The formula used is IMAX = (I * 100) / (100 + VD), where I is the busbar current rating and VD is the allowable voltage drop. Typical values for LV installations are given below in Figure G27. 1) These voltage-drop limits refer to normal. Instant voltage drop limits calculator: NEC and IEC compliant, auto-applies 3%/5% rules for lighting, feeders. Enter nominal voltage and optional measured drop; get pass/fail, max allowable volts, length helper instantly. Design by percent limits: compute max length, minimum size or actual drop. Voltage drop is the reduction in voltage along a bus bar due to its resistance. This standard defines the design verification, test requirements, and thermal performance of the assemblies.