Ovens and 30A fuse/feed

robopj

Diversity is not only about current, it is also about the time of operation of a load. This factor is complex and rarely taught to electricians, but is an important consideration for design.

A kettle boils in a few minutes, much too quickly to show much temperature rise in a cable. The same with a microwave, we usually use it for short periods on full power, but the same applies. Fuses and MCBs also have a “time - load to trip” characteristic, and all these together make domestic diversity very safe, trips do not normally occur however much you use the kitchen appliances together. The overall average consumption of the kitchen is much less than the total rating of all the appliances added up, and it is this average that best describes the heating effect of cables or the tripping of MCBs or blowing fuses. The power dissipation in a metre of cable is quite small, a few Watts maximum, and the thermal mas significant, so it takes a long time to heat, maybe an hour or more at maximum rated current.

As you do not trip your MCB the installation is perfectly satisfactory, and it is very unlikely the cables get overheated. I am more concerned with this £90 installation charge, what exactly do they undertake to do? I suspect that they just connect the oven to the nearest available power and leave, which may be somewhat unsatisfactory.

You also say you expect that modern appliances will take less power, not more. The consumption is fixed by simple physics, for example, boiling a kettle with less power probably takes more energy than doing the same job quickly, or at best the same amount, because heat loss is more. An oven has always been fairly well insulated, but now the consumer wants a glass door, that has very little insulating value, even if there are two layers because the space in between is well ventilated to keep the outside surface cool to the touch.

Very little is quite as simple as it seems!

David CEng etc.

robopj

Diversity is not only about current, it is also about the time of operation of a load. This factor is complex and rarely taught to electricians, but is an important consideration for design.

A kettle boils in a few minutes, much too quickly to show much temperature rise in a cable. The same with a microwave, we usually use it for short periods on full power, but the same applies. Fuses and MCBs also have a “time - load to trip” characteristic, and all these together make domestic diversity very safe, trips do not normally occur however much you use the kitchen appliances together. The overall average consumption of the kitchen is much less than the total rating of all the appliances added up, and it is this average that best describes the heating effect of cables or the tripping of MCBs or blowing fuses. The power dissipation in a metre of cable is quite small, a few Watts maximum, and the thermal mas significant, so it takes a long time to heat, maybe an hour or more at maximum rated current.

As you do not trip your MCB the installation is perfectly satisfactory, and it is very unlikely the cables get overheated. I am more concerned with this £90 installation charge, what exactly do they undertake to do? I suspect that they just connect the oven to the nearest available power and leave, which may be somewhat unsatisfactory.

You also say you expect that modern appliances will take less power, not more. The consumption is fixed by simple physics, for example, boiling a kettle with less power probably takes more energy than doing the same job quickly, or at best the same amount, because heat loss is more. An oven has always been fairly well insulated, but now the consumer wants a glass door, that has very little insulating value, even if there are two layers because the space in between is well ventilated to keep the outside surface cool to the touch.

Very little is quite as simple as it seems!

David CEng etc.