The diversity of Diversity

IMHO, the generally accepted method is to assume 100% of the largest socket outlet circuit, and 40% of the remaining circuits.

I do not feel that calculations based on the loadings of individual appliances are appropriate. The whole point of 13 amp sockets is that they are for general use by unskilled persons, whom may use anything with a matching plug.

In the particular example given, it also depends on the size of the house. It is in my experience most unusual to provide a dedicated 32 amp ring final for a loft space, unless this is in fact another habitable floor.

It is also unusual to provide more than two ring finals for the ground floor. One for the kitchen, and one for the rest of the ground floor is common. Any utility room being on either the kitchen ring or the on the other ring final, as seems best.

It may of course be that this is an unusually large house, perhaps occupied by an extended family or with live in staff. Duplicated washing machines and tumble dryers would certainly justify a dedicated 32 amp circuit.

So if this IS an unusually large home, with the loft used as living space, and the likelihood of an extended family and live in staff, then the “standard” calculation of “largest circuit, plus 40% of the remainder” is probably reasonable, and a single phase 100 amp service starts to look marginal. 

On the other hand if this is an average sized, single family home, with negligible load in the loft, and no real need for THREE ring finals on the ground floor, then I might base calculations on three “needed” ring finals, rather than on the five actually installed.

IMHO, the generally accepted guide, referenced above, is outmoded and a new approach arguably needed. Perhaps an updated guide is required ? I would favour something like 20 amps per dwelling, plus 5 amps per bedroom. For general purpose socket outlets only. Cookers, showers, EV chargers, and other loads on dedicated circuits being considered in addition.

A single bedroom flat would therefore need 25 amps for socket outlets (still install a standard 32 amp circuit, but assume a 25 amp load when calculating maximum demand)

A five bedroom house would need 50 amps under that system for socket outlets. (still install  two or three standard 32 amp circuits, but assume a total 50 amps when calculating maximum demand.)

IMHO, the generally accepted method is to assume 100% of the largest socket outlet circuit, and 40% of the remaining circuits.

I do not feel that calculations based on the loadings of individual appliances are appropriate. The whole point of 13 amp sockets is that they are for general use by unskilled persons, whom may use anything with a matching plug.

In the particular example given, it also depends on the size of the house. It is in my experience most unusual to provide a dedicated 32 amp ring final for a loft space, unless this is in fact another habitable floor.

It is also unusual to provide more than two ring finals for the ground floor. One for the kitchen, and one for the rest of the ground floor is common. Any utility room being on either the kitchen ring or the on the other ring final, as seems best.

It may of course be that this is an unusually large house, perhaps occupied by an extended family or with live in staff. Duplicated washing machines and tumble dryers would certainly justify a dedicated 32 amp circuit.

So if this IS an unusually large home, with the loft used as living space, and the likelihood of an extended family and live in staff, then the “standard” calculation of “largest circuit, plus 40% of the remainder” is probably reasonable, and a single phase 100 amp service starts to look marginal. 

On the other hand if this is an average sized, single family home, with negligible load in the loft, and no real need for THREE ring finals on the ground floor, then I might base calculations on three “needed” ring finals, rather than on the five actually installed.

IMHO, the generally accepted guide, referenced above, is outmoded and a new approach arguably needed. Perhaps an updated guide is required ? I would favour something like 20 amps per dwelling, plus 5 amps per bedroom. For general purpose socket outlets only. Cookers, showers, EV chargers, and other loads on dedicated circuits being considered in addition.

A single bedroom flat would therefore need 25 amps for socket outlets (still install a standard 32 amp circuit, but assume a 25 amp load when calculating maximum demand)

A five bedroom house would need 50 amps under that system for socket outlets. (still install  two or three standard 32 amp circuits, but assume a total 50 amps when calculating maximum demand.)