Maximum demand
- Thread starter Hiram
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Help Support Talk Electrician Forum:
rating of service fuse, if you know it
unless you have all fixed loads, you cannot work it out exactly.... so go for the most it can be (or so college lecturer says)
green-hornet
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Add together all used mcb's then multiply by 0.42
Gives you your maximum demand.
Gives you your maximum demand.
WHAT!!!!
(To ALL of the above)
1. One of the reasons for calculating MaxDem is to verify that the service fuse/tails are of sufficient size to supply the installation without overload. e.g. if you put a 30A floor heating system, a 9.5KW shower, a nice kitchen / utility with all mod cons, are you seriously going to write " erm....60A" as the maximum demand?????
QED - don`t use this method.
2. Clampmeter on tails. Yeah - cool. IF you could stand there for an "average" day, and read the peak usage. M.D. doesn`t change with seasons - usage does. Is the customer having a shower whilst you`re there? (You wish!). NO? then you are not reading the Maximum Demand.
QED again (sorry)
3. All used MCB`s X 0.42? Likely to be closer than the other 2, but still incorrect.
If you don`t possess the OSG; I`d suggest having it. Apart from anything else, it provides help in this area, as well as diversity. These two are somewhat connected; although it doesn`t come up as much. You are trying to determine / estimate how much current may be "reasonably expected" to be required at its highest. I tend to do the following:
Pretend it`s christmas. Seriously. Walk through each room ( the size of the property will give you a good idea as to how many people MIGHT be there for xmas dinner.)
The cooker will be on, as will the hob. Someone may be in the shower. etc. Add up the loads, room by room. Allow a little extra if you`re unsure.
n.b. Please be aware that MD is an arbitary figure. Your test meter CANNOT tell you this value. Sorry!
HTH
KME
(To ALL of the above)
1. One of the reasons for calculating MaxDem is to verify that the service fuse/tails are of sufficient size to supply the installation without overload. e.g. if you put a 30A floor heating system, a 9.5KW shower, a nice kitchen / utility with all mod cons, are you seriously going to write " erm....60A" as the maximum demand?????
QED - don`t use this method.
2. Clampmeter on tails. Yeah - cool. IF you could stand there for an "average" day, and read the peak usage. M.D. doesn`t change with seasons - usage does. Is the customer having a shower whilst you`re there? (You wish!). NO? then you are not reading the Maximum Demand.
QED again (sorry)
3. All used MCB`s X 0.42? Likely to be closer than the other 2, but still incorrect.
If you don`t possess the OSG; I`d suggest having it. Apart from anything else, it provides help in this area, as well as diversity. These two are somewhat connected; although it doesn`t come up as much. You are trying to determine / estimate how much current may be "reasonably expected" to be required at its highest. I tend to do the following:
Pretend it`s christmas. Seriously. Walk through each room ( the size of the property will give you a good idea as to how many people MIGHT be there for xmas dinner.)
The cooker will be on, as will the hob. Someone may be in the shower. etc. Add up the loads, room by room. Allow a little extra if you`re unsure.
n.b. Please be aware that MD is an arbitary figure. Your test meter CANNOT tell you this value. Sorry!
HTH
KME
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I cannot see how anybody can see what a maximum demand is you would have to be a physcic (thats not right) or a vet.
Batty
Batty
Did you mean Psychic? (very close bud.)
I agree that 0.42 of the circuits may give an approximation most times; but that is dependant on the circuit rating and loading.
e.g I have a 32A radial supplying underfloor heating. The MD of that circuit is 28A. For fixed heating loads - no diversity, hence 28A of load for the demand figure. Yet you`d have classed that as approx 15A. You`re already out by an immersion heater; if you know what I mean?
Similarly with showers. The MD of that circuit is far in excess of 42% of the MCB rating. Sorry - That could give rise to misleading results.
Calculate it guys. Do a bit of headwork!!!
I agree that 0.42 of the circuits may give an approximation most times; but that is dependant on the circuit rating and loading.
e.g I have a 32A radial supplying underfloor heating. The MD of that circuit is 28A. For fixed heating loads - no diversity, hence 28A of load for the demand figure. Yet you`d have classed that as approx 15A. You`re already out by an immersion heater; if you know what I mean?
