Distributing Boards

The main distributing board contains six fuses, one for each pole of the three sub-mains, and may be of any of the types shown in Chapter XIX (Electrical Accessories, Fixing And Connecting)., where the method of connecting to the circuits is also shown. The subsidiary distribution boards contain fuses and switches, one fuse on one pole of each circuit and one single-pole switch on the other pole of each circuit. The lamps are hung from ceiling roses by means of flexible wire, composed of two cables of 35/40 twisted together. The size of this flexible wire is determined from the following table, where it will be seen that the smallest wire which will safely carry 0.9 amperes is 35/40:-

Table Of Flexible Wires

Stranded Copper Wire.

Equivalent in Single Wire.

Carrying Capacity. Amperes.

23/40

22

0.6

35/40

20

1.0

70/40

18

1.8

130/40

16

3.2

Sizes Of Fuses

The size of a fuse wire depends upon several points, namely, the distance between the fuse terminals, the size of the fuse terminals, the material of which the fuse is made, and the current to be carried by the circuit which the fuse is to protect from overheating.

The distance apart of the terminals is not, as a rule, worth considering, as the fuse boxes are designed for various currents and voltages. The size of the terminals also affects the blowing of fuses according to the rate at which they carry away the heat generated by the passage of excessive currents, but this is a point which need not be considered as a rule, as the sizes of these accessories are standardised.

The material of which the fuse wires are made is a very important one, as may be seen from the following table. Still more important in determining the size of the fuse wire is the current it is required to carry. The following table gives the approximate fusing points of fuse wires of lead, tin, and copper.

Fuse wires are usually of such a gauge that they will "blow" when the current becomes 50 per cent, greater than that which the circuit it protects is intended to carry.

Approximate fusing Current in Amperes.

Approximate Standard Wire Gauge.

Tin.

Lead.

Copper.

I

37

35

47

2

31

30

43

3

28

27

41

4

26

25

39

5

25

23

38

10

21

20

33

15

19

18

30

20

17

17

28

25

16

15

26

15

14

25

35

14

13

24

40

14

13

23

45

13

12

22

13

12

21

60

12

21

70

20

80

Strip fuses above this

Gauge.

Strip fuses above this

Gauge.

19

90

18

100

18

120

17

140

17

160

16

180

16

200

15

225

14

250

14

275

13

300

13

Note

Pure copper wire (Column 4) makes the most reliable fuse, and must be at least 1 inch in length, measured between the terminal clips.

Thus in Plate VI. the separate lamp circuits carry 0.9 amperes, so that the fuses should blow when a current of 1.35 ampere passes through it. The wires for these circuits are 3/22, and will actually carry a current of 4.4 amperes (see table, page 180), so the fuses might be such a size as to blow when 6.3 amperes passed through them. The fuse wires in this particular case may be of tin of any gauge between 31 and 25 S.W.G. The sub-mains are all of 7/22 wire, which will carry a current of 8.7, and the fuses should blow when a current of 13.05 amperes passes. The wire for the purpose will be of tin No. 21 S.W.G.

The mains carry a current of 20.4 amperes, so that the fuse must blow at 30.2 amperes when No. 25 S.W.G. copper wire will be used. The main fuse, which is usually inserted by the supply company, is generally made of copper, and all the fuses on the branches are of lead or tin.

Table Showing Maximum Currents, Thickness Of Dielectric And Insulation Resistance For Copper Conductors, Insulated And Laid In Casing Or Tubing

Issued, July 1896, by the Institution of Electrical Engineers.

Table Showing Maximum Currents Thickness Of Dielec 187

Explanation Of Table

Column I

Gives the sizes of the conductors in common use. Cables are shown thus :-19/14, namely, 19 wires of No. 14 Standard further protection from mechanical injury or from vermin. B. - Those insulated with a material, as a dielectric,.

Wire Gauge. Column 2. - Gives the maximum current for situations where the external temperature is above 100° F. The maximum rise in temperature will be about 10° F. on large sizes. Column 3. - Gives the total length in yards of lead and return of each size of conductor causing a drop of one volt, when transmitting the current shown in Column 2. Column 4. - Gives the maximum current allowable in any situation. The maximum rise in temperature will be about 20° F. on large-sizes. Column 5. - Gives the total length in yards of lead and return of each size of conductor causing a drop of one volt, when transmitting the current shown in Column 4. Column 6. - Gives the minimum thickness of dielectric. This may be obtained for any conductor by adding 30 mils, to 1/10 the diameter of the conductor. Columns 7 and 8. - Give the insulation resistances in megohms for one mile of cable of Classes A and B respectively.

A

Those insulated with a material, as a dielectric, which is itself so impervious to moisture that it only needs.

which, in order to preserve its insulation qualities, must be kept perfectly dry, and therefore needs to be encased in a.

waterproof tube or envelope, generally of soft metal, such as lead, which is drawn closely over the dielectric.

The numbers of lamps given are based upon the current densities in Column 4. If the cables or wires are to be used in situations where they will normally be subjected to a greater temperature than 100° F.- e.g. in drying-rooms, hothouses, etc., the maximum permissible number of lamps will be less than that given in the above table, and may be ascertained by dividing the current density (Column 2) by the current required for each individual lamp. For example, a 16 c.p. 60 watt 100 volt lamp requires 60/100 = o.6 amperes, assuming the number of such lamps permissible on 3/20 lead to be required; from Column 2, 4.8-7-0.6 = 8 lamps.