When the lamps are strung along the main, however, as in fig. 826, it is sometimes necessary to choose the size of wire with regard to the drop, and in order to do this the main must be measured for either "ampere feet" or "lamp feet."
=Wire Calculations.=—The problem of calculating the size of wire will be presented here in as simple a form as possible, with explanation of the various steps so that any one can understand how the formula is derived.
In determining the size of wire, there are four known factors which enter into the calculation, viz.:
1. Length of circuit in feet; 2. Maximum current in amperes; 3. Drop or volts lost in the circuit, _in % of the impressed voltage_; 4. Heating effect of the current.
The calculation is based on the _mil foot_, which as previously explained, is a foot of copper wire one mil in diameter and whose resistance is equal to 10.79 ohms at 75° Fahr.
[Illustration: FIG. 829.—Wiring for lights requiring unusually long feeders.]
The first step is to find an expression for the resistance of the wire which may be later substituted in Ohm's law formula. Accordingly, the resistance of any conductor is equal to _its length in feet multiplied by its resistance per mil foot and the product divided by its area in circular mils_, thus:
length in feet × resistance per mil foot resistance in ohms = ————————————————————————————————————————— circular mils
feet × 10.8 or ohms = ————————————- (1) circular mils
(calling the resistance per mil foot 10.8 instead of 10.79 to facilitate calculation).
Showing the maximum number of 16 candle power 110 to 240 volt lamps in parallel that may be carried by the various sizes of wire without violating the underwriters' rules.
+——————————+———————+——————————————+————————————————+————————————-+ |Wire size |Amperes|3.1-watt lamps|3.5-watt lamps. |4-watt lamps.| |B. & S. | +——————————————+————————————————+————————————-+ | gauge | | At 110 220 | At 110 220 | 220 230 240 | | | | volts V. | volts V. | V. V. V. | +——————————+———————+——————————————+————————————————+—————————————+ |0000 | 210 | 462 924 | 412 825 | 722 754 787 | | 000 | 177 | 389 778 | 347 695 | 608 636 663 | | 00 | 150 | 330 660 | 294 589 | 515 539 562 | | | | | | | | 0 | 127 | 279 558 | 249 499 | 436 456 476 | | 1 | 107 | 235 470 | 210 420 | 367 384 401 | | 2 | 90 | 197 396 | 176 353 | 309 323 337 | | | | | | | | 3 | 76 | 167 334 | 149 298 | 261 273 285 | | 4 | 65 | 143 286 | 127 255 | 223 233 243 | | 5 | 54 | 118 237 | 106 212 | 185 194 202 | | | | | | | | 6 | 46 | 101 202 | 90 180 | 158 165 172 | | 8 | 33 | 72 145 | 64 129 | 113 118 123 | | 10 | 24 | 52 105 | 47 94 | 82 86 90 | | | | | | | | 12 | 17 | 37 74 | 33 66 | 58 61 63 | | 14 | 12 | 26 52 | 23½ 47 | 41 43 45 | | 16[3] | 6 | 13 .. | 11 .. | 20 21 22 | +——————————+———————+——————————————+————————————————+—————————————+