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Posted By Topic: Diversity Factor for 3Ph instantaneous hot water

Feb 14 2020 18:43


I'm doing a maximum power demand calculation for an apartment block (total 29 units) where the client wants to use a 3Ph instantaneous hot water (40A/Ph).

I was able to calculate the load required per unit using the 33% diversity mentioned in AS3000 table C1 for single dwelling.

I am a bit concern with regards to calculating the overall power demand. Based on table C1 the diversity for 10 units/ph (29/3 = approx 10) is 6A per living unit for instantaneous hot water which is also same for a typical hot water storage. So if I use a 40A/Ph instantaneous hot water or a typical 3kW (15A) hot water storage there diversity will still be the same? Which is 6A per living unit? So a bit hesitate since 40A is like 30A higher than 15A.

Hope someone can clarify this.


Feb 14 2020 19:41

Ok , so you've correctly applied note b; to arrive at 10 units / phase.
Then you've correctly found the contribution on row E (instantaneous water heaters) in column 4 (6 - 20 units per phase) and the figure is 6 A.

The entire point of a diversity is that not all the units will be using their w/h at the same time, and the table provides a 'deemed acceptable' guesstimation for this.

If instead of the huge central instantaneous w/h unit, you had a 3 kw storage w/h in each unit, then (row F) yes the contribution to MD would be 6A per unit. Again, not all units will be pulling their 3 kW at the same time.

The fact that these figures are the same is 9almost) co-incidental.

Remember that the MD is ONLY used for determining the size of cable required. It has NO bearing on what the ACTUAL current is at any particular moment; its just a method of working out the expected maximum loading.
Whatever the answer; the cable installed has to be absolutely protected against overload.
safe as houses