Calculating volt drop, cable selection and mcb selection on an 3kw motor

[daiplayer]

ok....when applying diversity...to socket outlets...the osg says : 100% first socket....40% of all other...Is this 13A ?

 

[malcolmsanford]

IMO no. I do like the OSG guide in many ways and it is a good tool, but this is one of the few pieces of advice that I personally don't agree with.

I'm not sure how they arrive at this, as it must be pure speculation what is plugged into a socket outlet final circuit. I think socket outlet diversity is purely down to the designer to decide.

 

[Sparky Ninja]

ok....when applying diversity...to socket outlets...the osg says : 100% first socket....40% of all other...Is this 13A ?

no it's 100% of current demand of the largest circuit + 40% of current demand of every other circuit (I assume you refer to domestic)

For example..

Consider a domestic installation comprising:

i an electric cooker (without socket on panel)
ii a 7.2 kW shower
iii two lighting circuits (10 x GLS fittings on each)
iv two socket circuits

Cooker consists of:
Hob with 4 x 3kW elements
Main oven 2kW
Grill/top oven 2kW

Therefore the total installed capacity = 16kW @ 240V this will result in an appliance demand of 16000/240 = 66A

From there work out the demand.. = 10+(0.3x56) = 26.8A


So..

Cooker = 26.8A
Shower = 30A (7200/240)
Lights 1 = 4.16A (10x100w = 1kW/240)
Lights 2 = 4.16A (10x100w = 1kW/240)
Sockets 1 = 30A
Sockets 2 = 30A


To apply diversity we take 100% of the largest circuit out of the equation and then multiply the remaining tally by 0.4 (40%). We then add the largest circuit back on to the resultant value.

So, in this case the largest circuit is the cooker. Although with demand the rated current 'appears' to be lower than the shower, it is still the largest rating and therefore the largest circuit.

Therefore we take away the cooker for now.... 26.8A
and tally the remaining.. 30A + 4.16A + 4.16A + 30A + 30A = 98.29A total.

98.29 x 0.4 (40%) = 39.33A

39.33A + 26.8A = 66.13A Installation demand.

 

[daiplayer]

ok...that baffled me to no end...lol...but will have a good look at it later...many thanks

 

[daiplayer]

aha...so thats the max demand for the entire installation yeah ? ...ok...got that..but how then do you work out the max demand/ diversity of just a socket circiut ?

 

[malcolmsanford]

What Widdler as done for you is a full design scenario,the type of thing that you would have to do on the 2391-20 course or even as a design engineer and is spot on.

If like me your a rag A**e spark you could use a rule of thumb method similar to what the DNO lads do, which is 0.4 domestic 0.6 commerical 0.8 industrial. What this means is an installation you add up your final circuit protection devices and multiply by the factor, so in domestic it would be 0.4. so if you did that in Widdlers installation you would get over 57 amps about 9 below the proper calcualtion. As you can see this is quite close for a rule of thumb, and both would tell you that in reality that installation would work on a 60amp Fuse as it describes a typical domestic house, but chances are you would fit an 80amp most likely for future proofing.

As I said in my OP there really isn't a tried and trusted method for calculating a socket circuit. This is why the OSG give guidance of socket circuit design by use of areas. If you had a kitchen which needed 8 double sockets you may decide the best design would be a A1 ring or perhaps two A3 radials. Where in the upstairs there are only 7 double sockets in bedrooms you might go for a single A3 radial, it is just a method the designer will judge is best

 

[pushrod]

aha...so thats the max demand for the entire installation yeah ? ...ok...got that..but how then do you work out the max demand/ diversity of just a socket circiut ?

Are you asking about mcb size for sockets circuits???

To a large extent you can't because you don't know what is going to be be plugged in - if you do know then you plan accordingly. There are however general guidelines based on the floor area being served by the circuit - look at table 8A p158 of the OSG.

 

[telectrix]

if your sockets are on a 2.5mm ring final assume a current load of 25A. if a 2.5mm radial assume 15A. that's my rule of thumb guide to calculating.

 

[daiplayer]

ok...thanks guys....it just threw me when trying to calc socket circiuts diversity. How can you calculate each soscket demand when you dont know whats being plugged in ? ... ..in this exam i must show my cable selection and the reasons for doing so.....I assume that saying ' socket are usaully 2.5mm so i'll just do that ' wont be acceptable.....Can i just refer to the osg (as pushrod so kindly pointed out) , table 8A pg 158 ? ..is this acceptable to the examiner ?

Many thanks for your responses All.

 

[Sparky Ninja]

How can you calculate each soscket demand when you dont know whats being plugged in ? ... ..

We don't, we just apply the rated current of the protective device.

Similar to cable selection where we would use In instead of Ib.

in this exam i must show my cable selection and the reasons for doing so.....I assume that saying ' socket are usaully 2.5mm so i'll just do that ' wont be acceptable.....Can i just refer to the osg (as pushrod so kindly pointed out) , table 8A pg 158 ? ..is this acceptable to the examiner ?

Never write done assumptions in an exam. Some exam scenarios are deliberately exaggerated to avoid this.

I can give you a couple of cable selection questions if you like??