General Points to be Considered in Design

1-    It is always preferred to put the distribution transformers in gardens as possible; yet the environmental constraints should be also fulfilled.

2-    For buildings of flats we usually use the diversification chart since the load profile between buildings is not necessary to be the same so we can't take a certain figure to be the diversity factor. On the other hand, villas can be considered as loads of the same load profile since all villas' inhabitants do behave in almost the same manner which is somehow luxurious; thus in villas we usually take the diversity factor to be 1.05à1.1

3-    Diversification is used for any node that supplies more than one node; i.e if the pillar feeds more than one feeder then to get the load of the pillar we consider diversification between these feeders. Same is done when considering distribution transformers and pillars.

4-    Since feeders have the same load profile; we use diversity factor of 1.05à1.1

5-    The locations of the transformers and pillars and the routes of the cables are chosen so that:

-       The maximum voltage drop between any transformer and the furthest consumer is  5% of the nominal voltage (220 V). to overcome this voltage drop taps on the high tension side of the distribution transformers are adjusted so that the consumer receives 220 V

-       The crossing between cables should be avoided as much as possible.

-       The routes of the cables should avoid street crossing as much as possible so that when maintenance in feeders is done we don't need to dig across the streets to get the cables out.

6-    As we mentioned before the distribution boxes are connected in loops and so does the coffree of the buildings. Thus if two coffrees or two boxes of different loads are connected together then it is recommended that both have the same c.s.a of feeders which suits the one with the larger load. This is very important so that if a fault occurs on the box with larger load; the feeder of the other box can withstand the overload safely.

7-    Low voltage fuses ratings are as follows: 2, 4, 6, 8, 10, 16, 20, 25, 32, 35, 40, 50, 63, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000 and1250 Amperes according to ABB pocket book(switchgear manual), 8^th edition.

8-    Standard ratings of pillars are 50 KVA, 100 KVA, 150 KVA, and 200 KVA

9-    Standard ratings of distribution transformers are 300 KVA, 500 KVA and 1000 KVA. Yet it is recommend using of 500 KVA transformers instead of 300 KVA transformers as much as possible since the former is slightly more expensive than the other (only 1000 or 2000 L.E more) while it is better for the new developing areas to use it due to its larger capacity. Thus in this project we will try to use the 500 KVA transformer as much as possible.

10- Standard ratings of street lighting pillars are 150 and 250 KVA

11-  Additional 25% spare equipments should be used in the design; i.e if the design shows the need of 4 cables then a fifth cable is added as a spare. In this project the extra equipments are not shown in the drawings but it is understood that they are found.

In the secondary distribution networks the c.s.a of the cables used shouldn't be less than 3 70 + 35 mm² or else the voltage drop will be severe and may be more than the permissible ranges.