Similarly with showers. The MD of that circuit is far in excess of 42% of the MCB rating. Sorry - That could give rise to misleading results.
Calculate it guys. Do a bit of headwork!!!
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As KME says.. you as the electrical engineer have got to make a reasonable assessment of maximum demand of the installation you are working on...
It is NOT a precise calculation because every installation is different..
also with domestic the same property could have a change of occupancy and max demand could go higher - or - lower because of the new requirements of new owners.. although the physical installation may be the same!!!?
which ever way you look at it DIVERSITY has to be taken into account!
which leaves two basic options...
1/ asses individual circuits applying diversity and add em up!
2/ assess the whole installation.. & take a percentage..
Method 2/ in a DOMESTIC! environment...
(commercial & industrial would need to be assessed on a bit more stringent manner)
RULE OF THUMB!
see page 97 On Site Guide,
Table 1B
Point 9.
Standard arrangement of final circuits in accordance with Appendix 8.
100% of largest circuit rating + 40% of every other circuit!
Personally I think this method can be a tad on the high side?
e.g.
1 x 40a shower
1 x 32a cooker
2 x 32a sockets
2 x 6a lights
1 x 16a imm heater
1 x 20a garden shed/garage.
(100% x 40a) + (40% x (32a + 32a + 32a + 6a + 6a + 16a + 20a))
=40a + (40% x 144a)
=40+57.6
=97.6A!
say for example if the shed is not in use 97% o the time... 20a is NOT going to be drawn! :|
and if the immersion heater is ONLY used as a back up to if the central heating packs in?....
thats another 13a out of the picture!
amending the above rule of thumb..
you could say...
(100% x 40a) + (40% x (32a + 32a + 32a + 6a + 6a))
=40a + (40% x 108a)
=40+43.2
=83.2!
Whereas compare with Green H method...
0.42 x 184a = 77a
which it probably is somewhere around 80a ?
Hopefully this helps illustrate how tricky it is...
BUT you must do a reasonable check. as KME pointed out,
check what IS in the property.
Not just what the cut out fuse rating is!
I have been on call outs where the main fuse has blown...
so in those cases Max demand.. MUST of exceeded cut out rating!? ; )
IMHO... Applying a bit of common sense, using the skills & knowledge you have learnt, you should be able to get a reasonable estimate.
Guiness Drink
It is NOT a precise calculation because every installation is different..
also with domestic the same property could have a change of occupancy and max demand could go higher - or - lower because of the new requirements of new owners.. although the physical installation may be the same!!!?
which ever way you look at it DIVERSITY has to be taken into account!
which leaves two basic options...
1/ asses individual circuits applying diversity and add em up!
2/ assess the whole installation.. & take a percentage..
Method 2/ in a DOMESTIC! environment...
(commercial & industrial would need to be assessed on a bit more stringent manner)
RULE OF THUMB!
see page 97 On Site Guide,
Table 1B
Point 9.
Standard arrangement of final circuits in accordance with Appendix 8.
100% of largest circuit rating + 40% of every other circuit!
Personally I think this method can be a tad on the high side?
e.g.
1 x 40a shower
1 x 32a cooker
2 x 32a sockets
2 x 6a lights
1 x 16a imm heater
1 x 20a garden shed/garage.
(100% x 40a) + (40% x (32a + 32a + 32a + 6a + 6a + 16a + 20a))
=40a + (40% x 144a)
=40+57.6
=97.6A!
say for example if the shed is not in use 97% o the time... 20a is NOT going to be drawn! :|
and if the immersion heater is ONLY used as a back up to if the central heating packs in?....
thats another 13a out of the picture!
amending the above rule of thumb..
you could say...
(100% x 40a) + (40% x (32a + 32a + 32a + 6a + 6a))
=40a + (40% x 108a)
=40+43.2
=83.2!
Whereas compare with Green H method...
0.42 x 184a = 77a
which it probably is somewhere around 80a ?
Hopefully this helps illustrate how tricky it is...
BUT you must do a reasonable check. as KME pointed out,
check what IS in the property.
Not just what the cut out fuse rating is!
I have been on call outs where the main fuse has blown...
so in those cases Max demand.. MUST of exceeded cut out rating!?
