Max Demand again...

[tankerman]

Hi guys, doing a rewire on my house, been a bit separation happy, and need to reduce the total load to under 80A for the incoming supply. (currently 60A but distribution coming to upgrade cutout)
House is 3bed semi, not big. under 95m2 in total. I originally designed it with
smoke alarm circuit 6A
upstairs lights 6A
Downstairs lights 6A
Ring up 32A
Ring down 32A
Kitchen ring 32A
Utility ring 32A
Cooker (7KW pre diversity) 32A
Hob (7.2KW pre diversity) 32A
Garage 20A
Shower 40A

With diversity applied to all, I end up with 156A. If I join lights upstairs with smoke alarm, up and down rings together, kitchen and utility together, reduce garage to 16A and shower to 32A, I still get 123A after applying diversity. This exceeds cons unit 100A max let alone my (soon to be)80A incoming supply.

Any thoughts? I appreciate this may be a basic question. I'm using building control at the moment and they will send an electrician to check my work at 2nd fix. I've worked with 3phase at sea for years, but houses are new to me. Thanks.....

 

[JUD]

As a rule of thumb add up all but the largest circuit breaker and multiply by 0.4 and then add on the largest.

With your proposed changes this gives you:

Upstairs lights = 6A (including smokes)
Downstairs lights = 6A
Up/Down Ring = 32A
Kitchen/Utility Ring = 32A
Cooker = 32A
Hob = 32A
Garage = 16A
Shower = 32A

(6 + 6 + 32 + 32 + 32 + 32 + 16) x 0.4 + 32 = 94.4A


Is 32A enough for the shower?

Can you not ask for a 100A fuse instead?

 

[tankerman]

Thanks Jud, asked.... denied.... Might try harder, every time I speak to them I get a different fob off to another department. It took 5 calls to get them to give me a work ref for the job to prove they were going to do it. Anyway.... I've run 10mm2 cable to the shower area, would prefer 40A but just tryin to get the load under 100A max.

If I fit a 80A main switch i the cons unit, what are the chances I would actually overload it with my proposed load?

 

[JUD]

What you've got to remember is you are talking about the MAXIMUM demand.

How often are you going to be in the shower while having the oven and hob on full blast and washing and drying clothes and watching TV.........

 

[tankerman]

Indeed, but how do you know the limits to max load. If i had to answer why my total load exceeded the rating of the main switch, wouldn't it come down to the individual (ie NICEIC inspector) as to whether he thought I was a danger or not?

I used to work on oil tankers, bit more black and white with ratings etc out there on the oggin! I want to get it right as I start out in this.

 

[JUD]

Use 100A main switch and use the max demand calculation above (94.4A).

 

[tankerman]

Cheers JUD, called distribution again and reckon I can get a 100A out of them. Now to rethink the cabling. I'd already run the cable in the 1st order of circuits I had planned. DOh! think twice, route once eh?..... Thanks for your time.

 

[dichroic]

That figure seems high for the loads you are intending to connect??


Purpose of final circuit fed from conductors or switchgear to which diversity applies
Diversity factor to be applied to the distribution circuit:

Lighting
66 % of the connected lighting load

Heating appliances
The first 10 A of the connected load plus 50 % of the remainder of the connected load

Cooking appliance(s)
The first 10 A of the rated current plus 30 % of the remainder of the rated current in excess of 10 A plus 5 A if a socket-outlet is incorporated in the cooker control unit

Instantaneous water-heaters
100 % F.L.of largest appliance plus 100 % F.L. of 2nd largest appliance plus 25 % full load of remaining appliances

Storage water heaters
No diversity allowable

Floor warming installations
No diversity allowable

Thermal storage space heating
No diversity allowable

Socket-outlets and stationary equipment supplied by a standard arrangement
100 % of current demand of largest socket-outlet circuit plus 40 % of current demand of every other socket-outlet circuit

 

[Des 56]

Socket-outlets and stationary equipment supplied by a standard arrangement
100 % of current demand of largest socket-outlet circuit plus 40 % of current demand of every other socket-outlet circuit

These guides or suggestions fall down badly, especially the socket circuits,which are wild in their estimation of load

A typical domestic situation
Kitchen ring 32.0 amps
Utility ring 12.8
Up ring 12.8
Down ring 12.8

Total load suggested by using guide 70.4 amps
Have a shower whilst the spuds are cooking and hey presto the 1361 is fighting for its existence

It doesn't happen, does it ? ever.


