Maximum Demand

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norfolksparks

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I know this is always discussed to death but i could never find a real solution to working this out. I know how to do it from the tables taking into account diversity etc but i changed the board in my own house and doing it properly, maximum demand came out at 117 amps with the supply fuse being 100 amps. That's without taking into account the other dist board that feeds my 9.5kw all electric central heating boiler!

Anyway had my Elecsa assesment last week (passed) and used this board for my assesment. The two guys that came out were very thorough, professional, and helpful. Had a bit of a chat with them after and i asked the assessor the easiest way to work out maximum demand. He said that there is no real accurate way of doing it, too many variables, he went on to say that he used to work for the electricity board and they did it a different way.

He said add all the values of the breakers together then multiply them by 0.4 (think he called it the 5 amp rule but not sure why!). I tried this method and the maximum demand came out at a more reasonable 69 amps, this is the method i will be using from now on. Good enough for him, good enough for me! :D

 

Stoday

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You can't even start to estimate maximum demand until you've defined it.

Is it the instantaneous demand over one cycle? Switch your computer on and you typically have a demand of 18kW (80A).

Of course, you have to allow for diversity so you can have a whole house with 18kW on the consumer side of the meter.

That translates to 5kW on the company side of the meter because they define maximum demand as twice the number of kWh consumed in half an hour.

If you are setting up a development of several tens or hundreds of houses, each house would contribute about 2kW to the site maximum demand, after diversity.

At the bulk supply point, where the kWh are attributed to the various energy suppliers, diversity is maxed out and the demand is 1kW per house.

 

norfolksparks

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A lot of installations come out with maximum demand over the rating of the main fuse, i've even read (on another forum) that the NIC advise you to put down the rating of the main fuse on the test sheets. The Elecsa guy didn't fail me because he agreed with what i was saying and he was happy that i am a competent electrician who is able to test and inspect his own work. They questioned me for two and a half hours on both the 16th and 17th edition (once they knew i got 100 per cent on the 17th), inspected two of my installations and got me to carry out various tests on the main installation, cross referencing the readings with the results that i put on the test sheets.

 

davetheglitz

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Just a thought - but for any one installation you could have several radials all fed off the same fuse. If you follow the ideal of segregating the circuits then the perceived max demand goes up due to the number of fuses - but in actual fact the demand from the householder hasn't changed.

To me it seems to show the arbitary nature of the measure where it is possible to 'improve' an installation by combining circuits!

In my opinion - if a house doesn't have electric heating you are hard pushed to get a max demand of over 60A - possibly 80A with a shower.

How often does anyone come across a blown main fuse caused by overload rather than a fault? Personally I haven't yet - and often houses have 2 - 3 electric showers fitted and electric cookers.

To my mind the 0.4 rule sounds like a useful guide but is as flawed as any other measure.

 

binky

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I kind of gave up worrying about this a while ago having been to properties with max demand in excess of 200 Amps on a 60A fuse - its very easy to exceed fuse size especially when you split circuits down to improve separation of circuits. For instance I like to keep kitchens on a seperate circuit to downstairs sockets and upstairs sockets that instantly gives me 3off 32 Amps circuits before putting garage on its own supply, etc, etc. The only thing I really take much notice of is power hungry devices such as showers and cookers. I recently quoted for a re-wire on a large property with 4 electric showers (10Kw) aswell as cookers etc, but worse owner had young family so You could imagine Xmas with grannies, relatives, kids and turkey in oven...... Suggested owner might like to consider gas showers or a second phase. Never heard back, but would love to know if they did blow main fuse. Generally upto about 120 Amps I ignore, but I do also tend to down rate rings in lounge / dining room areas to 16 or 20A MCBs and similarly bedrooms where quite frankly 32 Amps will never be needed.

 

jckent

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With modern 10Kw showers and many houses having 1 or 2 on-suite shower rooms, we may well find the case for the need of a 2nd phase within the house more common.

I have visited a few newer houses with 3 phase coming in as standard.

