MAX DEMAND LOAD

[trooper]

Hi
i am currentley doing my part p
on the new installation certificate in Supply Characteristics,
there is a box to state the max load demand....
on a consumer unit with a 100a main switch...
4 x 32a mcbs rings
1 x 16a water heater mcb
4 x 6a mcb for lights + smoke det'
40a mcb cooker
...I've noted from the original certificate issued that max demand is stated at 60a???
Even with diversity its over 100a ???
or am i getting confused!!!!
help on this is appreciated..

 

[Maxxy]

Is this a practice paper, or one you've seen on a job?

 

[trooper]

1 ive seen on a job, the house has just been rewired completely,
and the sparky got his registered mate to do the testing and certifying

 

[Maxxy]

And you worked yours out using the OSG im guessing.
Its the way i was taught in college, but since then ive seen out in the real world (and been told many times) that the OSG is very OTT in this regard.

An example could be a 5 bedroom house. You use the OSG and find its maximum demand is 130 Amps...but theres only one person living there and their measured maximum demand is only 40Amps.

As such, a few people ive seen put down around 40-50 amps for a 3 or 4 bedroom house, depending on whether or not its got a power shower.

In reality, theres not much chance of someone cooking, having a shower, boiling the kettle and making toast, aswell as using an electric heater at the same time. Not that im saying it doesnt happen! But lets hope they arent making toast in the shower!

 

[trooper]

but could i legally put 60 amps down, or would i be expected by an assessor to hold back the certificate till the max demand is reduced in some way??

 

[Maxxy]

1 ive seen on a job, the house has just been rewired completely and the sparky got his registered mate to do the testing and certifying

Getting someone else to certify your work? tut tut.

Dunno how a competent scheme would see it, hopefully someone with an approved body will see this post.

 

[trooper]

thats the criteria of corgi, to b assessed i have to show him some electrical
work( in this case the radial to the immerssion on the megaflo i fitted),
but the work had to b certified by a registered sparky..(funny i know) so looking at his
certificate i was wondering why 60a max demand!!!!
knowing my luck i'll get asked it by the assessor...
So is there a calculation u can think of that has made him come up with 60a???

 

[Taylortwocities]

I've got an approved body, does that count?

 

[Jaymack]

The method of calculation of diversity as found in the OSG is very conservative. In reality the maximum demand is far lower than this. If you really want to know, put a recording clamp meter on the tails. I would guess that most installations peak at around 30-40 amps.

The only exception would be on Christmas Day when the turkey is in the oven, all the cooker rings are on, the washing machine is going, the kids are playing with their new electronic toys dad says to mum "put the kettle on while I have a shower". You may then pop an old fatigued company fuse. There is a massive margin of safety in tabulated cable ratings. So if the installation conforms to BS7671 and the terminations on the consumer unit are tight there should be no problems. The peaks will be only of short duration.

A 100A fuse will carry 200A long enough to boil a kettle, and 150A long enough for a good shower, so it won't really pop. (check the curves) And that 'Christmas Day' scenario isn't that common either, or we'd be losing substations like a string of fairy lights - remember that typically the transformers are sized on a working load of only couple of kilowatts per house -a logic which they extend to some extent to the sizing of the street conductors too, (only for substations or cables feeding 20+ properties though, to allow load peaks to smooth out)
Remember also on the cooker, the rings are not on all the time - their thermostatic controllers see to that, so once the water in the pan is boiling, whoever turns down the temp, they just ping on and off, with perhaps 10 to 20% duty cycle.
Don't panic - many houses calculate up at >200A and run quite happily on a 100A, or even a 60A service fuse. And if not, and one day it does pop, then the supply company will come out and fit another one, or the next size up (80A, 100A 125A...)

You can estimate it, which is what experienced electricians normally do. or: -
You can work it out using the OSG for reference regarding to diversity etc; or: -
You can just put the main fuse size down, such as 60 Amp, 100 Amp etc; which many do.

Jaymack

 

[Adam_151]

Deja vu anyone?, I'm sure I've read those words posted by jackmack somewhere else before...

 

[trooper]

thanks for that, it sounds as if the assessor wouldnt be over concerned on that point then!!!(but knowing my luck!!)

 

[Maxxy]

I've got an approved body, does that count?

I notcied id stepped right into that after posting, but figured i'd leave it to see how quick people on the forums are lol

 

[securespark]

I've got an approved body, does that count?

I've got an approved body, too - Mrs Secure approves of mine!