Isofootcandle curves are based on illumination at 87-1/2% of nominal battery voltage (end of battery life.) Illumination levels at the beginning of the discharge cycle (Beam penetration) will be approximately 20% greater than shown. The National Electrical Code limits voltage drop to a maximum of 5% of nominal. The table shows the maximum length of a given battery circuit, assuming that the total load is concentrated at the end of the load circuit.

If loads are uniformly spaced (equal watts, equal distances) along the circuit path, all of these distances may be increased, based on the number of heads or fixtures on a given circuit. This may permit a savings by the use of a smaller wire size (see wiring distance formula table.)

DISTRIBUTED LOAD CIRCUIT DISTANCE ADJUSTMENT TABLE

For example, a 36 foot long, 6V circuit has 7W heads spaced 12 feet apart. According to the voltage drop table, #10 wire must be used at 43 feet for a 5% voltage drop. But, multiplying the 27 feet for #12 wire by 1.5, a 40-1/2 foot wire run is acceptable. Therefore, #12 wire may be used and still meet the 5% voltage drop limitation.

Wire size determinations can be made for non-uniformly spaced loads. But for ease and safety, it would be advisable to use the voltage drop tables.

For a precise determination of the maximum length of wire run for a concentration load (before applying the adjustment for uniformly distributed loads), divide the total load in watts into the constant at the bottom of each row. (Example: the maximum wire run for #12 wire on a 12V system, with a 44W load, is 2267 ÷ 44 = 51W; on #10 wire, 3604 ÷ 44 = 82W.)