; )IMHO... Applying a bit of common sense, using the skills & knowledge you have learnt, you should be able to get a reasonable estimate.
Guiness Drink
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So what you are saying, in a nutshell, demand can theoretically exceed supply
(briefly)
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Thats when the fuse blows!!So what you are saying, in a nutshell, demand can theoretically exceed supply
That is the general concept of why we need fuses/MCBs...
there is NOTHING stopping some plonker trying to drawn too much power.
[max demand] through a circuit, or a whole installation.
other than the fuses.
Regulation 131.8...
check rating of existing installation .. BEFORE adding any more to it!
{beer paraphrase version!}
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or stuff melts...............
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assuming... Demamd is demand?
{dont you just luuuuv Typo's?}Applaud Smiley:x
with an abacus or slide-rule....
oh bum!!Blushing
Am i showing my age again?
Deke remembers Abacus's & slide-rules?
and log-tables probably?
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Log tables?assuming... Demamd is demand?{dont you just luuuuv Typo's?}Applaud Smiley:x
with an abacus or slide-rule....
oh bum!!Blushing
Am i showing my age again?
Deke remembers Abacus's & slide-rules?
and log-tables probably?
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Just to throw another spanner in the works of how you doooze your calcs
consider a rewire on a property with old wylex 4-way CU.
1 x 30a cooker
1 x 30a sockets
1 x 15a imm heat.
1 x 5a lights
All run of 60a main fuse... all ok worked fine for years....
"apparent" max demand = 30 = 0.4x(30+15+5)=50a
Do rewire.. fit new CU....
1 x 32a cooker
2 x 32a sockets
1 x 16a imm heat
2 x 6a lights
1 x 6a smoke detectors.
"NEW" apparent max demand=
32+0.4x((32+32+16+6+6+6)=71.2amps!!!!?
but ONLY extra load is smoke detector circuit which in reality probably takes less than 1 or 2 amps? ?:| :|
Mr common & Mrs sense needed!
B-)p.s. the above two scenarios on the GH 0.42 method give...
0.42x(80a)=33.6a IMHO a tad low...?
0.42x(130a)=54.6a Maybe not far off?
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Applaud SmileyApplaud SmileyApplaud SmileyApplaud Smiley:^O:^O:^O:^O:^O:^O:^OLog tables?
:x :x:x
green-hornet
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Maximum Demand is NOT as is sometimes assumed the rating value of the distributors cut out.
For smaller installations Ie:- A house, guidance on estimating maximum demand is given in IEE Guidance note 1: selection and Erection.There are however other methods by which maximum demand current may be assesed, and these are not precluded provided they give realistic values.
My calculation method if rounded up (as it should be) would be 80 amps.
Which is a realistic figure given that with diversity applied 83 amps could be
the figure you calculate.
Just as a note most larger installations employ the use of meters to monitor the demand over a longer period of a week or even two or more.
This would then give a more accurate assesment of maximum demand than a calculated figure.
Each circuit would have its own calculation, some are not allowed any diversity at all.
Part L or part L2 of the building regulations would put most lighting calculations way off the actual mark.
For house installation certificates I would continue to use the 0.42 method.
However for industrial demand I would look very closely into what is actually being used and do the recordings over a period of time.
If it was a new build then I would calculate all circuits apply diversity and give the design maximum demand.
I hope that clears some concerns any one may have.
For smaller installations Ie:- A house, guidance on estimating maximum demand is given in IEE Guidance note 1: selection and Erection.There are however other methods by which maximum demand current may be assesed, and these are not precluded provided they give realistic values.
My calculation method if rounded up (as it should be) would be 80 amps.
Which is a realistic figure given that with diversity applied 83 amps could be
the figure you calculate.
Just as a note most larger installations employ the use of meters to monitor the demand over a longer period of a week or even two or more.
This would then give a more accurate assesment of maximum demand than a calculated figure.
Each circuit would have its own calculation, some are not allowed any diversity at all.
Part L or part L2 of the building regulations would put most lighting calculations way off the actual mark.
For house installation certificates I would continue to use the 0.42 method.
However for industrial demand I would look very closely into what is actually being used and do the recordings over a period of time.
If it was a new build then I would calculate all circuits apply diversity and give the design maximum demand.
I hope that clears some concerns any one may have.
