Fixed load

[studentspark]

I thought a fixed load was an immersion, and shower, a cooker. Any one appliance that has a maximum load.

The cooker and the shower, have a load that can be altered. But are still classed as a fixed load as you can't go over its kW rating?

Am I wrong?

I ask in relation to the BS3036 fuse.

I have always thought that the 0.725 Cf factor applied, to any circuit protected by a BS3036.
But looking at equation 5 in BS7671 the correction factor for the BS 3036 is not used.

As it can not be overloaded, the design current is used in any cable calculations.

But I thought the historical reason why immersions were generally on 2.5mm2 and cookers were 6mm2 cable, was down to the correction factor.
If condition allowed, 1.5mm2 and 4mm2 would be adequate. for a 15A and 30 A fuse
But if overload protection is not required, why the increased cable size.

Thanks .

 

[EFLImpudence]

Am I wrong?

No.

I ask in relation to the BS3036 fuse.
I have always thought that the 0.725 Cf factor applied, to any circuit protected by a BS3036.
But looking at equation 5 in BS7671 the correction factor for the BS 3036 is not used.
As it can not be overloaded, the design current is used in any cable calculations.

Ok.

But I thought the historical reason why immersions were generally on 2.5mm2 and cookers were 6mm2 cable, was down to the correction factor.
If condition allowed, 1.5mm2 and 4mm2 would be adequate. for a 15A and 30 A fuse
But if overload protection is not required, why the increased cable size.

I suspect it was lack of knowledge or it probably says do that in the OSG.

You might ask why is 2.5mm² still used for immersions and 6mm² (if not 10mm²) still used for cookers when protected by MCBs.

 

[JohnW2]

I thought a fixed load was an immersion, and shower, a cooker. Any one appliance that has a maximum load. ... The cooker and the shower, have a load that can be altered. But are still classed as a fixed load as you can't go over its kW rating? ... Am I wrong?

Is the term not usually used to simply describe a 'hard-wired load', as opposed to 'unknown' loads which may be plugged into a sockets circuit?

Virtually any load 'in normal operation' presents a 'variable load', with a 'maximum' (and, as with an immersion etc., a load which will often be zero).

I'm not sure that I understand your concern.

Kind Regards, John

 

[studentspark]

You might ask why is 2.5mm² still used for immersions and 6mm² (if not 10mm²) still used for cookers when protected by MCBs.

Its just whats been done, habit, lack of understanding, and confidence. Even knowing that the smaller cable size will be adequate, you still go a size up.

I'm not sure that I understand your concern.

No big concern, just a realisation , that on a fixed load with a BS3036 you don't apply the factor.
Doesn't really matter. Im not doing and calculations for cables on a BS3036.

 

[EFLImpudence]

Its just whats been done, habit, lack of understanding, and confidence. Even knowing that the smaller cable size will be adequate, you still go a size up.

Larger cables than necessary seem to be a "fashion".

 

[flameport]

But if overload protection is not required, why the increased cable size.

A cooker (as in a freestanding deal with 2 oven compartments and several hot plates on top) can cause an overload.

The circuit will be designed taking diversity into account, which is why 99% of the time a 32A (30A) circuit with similar rated cable is suitable even though the total rating of all of the elements is more than 32A.

The overload situation would be where one or more of the thermostats failed, resulting in element(s) being on 100% of the time, and the load exceeding 32A for however long the thing remained on, and in that situation the fuse would need to disconnect before the cable overheated, so the correction factor is applicable for a cooker circuit.

A single oven, electric shower or immersion heater cannot overload, as even with the thermostat failed closed, the load is still only the rating of the single element.

In the days when 3036 fuses were common, cooker circuits almost always had a socket outlet included, which gives even more options for overloading. If you go back far enough, that socket outlet was the only one in the whole kitchen.

 

[studentspark]

The overload situation would be where one or more of the thermostats failed, resulting in element(s) being on 100% of the time,

Thanks that make sense, with multiple elements.