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Hi all, im new the the forum.

Curently doing the city and guilds 2365 level 3 course. Got a question regarding the 305 design assignment.

When filling out the table to calculate the voltage drop and calbles sizes, do i use the design current etc before or after the diversity factor?



Design current (Ib) is the acceptable current after diversity has been taken into account
An example might be:
12kW cooker total current flowing if all elements were on simultaneously = 12000/218.5 (worst case) = 54.9A.
That would require a 10mm2 cable
Apply diversity owing to rheostats switching in and out and we have (54.9 - 10)*30% +10 + 5 (if appropriate) = 23.47A
You might get away with a 2.5mm2 cable .

To become an electrician you have to do this stuff for yourself .......
I only recall ever applying diversity to things like cookers, or anything above 13a devices. Never for rings, radials or lighting circuits.

We went over and over this stuff until it sunk in long enough to pass the module and it only came up in a few questions when we finally sat the test.

Apart from electrical science where you really need to apply physics based maths it was all study your notes for X module, take test, pass test.


In question 4 its to do with cable calculations so you select the right csa & protective device, fault current etc.

Question 6 if I remember correctly asks you to show loads before & after diversity is applied so you can calculate max demand etc.


I haven't seen the latest set of practical papers. but from memory they were very basic and simplified.

Diversity comes into its own when designing large electrical installations where the designer has to take into consideration the loading in large complex systems where there maybe 20 to 30 ring final circuits besides a selection of more basic radials such as condensers, heating and HVAC systems.
C&G uses the OSG which covers the very basic circuits and loading which are applicable to circuits protected by up to 100A service fuses. at 16kA maximum

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