existing electric shower re max demand

[ianphilip007]

hi

i have a 60 amp main fuse supplying 63a main switch in cu. my question is about OSG table 1b max demand figures (calculations). When applying these figures ie no diversity for shower, no diversity for first ring main it seems that any installation with a 60a main fuse cannot have a electric shower as it is impossible for the calculated max demand to ever be below 60a. But how many 60a installations run perfectly normally with electric showers installed?

so i have a pre-existing 8.5kw shower currently installed in my own cu and depending on which method i use to calculate max demand it ranges from 95a(osg) to 54a. (40+30+30+30+5) x0.4 (rule of thumb)

now in using my own expieirience and kwowledge of how the installation is used i would bet that the max demand would be no more than 45-50a. With all loads switched on including shower/cooker/ringmain/lights clamp meter reading 57A

As this is for my elecsa assessment i wondered if i should dissconnect the shower circuit so that max demand as per osg table 1 can be met. Or leave as is and explain my reasoning

 

[malcolmsanford]

The OSG is a guide and it even tells you in the introduction to that section that there are many ways of calculating and applying diversity.

That 0.4 rule of thumb is the one I use, and as you have done both that and the OSG guide then the 0.4 rule of thumb would seem to be a more realistic one

Once again Ian I doubt very much if the assessor will be at all interested in the diversity of the installation, and unless he is happening to read this thread, and then say i'll ask this lad about it, won't even mention it

Leave it connected mate .

 

[ianphilip007]

Thankyou malcom i always forget " its only a guide" and as you state it does say that there are many ways of calculating diversity. I feel the osg method just seems to produce a very high figure and not that realistic in my opinion to real life situatons,compared to the way i was taught over 20 years ago which always seems to be much closer. i shall leave it connected and record a max demand of 54A

 

[malcolmsanford]

To be honest mate I would put max demand down as the fuse capacity which in your case is 60amps.

In fact if the carried itself is labeled 100amp then I would put that down, just to show you haven't been a very naughty boy and removed the fuse to look.

 

[GLENNSPARK]

as malcolm has said ian....just ride it mate.....that assessor is more interested in gettin home early...lol.....

 

[brman]

To back up what the others have said, I asked my elesca assessor about this. I showed him my cert which had max demand as 80A (the main fuse rating) and my diversity calcs (by the OSG method) which showed about 115A and asked him what he would do.

If I remember right he just shrugged, asked if it had ever blown the fuse and then changed the subject

 

[daimack]

I'm feeling your pain Ian as I'm in the same position. I am doing a rewire and CU change which will also be for an elecsa assessment. With two lighting radials, two RFC's and a cooker and hob (no shower in my case) I'm getting a MD of around 68A using the OSG method, or a more realistic 50A by the 0.4 rule of thumb method. Like you this is on a 60A main fuse, but is hardly an unreasonable or unusual demand...

Let us know how your assessment goes mate, just in case I need the heads up to temporarily disconnect my cooker and get a camping stove out :wink5:

 

[alanl]

You are looking at it the wrong way. I doubt a typical house ever draws 40A for more than 5-10 minutes. A 8.5kW shower would be the biggest current using device in the house, that is on for what, 5-10 minutes? Even with the cooker on, you would not be drawing 60A. Domestic properties rarely have constant heavy loads - oven comes on full power for 5 minutes, then they are up to temperature, and only come back on for a minute at a time, or a constant, lower current demand to keep the oven hot.Ring final circuits - have you ever calculated what power they use? Apart from the kitchen/utility, there is very rarely any need to use to use a RFC, a radial on 16A is usually too much, my house uses around 700Watts on the upstairs sockets, when everything is on. Living room, maybe 1000watts with TV and recorder on - there is very little heavy current using equipment in a domestic house, washer, dryer, oven and shower are the 'big' ones, and even when all are on together, which is unlikely, your maximum demand is unlikely to be 60 Amps.Industrial/commercial is different, as loads may be connected for many hours, so account must be taken of this, but that is another subject.

 

[SPARTYKUS]

As mentioned somewhere else in another thread, even a 'meager' 60A main fuse will grumpily pull 200A for a minute or so.

Very very very unlikely a main fuse blows because of problems above. Only ever seen blown through (massive) fault or.....a very silly sparky.

 

[PB1]

It's interesting to get a clamp meter around the meter tails on occasions like this and switch things on to see what they actually draw. I did this a while ago when a couple wanted a new hot-tub circuit and they only had a 60amp cut out. I was concerned that their max demand might be too high. The actual current drawn by their shower set at its usual setting, cooker, washing machine, kettle etc was only around 47amps, much less than I had calculated.

 

[daimack]

Thanks gents, I think this is the issue that both the OP and I had; all common sense assessment of the demand would indicate that all was fine, but following the OSG guidance (and, as was rightly pointed out, it is only guidance) put our MD well above what we thought it should be. Certainly in my case the real issue was how to explain my rationale to the Elecsa assessor, and you have given me considerable help in that respect. The clamp meter reading is interesting too, and its good to know that this bears out the common sense approach. Cheers all,

 

[drew35]

Virtually every time I work out maximum demand using the OSG it comes out way over the main fuse rating, so I also just tend to put 60a or 80a whatever the main fuse is. This calculation would work in the days of one power circuit, one lighting, one cooker, and one immersion, but not when you have a twenty way consumer unit in a six bed house, still with a 60a main fuse.