b) How to calculate allowable current of wires & cables

First, based on the previous calculation, estimate temperature Τ[℃] meeting Ρ_a = Ρb, and then calculate allowable current I using P at this estimated temperature. The graphs (1-2-1~1-2-8) in the next section show I values calculated for typical wires and cables. The notation “at atmospheric pressure” means that temperature rises when wire is laid horizontally in the air at 20℃ with no airflow (no wind).

1. Determine the “rise in conductor temperature” (ΔΤ) based on ambient temperature, upper-limit temperature rise due to other adjacent components, or maximum continuous operating temperature for the insulation or conductor. Typically, the rated temperature of the wire is taken as the upper-limit temperature.

   Table 1-2-1 Maximum continuous operating temperature for wire material

   
2. Read current [A] directly off of the graph by erecting a perpendicular to the x-axis at a given point of ΔΤ (rise in conductor temperature provided in ①) until it hits the conductor outer diameter line or wire type line.
3. Allowable current in a vacuum is about 1/2 to 1/3 that under atmospheric pressure. Figure 1-2-2 shows examples of calculation. For use in high-pressure gas and under low pressure at high altitudes, multiply the correction coefficients shown in Figures 1-2-9 and 1-2-10, respectively, by the value for atmospheric pressure.
4. For use in the form of a bundle of many wires or cables, or in a parallel configuration, multiply the correction coefficient for the bundle.
5. For use at high frequency (400Hz or higher), multiply the correction coefficient for AC resistance √(1/Ks) by the current value read in the figure.
6. For such conductors as copper alloy wires for which conductivity constants are not 100%IACS, multiply the correction coefficient for conductivity (%IACS) √(Conductivity/100) by the current read in the figure.

1. Increases as the conductor cross-sectional area becomes larger;
2. Increases as pressure rises, and decreases as pressure falls; when in a vacuum, it is about 1/2 to 1/3 the value under normal atmospheric pressure;
3. Increases in Junkosha Fluoropolymer Wires which have a larger ΔΤ; In other words, it is possible to make the cross-sectional area of them smaller when the same allowable current is desirable; and
4. Little depends on the type or thickness of insulation when ΔΤ is the same; the current read from the conductor outer diameter or cross-sectional area can be used as is even when the insulation thickness is somewhat different.