Cooker Connection

uksparks · May 30, 2014

Hi,

Ok, let me know what your thoughts are as to MCB size, cable size of cable from cooker to wall, nothing more, just let me know, then ill tell you what I have just found.

The cooker uses 10.4kW to 11.6kW, its free standing, 4 rings and double oven, new oven, well bought it off someone, but still has all labels on it etc, looks like its been used once. So lets just say on christmas day when you have stuff warning in the top oven, turkey in the bottom, and all 4 rings on boiling potatoes etc, the point is, it "could" draw 11.6kW.

1. What size MCB
2. What size cable for circuit
3. Size of cable from cooker to outlet.

Richard Burns · May 31, 2014

I worked out the calculations for a 14.8kW cooker (5 rings and two ovens) (there is a graph on another thread somewhere) and if both ovens were up to temperature then the average current was 25A with everything on and this was the case for over 100 minutes.

Knobhead · May 31, 2014

telectrix said:
trust a sat. nav.? in your dreams. sat navs have taken me up farm tracks to locked gates, down roads with signs on saying unsuitable for vehicles, told me to take 3rd exit at a roundabout that was traffic lights, 5th exit from a roundabout that only had 4 exits. trust??? i'd rather trust harold shipman with my grandmother.

You’ve got to love sat-nav.

Not long before I retired from the foundry I spotted a nice shiny new Volvo estate driving off in to our scrap stock ground.
Walked back to the workshop, got the van and followed him. When I caught up with him he was getting the jack out to change a shredded tyre.
I couldn’t bring myself to give him a rollicking for trespass, both he and the car were covered in ---- by this time.
He showed me his sat-nav and yes he was following the road shown. It was a private works road that hadn’t been used since 1965 when the bridge over the canal was taken out.
Helped him change the wheel and told him to follow me. We got about 50 yards and a second tyre was shredded. He was near in tears when I took him to the workshop and made him a drink and to wait for the AA.

According to Google earth, my flat is three miles away from where I left it this morning.

Again at the foundry, the great white chief asked me to set on his laptop for the dreaded sat-nav. Apparently our workshop was 2’ below sea level and travelling northwards at 1MPH.

uksparks · May 31, 2014

Richard Burns said:
I worked out the calculations for a 14.8kW cooker (5 rings and two ovens) (there is a graph on another thread somewhere) and if both ovens were up to temperature then the average current was 25A with everything on and this was the case for over 100 minutes.

Pidgin be interested to see that graph for future reference if you ever find it, I'll see if I can find it, but you may remember the thread better.

Richard Burns · May 31, 2014

This was a 14.8kW cooker with five rings and two double ovens all switched on at the same time (which would never really happen as the ovens would be on first, then the rings on for the last half hour or so). Some assumptions have been made about how often the rings and ovens go on and off so this is not a precise assessment (though generally taking the worst cases) but gives you the idea. A 32A breaker would never trip.
(the ovens start cycling around 20/30 mins in)

[ElectriciansForums.net] Cooker Connection

uksparks · May 31, 2014

What did you produce this data with just out if interest? A take it it's an expensive fancy bit of kit...

Richard Burns · May 31, 2014

Why would you think that? I do not do expensive!

This is calculating the current usage every 10s in a table then converted to a graph in excel.
simple basic easy cheap

uksparks · May 31, 2014

Ah very clever, but how did you get the data for the bottom chart then?

I presumed you used some fancy power analyser or something!

Richard Burns · May 31, 2014

I took it that the current flowing though the MCB at 32A is generating heat at a rate that is equal to the the heat dissipation for the MCB (as it is designed never to trip at 32A) so as the current increases the heat generation increases but the heat dissipation remains the same (approximately), based on the time current curve for a 32A MCB I correlated the timings to the heat generation from the current (less the heat dissipation) and then calculated the overall heat generated in the MCB over time (again every 10s), because I had then got fed up with working out the calculations I failed to extend the calculation to account for the zero rate of heat dissipation once the current drops below 32A (therefore the heat in the MCB goes negative on the graph whereas it should stop at ambient temperature).

Silly Sausage · May 31, 2014

Good stuff Richard!
Could you plot the curves for average Current & kW on the same graph?

Richard Burns · May 31, 2014

Archy Styrigg said:
Good stuff Richard!
Could you plot the curves for average Current & kW on the same graph?

I can't quite follow there, the graph shows the current and kW on the same graph.
this is the average over each ten seconds, what would you want as the period of time for the average?
If you took 100 hours then the average current is 29A and the average kW is 6.6kW.

joe.ninety · May 31, 2014

Richard Burns said:
I took it that the current flowing though the MCB at 32A is generating heat at a rate that is equal to the the heat dissipation for the MCB

Is the heat being generated not simply a product of the current being drawn at that particular moment, vs the resistance of the MCB?

Richard Burns · May 31, 2014

joe.ninety said:
Is the heat being generated not simply a product of the current being drawn at that particular moment, vs the resistance of the MCB?

Yes that is it.
When the MCB is carrying a constant 32A it will never trip on thermal overload, this is equivalent to about 7.3 kW load.
Therefore the heat generated by 32A passing through the MCB is dissipated to the surroundings at the same rate as it is being generated.

If the MCB were carrying 33A, theoretically the MCB thermal overload may heat up very slowly and eventually trip, but the rate of heating above the rate of dissipation would be only that equivalent to an extra 1A passing through the MCB.
As the current increases the thermal overload will heat up faster and so trip faster.

When the current is lower than 32A then the heat dissipates faster than it builds up so the MCB will stay at a temperature below the tripping temperature.

If the MCB takes a current of 58A for one minute then the thermal overload will be getting close to the temperature of tripping, if the current then drops to less than 32A the thermal overload will start to cool down, however if the current drops to say 40A then the rate of heating is slower than with 58A but still increases so it will take longer to trip but will still trip eventually.