Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, concerning a domestic installation and diversity, is answered by SELECT:
Question: I'm dealing with an existing domestic installation with the following circuits. What is the maximum demand and diversity for this installation? The installation comprises:
- Two ring circuits each with a 32A MCB
- Two lighting circuits each with a 6A MCB
- One water heater (thermostatically controlled) with a 16A MCB
- One cooker circuit with a 32A MCB
To the best of my knowledge, I have made an accurate assessment of maximum demand.
My concern is that the rating of the cut-out fuse is not suitable, even though I have taken diversity taken into account. The fuse is a 60A BS1361 Type2a.
Using Appendix 1, Table 1B in the On-Site Guide, I have carried out the following:
- Two rings - 32 + (32 x 0.4) = 44.8A
- Two lighting circuits - 12 x 0.66 = 7.92A
- Water heater - 16A (no diversity allowable)
- Cooker circuit - 10 + (22 x 0.3) + 5 = 21.6A.
So, the maximum demand with diversity taken into account is 90.32A. Surely, in this instance, a 100A main fuse should be used, rather than a 60A fuse to satisfy the criteria of Ib must be equal to or less than In?
Perhaps I have missed something out. I would be grateful for a comments and advice on this matter.
Answer: You have based your calculation upon the rating of the protective devices rather than the current demand of the circuits themselves.
Return question: Thanks to SELECT for the previous answer concerning maximum demand & diversity. The answer pointed out that I based my calculation on the rating of the protective devices rather than the current demands of the circuits themselves.
This time I have taken the answer from SELECT into account and again used Table 1B from Appendix 1 of the On-Site Guide for guidance (note, for the two ring circuits, I have assumed that the occupiers of the property will vary their use of the circuits from time to time, and for this reason I have taken the design current of one ring to be equal to the protective device rating, and applied diversity to the second ring in accordance with Table 1B of the On-Site Guide).
Max demand with diversity taken into account:
- Lights - 2.87A
- Rings - 44.8A
- Water heater - 12A (no diversity allowable)
- Cooker (which has a socket incorporated in the control unit) - 15.9A
Total = 75.57A.
But the 60A cut-out fuse does not appear to be suitable. I would say that an 80A fuse should be used in this instance. Have I made an error here, or is this an error from the distributor? Should an 80A cut-out fuse be used instead?
Return answer: Estimating maximum demand can be difficult and is rarely made accurately. The designer can opt to use other methods than those in the On-Site Guide - i.e. use his/her knowledge of what the actual current loading is likely to be connected to a circuit.
If using Table 1B from the On-Site Guide, I recommend that you apply diversity to the lighting circuit (as you have done), then use Row 9 of the table - you will find this drops the anticipated maximum demand to just over 60A and will not overload the 60A fuse. Therefore, the 60A fuse is satisfactory.
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