 

steptoe

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I kind of gave up worrying about this a while ago having been to properties with max demand in excess of 200 Amps on a 60A fuse - its very easy to exceed fuse size especially when you split circuits down to improve separation of circuits. For instance I like to keep kitchens on a seperate circuit to downstairs sockets and upstairs sockets that instantly gives me 3off 32 Amps circuits before putting garage on its own supply, etc, etc. The only thing I really take much notice of is power hungry devices such as showers and cookers. I recently quoted for a re-wire on a large property with 4 electric showers (10Kw) aswell as cookers etc, but worse owner had young family so You could imagine Xmas with grannies, relatives, kids and turkey in oven...... Suggested owner might like to consider gas showers or a second phase. Never heard back, but would love to know if they did blow main fuse. Generally upto about 120 Amps I ignore, but I do also tend to down rate rings in lounge / dining room areas to 16 or 20A MCBs and similarly bedrooms where quite frankly 32 Amps will never be needed.
I would agree with this, tho the easiest way IMHO to comply 17th is simply to use radials.

max demand is a hard one to calc, rule of thumb(?) is 45A, tho most modern houses do draw much more, but seldom more than 80A average in any week, usually nearer 60A peak.

 

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I know this is always discussed to death but i could never find a real solution to working this out. I know how to do it from the tables taking into account diversity etc but i changed the board in my own house and doing it properly, maximum demand came out at 117 amps with the supply fuse being 100 amps. That's without taking into account the other dist board that feeds my 9.5kw all electric central heating boiler!Anyway had my Elecsa assesment last week (passed) and used this board for my assesment. The two guys that came out were very thorough, professional, and helpful. Had a bit of a chat with them after and i asked the assessor the easiest way to work out maximum demand. He said that there is no real accurate way of doing it, too many variables, he went on to say that he used to work for the electricity board and they did it a different way.

He said add all the values of the breakers together then multiply them by 0.4 (think he called it the 5 amp rule but not sure why!). I tried this method and the maximum demand came out at a more reasonable 69 amps, this is the method i will be using from now on. Good enough for him, good enough for me! :D
Doing maximum demand properly?

Spose the only REAL way would be to monitor maximum current over a 12month period and note the actual peaks of demand when they occur.

Lets face it...

It is difficult to work out the actual maximum demand for a single circuit such as a cooker.. or lights.

The only things you could categorically specify demand for are

1. Items that are permanently switched on [fridge / freezer / smoke detectors / Alarm system etc..] However in terms of actual loads these items are virtually negligible..! :(

2. Fixed items of equipment that are controlled by a timer clock at specified points in the day.

Anything that involves the ability to plug any appliance in at any time, or switch any light off or on at any time..

then a reasonable diversity calculation must be taken into account..

Also as pointed out you cannot simply ADD all of the MCB/RCBO ratings

for example a 14way CU fully equipped could only on average have a 7.1amp load per circuit!

Modern boxes will often have numerous circuits to meet the requirement to "Minimise Inconvenience" in the event of a circuit tripping..

e.g.

Smokes may be on there own 6a MCB. (actually 3a FCU supply)

External lighting may be on its 6a MCB.

A boiler may be on its own 15a or 10a radial. (actually 3a FCU supply)

External sockets or shed may be on its own 16a OR 20a MCB

Freezer may be on 16a radial.

Obviously none of the above will be anywhere near total load of circuit.

Showers and cookers will be switched OFF for a large proportion of the day!

I will often put downstairs sockets on a 20A (although 16A may be ok)

The above assumes kitchen/utility are on there own 32a!

Upstairs sockets I normally put on a 20A... however a 16A or even a 10A may be adequate!! ?:|

I think I have read somewhere that Maximum demands relates to a maximum current required over a continual 30minute period! (open to anyone to verify / reject this point?).

It is one of those things that is very much a value judgement.

As long as you can justify your reasoning and you design your installation accordingly then that is ok!

Obviously easier on New installation or rewire as compared to PIR on exsiting installtion!..

Hope this makes sense ... getting late... brain nicley mellow from beer... off to bed soon.. contented & relaxed:D:D:D:D:p:) :x

 

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As an extra point..

The only job I have ever worked on where the main fuse had blown..

was a converted house that was now a hairdresser with 3x electric shower + other normal hairdresser stuff!

was on a 60Amp fuse... blew...

I had to put in 25mm tails .. before supply company would organise fuse upgrade to 100a!

 
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