My First Job - help

[Trevor R]

Hi Guys - my first job and I have been asked to put sockets \ lighting into a small building 100ft from the house - will
just be 4 double sockets & 700w of lighting plus 2 electric rads - my concern is overloading back at the cu.

It is a Crabtree Starbreaker 10 way - there is 1 spare way which is not on the RCCB side of the board so I will swop
it about so that is (assume I can do this) - this is where I shall put the SWA to the external building.

My concern is overloading - the CU already has 2 x 6A lighting \ 1 x 16A heating \ 3 x 32A Rings \ 1 x 32A Hob \
1 x 32A shower \ 1 x 16A marked BT socket. Adding this up 204A - applying diversity gets me back to 114.3A still
more than the board 100A. The rules are that the CU has to be able to take the full load.

What do I do next guys - have a number of ideas - additional CU? check that the ratings are actually needed?

 

[Engineer54]

Ok, granted im sure im younger and less experiences then you, the former being a definite bonus im sure , and so your undoubtedly right, that my judgement on such matters will not be up to your standards, but all i was asking buddy, is going by the applied diversity calcs, based purely on the figures the OP has provided, and not having inspected the property to find out the exact set up and what does what and when and how, we all have a different result. all i was after was the calculations these guys had used to get their figures. to see if i was going about it wrong or vice versa.

Wasn't trying to be funny with you or anyone else Wade, it's just one of those quirky things that often catches out even some experienced sparks. Applying meaningful diversity assessments comes with experience. The formula's given in the Reg's and other official publications are for guidance only and are often leaning very much on the safe side.... Very few design Engineers will use those figures given in those publications, (with the exception of cooking loads)


As suggested, often on existing installations, some are only convinced by clamping the supply phase tail(s), ....even then, some sparks find the resultant measured load hard to believe!!...lol!!

 

[Des 56]

all i was after was the calculations these guys had used to get their figures. to see if i was going about it wrong or vice versa.

I just followed a link by Kung for diy electrics in the home
Diversity was explained with various methods

Below is an excellent example of why engineer 54 was trying to steer you away from relying on figures in the on site guide
I hope it gives you the reasoning behind the varying figures you have found in this thread

Here is the copy/paste I hope it allows you to see his reasoning based on experience

Part of the cause of this apparent problem is the assumptions upon which the calculations are based. These are now somewhat outdated and don't reflect modern usage for many properties. The guidance in the OSG for example has remained unchanged for many years, and yet in that time we have seen:

• an "explosion" of use of small low load devices in the home;
• a large growth in the numbers of socket outlets deemed adequate
• The introduction of the 17th edition of the wiring regs which has tended to further increase the number of circuits used due to the increased reliance on RCDs and the need to maintain discrimination under fault conditions; i.e. Although there has been a general increase in the number of circuits provisioned, there has not been a corresponding growth in consumption.
• a general fall in the numbers of houses that rely on electricity for extensive day time heating as oil or gas fired central heating has become almost ubiquitous.

Couple this to the reality that the incomer fuse will typically support quite significant overload for short durations, and the supply impedance in the majority of properties is high enough that there will be a small amount of load shedding[1] at high loads, and it becomes apparent why they are not fusing on a regular basis. (note that the load shedding effect can not be relied upon as a design parameter since it is not under the designers control, and is subject to change without notice -


[h=3[B]]Alternative Method: Applying a judgemental approach[/h]An alternative approach to assessing load is to make an engineering judgement based on more detailed knowledge of the installation, and can take into account:[/B]

• the operating time profile of the load
• the coincidence or simultaneous operation of individual loads with other loads
• the seasonal demands of heating and cooling loads
• the allowance, if any, for spare load capacity based on the users needs.

Examples of these judgements could be:

• Where socket circuits are concerned, further reductions in load could be assumed after the first two are accounted for at 100% and 40% - say counting the remainder at 20%. Alternatively one might wish to count all socket circuits at only 40% loading.
• A load like an electric shower one might assume is unlikely to run for more than 12 mins out of every 15, and hence a reduction of 12/15ths would be arguable.
• It may be practical to ignore complete circuits where there loading is likely to be mutually exclusive (i.e. heating and cooling circuits are unlikely to run concurrently)

Any such judgement will be specific to a particular installation, and will take a little more time and investigation than a straight numerical exercise like that presented earlier.

 

[wade88]

Ah Engineer i know you wernt being funny pal i know better then to take exception to anything anyone says on here - at the end of the day we are all here for a reason and advice and banter seem to go hand in hand, i appreciate your input as much as any other mate.

thanks for that post Des, i do understand how unrealible such calcs can be, quite frankly the figure you will end up with as a result could be 20 amps or more + or - either side of the result. so not a lot of good. I was just more interested in how if we were all doing the same calc, all got different results? were all a bunch of divs maybe?