Reality of average household usage
at peak" wife gone wild" time, with washer
and making occasional cuppa,whilst kids are in computer or X- box land
20-30 amp if you want to be generous and a hell of a lot less than 70+ amps

Take a clamp meter and see which of the above is more in tune with actual loading,you will probably find the on site guide is living in a different galaxy never mind planet

If I was doing a full installation unless they started mentioning 2 No showers,electric cooking,water heating and loaded garages or other, loading or the need for diversity calculations in a domestic is hardly worth thinking about
Theres plenty of numbers between 80 and 100 for nearly all standard domestic installs without actually calculating anything at all

 

[dichroic]

These guides or suggestions fall down badly, especially the socket circuits,which are wild in their estimation of load

A typical domestic situation
Kitchen ring 32.0 amps
Utility ring 12.8
Up ring 12.8
Down ring 12.8

Total load suggested by using guide 70.4 amps
Have a shower whilst the spuds are cooking and hey presto the 1361 is fighting for its existence

It doesn't happen, does it ? ever.


Reality of average household usage
at peak" wife gone wild" time, with washer
and making occasional cuppa,whilst kids are in computer or X- box land
20-30 amp if you want to be generous and a hell of a lot less than 70+ amps

Take a clamp meter and see which of the above is more in tune with actual loading,you will probably find the on site guide is living in a different galaxy never mind planet

If I was doing a full installation unless they started mentioning 2 No showers,electric cooking,water heating and loaded garages or other, loading or the need for diversity calculations in a domestic is hardly worth thinking about
Theres plenty of numbers between 80 and 100 for nearly all standard domestic installs without actually calculating anything at all

Des, The maximum demand of an installation is the maximum rate of consumption expressed in amperes, kW or kVA. It is generally taken as the average rate of consumption over a period of time, usually 30 minutes but in some cases 15 minutes or even an hour, depending on the tariff and electricity meter.

I understand where you are coming from with your comments Des but you seem to missing the fact that we are talking about "maximum demand" and not "average demand".

Dichroic

 

[Des 56]

I was also talking of maximum demand

When I gave the example of the install with 4 rings,a wife using appliances, a cooker on the go,a cuppa brewing and kids chipping in,it was to give an example of real world maximum demand.

The way many comply with the 17th ed in domestic installation is with twin rcds and ring circuits and lighting radials in perhaps greater quantity than in previous set ups

The on site guide information,adding set quantities for a first ring circuit and large percentages for the remainder, maximum demand of an installation that has many circuits instead of few,these installations give a wildly in accurate guide to theoretical maximum demand and expected maximum demand, if the guide is observed to its letter

In my opinion,guessing what the maximum demand is likely to be,is more a reflection of the experience of the installer, to do that guesstimate, than follow very poor general advise, to a subject that is, by its very nature, almost impossible to make accurate

The theoretical possible maximum demand of the installation is likely never to be a figure worth concerning ourselves with
The likely average figure would give a much more realistic load expectancy and concerns over supply capacity are better based on those more realistic figures

 

[Scott F]

I understand where you are coming from with your comments Des but you seem to missing the fact that we are talking about "maximum demand" and not "average demand".

Dichroic[/QUOTE]



Diochroic Ask Desi how old he is and how long he has been in the game .

The answer is probably very long .

I also tend to agree with him .

Got a clamp meter on a property with every thing on - ground source heat pump , hob, 2 ovens, washing

machine and tumble was round about 47 amps. Lets just say I was surprised ......

If you have nothing else then a guide is all well and good but again I agree with des

 

[Scud]

Des, The maximum demand of an installation is the maximum rate of consumption expressed in amperes, kW or kVA. It is generally taken as the average rate of consumption over a period of time, usually 30 minutes but in some cases 15 minutes or even an hour, depending on the tariff and electricity meter.

I understand where you are coming from with your comments Des but you seem to missing the fact that we are talking about "maximum demand" and not "average demand".

Dichroic

I'm not sure what tariff you're on has to do with anything. Half-hourly metering is not used in domestic properties. MD tariffs are industrial/commercial tariffs. A single rate meter doesn't take readings over any sequenced time scale.

 

[dichroic]

I understand where you are coming from with your comments Des but you seem to missing the fact that we are talking about "maximum demand" and not "average demand".

Dichroic

Diochroic Ask Desi how old he is and how long he has been in the game .

The answer is probably very long .

I also tend to agree with him .

Got a clamp meter on a property with every thing on - ground source heat pump , hob, 2 ovens, washing

machine and tumble was round about 47 amps. Lets just say I was surprised ......

If you have nothing else then a guide is all well and good but again I agree with des[/QUOTE]



My apologies guys i was just trying to help.

By the way ask me how old i am and how long i have been in the "game" ??

Ask me what i do for a living ??

Regards

Dichroic

 

[bugsy]

Connected loads will not nessacerily equate to maximum demand., I very much doubt you reach the 60a rating of the cut out