The diversity of Diversity

I don't think that this is unusual. The more circuits you have, the bigger the apparent problem. To my mind, the value of calculating loads is to divide the circuits as evenly as possible between both RCDs. An ordinary house just isn't going to draw 100 A for an appreciable amount of time. Just think of the bill if it did!

What you should not do, incidentally, is apply diversity to diversity.

How big is the place - as a rule of thumb for a house, 32A per 100sq metres seems to be another figure- as to be honest much more than this and the rooms overheat and then you need air-con. 

If the area could be served by one ring, but it happens it has been split into more circuits for convenience or to reduce problems of earth leakage, then the actual load is not really increased.

If it reassures you, realise that short duration loads like toasters (unless they do B and B) will not matter, as the time to heat the wiring or the company fuse is far longer than the time it takes to do 4 slices. (look at the time-current breaking curves for a house fuse - a 100A fuse will pull 200A for quite a while ) In that sense loads of twice expected for less than  5 mins are not going to worry anyone.

regards

Mike " 4 rings ? " PJ 

;-) 

Thanks Chris. I've tried to split the circuits sensibly as the highest demand areas are the kitchen and utility and then it's sensibe to split by floor.

I've seen someone suggesting using a second CMU with 25mm tails when exceeding a 100A theoretical demand and whilst that fixes the problem of the 100A main CMU switch, it doesn't resolve the 100A DNO fuse. I know in practice these don't blow and anyway, as you say, the real demand will be much less than the diversity calculation.

mapj1: 
 

How big is the place - as a rule of thumb for a house, 32A per 100sq metres seems to be another figure- as to be honest much more than this and the rooms overheat and then you need air-con. 

If the area could be served by one ring, but it happens it has been split into more circuits for convenience or to reduce problems of earth leakage, then the actual load is not really increased

regards

Mike " 4 rings ? " PJ 

;-) 

It's quite large, but not crazy and its very well insulated (new build) so there's no way it will realistically be burning many kW as you say (no bitcoin mining :-).

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.)

broadgage: 
 

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.)

I like your method as it makes sense for how people life. A large area house doesn't in itself mean high power usage (outside of heating), but a large number of bedrooms can.

My scenario is a 5bed with additional loft playroom / bedroom with toilet.

Having the utility and kitchen on the same circuit seems almost OK, but I worry about the possible scenario of:

• Washing Machine 2kW peak → 8.7A
• Drier 2kW peak → 8.7A
• Iron 2.8kW peak → 12.2A
• Kettle 2kW → 8.7A

If someone turns on the kettle in the kitchen during a mammoth laundry session in the utility the peak current would be 38.3A. That would exceed a single 32A ring, hence the thought to split them.

Ignoring electric showers, EV charging and ASHP, demands these days are way below historic were a bar heater and a handful of incandescent bulbs would get the meter disc spinning. 

WireWeHere: 
Having the utility and kitchen on the same circuit seems almost OK, but I worry about the possible scenario of:

• Washing Machine 2kW peak → 8.7A
• Drier 2kW peak → 8.7A
• Iron 2.8kW peak → 12.2A
• Kettle 2kW → 8.7A

If someone turns on the kettle in the kitchen during a mammoth laundry session in the utility the peak current would be 38.3A. That would exceed a single 32A ring, hence the thought to split them.

I don't know whether the heating elements of driers are on constantly, but let's assume that they are. A washing machine will draw most power when heating the water, but that will not be for long in a modern machine, which uses less water than they used to do, and may well only be heating to 30 deg C. Once up to temperature (and steam generation if appropriate) the iron's thermostat will be clicking on and off. The kettle will be on for only a few minutes.

Even if everything was on maximum load, it wouldn't last for long enough to trip. But ask yourself whether there really would be one load in the washer, one in the drier, and a pile of ironing to do. It's unlikely.

These appliances,

• Washing Machine 2kW peak → 8.7A
• Drier 2kW peak → 8.7A
• Iron 2.8kW peak → 12.2A
• Kettle 2kW → 8.7A

Would in my view be fine on one 32 amp circuit.

The drier is a long hour load and would typically use 10 amps for hours at a time.

Washing machine might be 10 amps for about 10 minutes when heating the water, much less for rest of the cycle.

A modern electric clothes iron is likely to average about 5 amps, under thermostatic control.

A kettle is only used briefly. A very brief overload is possible, if the kettle is used whilst the washer is heating, AND whilst the iron is on via the thermostat. An overload for long enough to operate a 32 amp MCB is exceedingly improbable.

broadgage: 
An overload for long enough to operate a 32 amp MCB is exceedingly improbable.

Interesting, for my understanding is there a trip time vs overcurrent curve for MCBs? I know fuses go with the square of current but that'll be because they are thermal based. Are MCBs?

Interesting, for my understanding is there a trip time vs overcurrent curve for MCBs? I know fuses go with the square of current but that'll be because they are thermal based. Are MCBs?

MCBs have both a thermal element operating on a bimetallic strip (which trips faster on large overloads, slower on smaller ones - rather like a fuse) and a magnetic element which almost instantly triggers a trip once the current reaches a pre-determined high level (e.g. somewhere between 3x and 5x the MCB's rating for a B-type MCB).

   - Andy.