to the 16th Edition IEE Regulations
   
   
   
 
 

chapter 5
Earthing

chapter 6
Circuits

Circuits
  6.1. - Basic requirements for circuits 6.4 - Industrial socket outlet circuits
  6.2 - Maximum demand and diversity 6.5 - Other circuits
  6.3 - BS1363 socket outlet circuits 6.6 - Circuit segregation


6.2.3- Applied diversity

Apart from indicating that diversity and maximum demand must be assessed, the
Regulations themselves give little help. Suggestions of values for the allowances
for diversity are given in {Table 6.2}.
Distribution boards must not have diversity applied so that they can carry the total load connected to them.

Example 6.1
A shop has the following single-phase loads, which are balanced as evenly as possible across the 415 V three-phase supply.

2 x 6 kW and 7 x 3kw thermostatically controlled water heaters
2 x 3 kW instantaneous water heaters
2 x 6 kW and 1 x 4 kW cookers
12 kW of discharge lighting (Sum of tube ratings)
8 x 30 A ring circuits feeding 13 A sockets.

Calculate the total demand of the system, assuming that diversity can be applied. Calculations will be based on {Table 6.2}.

The single-phase voltage for a 415V three-phase system is 415/ 3 = 240 V.

All loads with the exception of the discharge lighting can be assumed to be at unity power factor, so current may be calculated from

l = P
  U

Thus the current per kilowatt will be

1000 A = 4.17 A
  240  

 

Table 6.2 - Allowance for diversity

Note the following abbreviations :
X is the full load current of the largest appliance or circuit
Y is the full load current of the second largest appliance or circuit
Z is the full load current of the remaining appliances or circuits

Type of final circuit   Type of premises  
  Households Small shops, stores, offices Hotels, guest houses
Lighting 66% total demand 90% total demand 75% total demand
Heating and power 100% up to 10 A + 50% balance 100%X + 75%(Y+Z) 100%X + 80%Y + 60%Z
Cookers 10 A + 30% balance + 5 A for socket 100%X + 80%Y + 60%Z 100%X + 80%Y + 60%Z
Motors (but not lifts)   100%X + 80%Y + 60%Z 100%X + 50%(Y+Z)
Instantaneous water heaters 100%X + 100%Y + 25%Z 100%X + 100%Y + 25%Z 100%X + 100%Y + 25%Z
Thermostatic water heaters 100% 100% 100%
Floor warming installations 100% 100% 100%
Thermal storage heating 100% 100% 100%
Standard circuits 100%X + 40%(Y+Z) 100%X + 50%(Y+Z) 100%X + 50%(Y+Z)
Sockets and stationary equip. 100%X + 40%(Y+Z) 100%X + 75%(Y+Z) 100%X + 75%Y + 40%Z

Water heaters (thermostatic)

No diversity is allowable, so the total load will be:
(2 x 6) + (7 x 3) kW = 12 + 21kw = 33kw
This gives a total single-phase current of I = 33 x 4.17 = 137.6 A

Water heaters (instantaneous)

100% of largest plus 100% of next means that in effect there is no allowable diversity.
Single-phase current thus = 2 x 3 x 4.17 = 25.0 A

 

Cookers

100% of largest = 6 x 4.17A = 25.0 A
80% of second = 80 x 6 x 4.17A= 20.0 A
  =100  
60% of remainder = 60 x 4 x 4.17 A = 10.0 A
  =100  
Total for cookers
=
55.O A

Discharge lighting

90% of total which must be increased to allow for power factor and control gear losses.
     
Lighting current
= 12 x 4.17 x 1.8 x 90 =
81.1 A
 
100
 

Ring circuits

First circuit 100%, 50 current is 30 A
75% of remainder
= 7 x 30 x 75 =
157.5 A
 
100
 
Total current demand for ring circuits = 187.5 A
Total single phase current demand= 486.2 A
Since a perfect balance is assumed, three phase line current = 486.2 A
    =3
 
=
162 A

 

 

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Extracted from The Electricians Guide Fifth Edition
by John Whitfield
Published by EPA Press Click Here to order your Copy

Click here for list of abbreviations