on some of these appliances see what you actually get!

The Dishwasher will NOT be taking 3KW all of the time it is switched on...

draining water out.. just the pump..

some of the rinse bits just the cold water shooting in.

The 3k element will only heat bits of water for a short time!

(our dishwasher is on at the moment..plus some lights & some other bits.)

the whole house is currently only drawing 4.5amps! <1.5k!

- - - - - - -

The 1.5kW water heater is on for hot warming coffee..

but again this will be switching itself via a thermostat of some sort to stop the coffee boiling!

as me mom said.

"Coffee boiled is coffee spoilt!"

With a high power toaster. it will only be on for a short time.

or your toast is burned!

the chances are the time whilst the DW is drawing full power..

The toaster will have finnished & popped the toast up

;)

 

TTbangbang

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Thanks for the explanation Specs - I'll stick with standard circuits. I must stop thinking of RCD tripping as eternal damnation and be realistic. If it did trip, it'd be easy to turn everything off and send everybody out of the kitchen!

Thanks for atavar link Admin2 - can't make my mind up what to have!

 
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