Formula For Cable Sizing Jun 2026

Just because a cable can carry the current without melting doesn't mean it is efficient. Current flowing through a cable causes a drop in pressure (voltage). If the drop is too high, equipment won't work.

The Ultimate Guide to the Formula for Cable Sizing: Safety and Efficiency formula for cable sizing

High resistance in long cable runs causes voltage to drop, which can damage equipment. The permissible limit is typically . Just because a cable can carry the current

You divide the breaker size by these factors. If the factors are less than 1 (e.g., 0.8), the required tabulated current ($I_t$) goes up . This forces you to select a larger cable. The Ultimate Guide to the Formula for Cable

Before picking a cable, you need to know how much current will flow through it.

Note: For motors, the NEC standard typically requires sizing the cable to 125% of the full load current ( Iflccap I sub f l c end-sub 2. Determine Required Current-Carrying Capacity ( Izcap I sub z Once you have the design current ( Ibcap I sub b ), you select a protective device ( Incap I sub n ) such that