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C

40 LAMPS

CENTRE,90 FEET

SIZES OF CONDUCTORS FOR INCANDESCENT

CIRCUITS.

The most accurate method of determining the proper sizes of incandescent lamp conductors is to refer all measurements back to the dynamo, converter, or street tap.

To illustrate, suppose we have an installation of 150 lights, consisting of a feeder or dynamo main 20 feet long (to distributing point) and several mains A, B, and C, their lamps and lamp centres being respectively 60, 50, and 40 in number, and 38, 60, and 90 feet from the end of the feeder. Let us calculate the sizes of the feeder and one main, and of one branch having 12 lamps with centre 20 feet from the main, the branch starting 18 feet from the distributing point. (See cut.)

To find the size of the branch wire, refei to the appropriate table with 20+ 18+ 20 feet, or 58 feet for 12 lamps.

To find the size of the main, imagine the branches on one side to be revolved (or lay them out thus on a diagram) so that all are on the same side of the main; then estimate or calculate the lamp centre of the resultant group, which in this case we will suppose to be 23 feet from the main and 38 feet from the distributing point measured along the main, and refer to the table with 20+ 38+ 23 feet for 12 +30 + 18 lamps, or 81 feet for 60 lamps.

To find the size of the feeder, suppose the mains to be revolved about the distributing point so that they all overlap, and with all the branches on one side of the overlapping mains; then estimate or calculate the lamp centre of the resultant group (comprising all the lamps), which in this case we will suppose to be 20 feet from the overlapping mains measured at right angles, and 48 feet from the distributing point measured along the main, and refer to the table with 20+48 +20 feet or 88 feet for 150 lights, or for the largest number of lights that will ever be used at one time.

In simple cases the quantities may be estimated, either directly (especially for branches) or from rough diagrams, and for more complex cases or where a perfectly accurate result is desired, the following rules are given :

For BRANCHES, follow the method given above.

For MAINS, multiply the number of lamps on each branch of a main by the distance of their lamp centre from the distributing point, always measured along the lead of the main and branch; add the products thus obtained for all the branches on the main and divide by the whole number of lamps on the branches. Add the length of feeder and refer to the table with the resultant distance and lamps.

EXAMPLE. (See cut, main A.)

(18 +20) × 12 =

456

(33 + 30) × 30 =

1890

456 + 1890 + 1350 +20=81 feet for 60 12 +30 + 18

lamps.

For FEEDERS, add the sum of the prod

ucts obtained as above for all the mains, divide by the entire number of lamps on the feeder, add the length of the feeder, and refer to the table with this distance and all the lamps on the feeder, or the largest number that will ever be used at one time.

EXAMPLE. (See cut.)

Main A. 456 + 1890 + 1350 = 3696

Care must be taken not to confound a lamp centre (so called) with a geometrical centre. For example, suppose a series of branches of equal length radiating from the end of a main like the spokes of a wheel, and having lamps at equal intervals. Here the geometrical centre is the radiating point, while the lamp centre is on a circle passing through the centres of the various groups, or the length of the radius from the radiating point. In the case of the main A given above, the geometrical