Diversity and Sub-Main Calculations

Diversity and Sub-Main Calculations Points : diversity and sub-main calculations, diversity, use diversity table, use a power load for per socket outlet, use a base load per ring circuit and try to anticipate other heavy loads Having detailed the final circuits we have now to consider how each of these fuse-boards are to be supplied. Diversity Table lB (copy below) within the lEE On-Site Guide, Appendix 1, Allowances for Diversity gives guidance with regard to typical allowances for diversity which may or may not be applied as you feel appropriate. The lighting for example is reasonably simple. Type of Premises

Type of Premises
	Purpose of final circuit fed from conductors or switchgear to which diversity applies	Individual household installations including individual dwellings of a block	Small shops, stores, offices and business premises	Small hotels, Boarding houses guest houses, etc.
1.	Lighting	66% of total current demand	66% of total current demand	75% of total current demand.
2.	Heating and power (but see 3 to 8 below)	100% of total - current demand up to 10 amperes + 50% of any current demand in excess 10 amperes	100% of largest appliance +75% f.l. of remaining appliances	100% f.I. of largest appliances +80% f.l. of second largest appliances +60% f.l. of remaining appliances.
3.	Cooking appliances	10 amperes + 30% f.I. of connected cooking appliances in excess of 10 amperes + 5 amperes if socket-outlet incorporated in control unit	100% f.I. of largest appliances +80% F.I. of second largest appliances +60% f.l. of remaining appliances	100% f.l.of largest appliances +80% f I. of second largest appliances +60% f.l. of remaining appliances.
4.	Motors (other than lift motors which are subject to special consideration	not applicable	100% F.I of largest motor +80% F.I of second largest motor 60% f.I. of remaining motors	100% f.l. of largest motor +50% f.l. of remaining motors.
5.	Water-heaters (instantaneous type)	100%fI.Iof largest appliances +100% f.I. of second largest appliances +25% f.l. of remaining appliances	100% f.l. of largest appliances +100% f.I. of second largest appliances +25% f.l. of remaining appliances	100%f.I.of largest appliances +100% f.!. of second largest appliances +25% f.I. of remaining appliances.
6.	Water-heaters (thermostatically controlled),		no diversity allowable
7.	Floor warming installations		no diversity allowable
8.	Thermal storage space heating installations		no diversity allowable
9.	Standard arrangement of final circuits in accordance with Appendix 8	100% of current demand of largest circuit +40% of current demand of every other circuit	100% of current demand of largest circuit +50% of current demand of every other circuit	100% of current demand of largest circuit +50% of current demand of every other circuit
10.	Socket-outlets other than those included in 9 above and stationary equipment other than those listed above.	100% of current demand of largest point of utilization +40% of current demand of every other point of utilisation	100% of current demand of largest point of utilization +70% of current demand of every other point of utilisation	100% of current demand of largest point of utilization ÷75% of current demand of every other point in main rooms (dining rooms, etc.) +40% of current demand of every other point of utilisation

For the purpose of this Table, an instantaneous water-heater is deemed to be a water heater of any loading which heats water only while the tap is turned on and therefore uses electricity intermittently.

It is important to ensure that the distribution boards and consumer units are of sufficient rating to take the total load connected to them without the application of diversity.

Take as an example, a lighting fuse-board with a typical connected load of say 60 Ampere. We might decide to apply the diversity factor of 0.9 and hence use 60 x 0.9 = 54 Ampere as our design current for the sub-main cable which feeds this board.

However small power is not quite so straight forward, nor so readily accepted as the correct technique to use. In fact at least three design techniques seem to have evolved each with their own relevant advantages/disadvantages: 1: Use Diversity Table This is the only method at least recognised or supported by the Wiring Regulations]

In fact although the I.E.E. Make it clear that we should not apply diversity in the final circuit they do not make it clear quite where in the installation we should apply this table. It is probably intended to be applied at or near the main intake position and of course the allowances for diversity must only be applied once throughout the installation, but there is no reason why it should not be used at sub-main level.

For example if a fuse-board provides 6 x 32 A fuse-ways then the total current in the sub-main cable will be given from
100% of the largest fuse - 1 x 32 A = 32 A
50% of the remainder 0.5 x 5 x 32A = 80 A
Total 112 Ampere This method can on 6ccasions tend to overestimate the current requirements for a fuse-board. 2. Use a Power Load for per Socket Outlet Most people use 250 Watts per socket outlet, but of course this figure should be estimated from previous experience of a similar design.

Taking our theoretical fuse-board above, suppose each ring circuit supplies 10 socket outlets then the total estimated current drawn by the fuse-board will be 6 x 10 x250 Watts = 15 kW which will give a current of 15 x 1000/240 = 62.5 Ampere
This method tends to underestimate the total sub-main current unless there are a large number of socket outlets. 3. Use a Base Load per Ring Circuit and Try to Anticipate other Heavy Loads This is the method I prefer since it tries at least to predict the probable use of the installation. For example, allow a base load per ring or radial circuit - I usually choose I kW which will allow for all the smaller loads such as;

Desk Lamps - 100 Watts
Calculators - 20 Watts
Typewriters - 200 Watts
Computers - 100-300 Watts
Fax Machines - 200-300 Watts
Televisions - 300-500 Watts
Videos - 200-300 Watts

Then is try to anticipate other heavy load - water and space heating being typical - such as the use of electric kettles or portable electric fan convectors in say a manager’s office.

Electric Kettles- 1500-1500 Watts
Portable Heaters - 2 500-3000 Watts
Large Photocopiers - 2000-3000 Watts, etc.

So once again our theoretical board might contain;
6x.1 kW=6 kWplus
2 x Electric Kettles 6 kW
3 x Electric Fires = 9 kW
1 Photocopier = 2 kWv Total 23 kW

From which we would get an anticipated current of 2300/240 =95.8-say 100 Ampere

As you can see, the variation between the methods is quite large, from 62 ampere to 112 ampere and I suspect in reality the actual current lies somewhere between the two. Remember you are not trying to estimate the average current but the likely maximum that will occur over a long enough period to cause significant cable heating, say in excess of 1 hour’s duration. However, the actual current can only be predicted with any degree of certainty by discussions with the client and by asking him (or her) “What will you connect to the sockets?”, and then by attempting to allow for these loads. Of course we also have to be rational about our loads, for instance the fuses and circuit breakers come in discrete sizes and in the above example the choice is really only between the 100A fuse and the 1 60A fuse.