UL 508A - Sizing of Feeder Circuits

There were several methods for sizing feeder circuits until April 2020.

NFPA in 2018 edition provided:

• a general approach (§430.62);
• an approach for "standard" Industrial Control Panels (§409.62);
• an approach for Industrial Control Panels specific for Industrial Machineries (§670.4).

UL 508A in 2018 edition provided:

• an approach for "standard" Industrial Control Panels (§32.3.1);
• an approach for Industrial Control Panels specific for Industrial Machineries (§66.7.4).

At the time of writing, NFPA 2021 has not yet been published, while for UL 508A April 2020 update led to the unification of the two approaches mentioned above into a single one that represents the unique method for all types of industrial control panels.

The most important requirements made with this unification are:

• Sizing of feeder conductor (§66.5): it is stated that the minimum ampacity of the feeder conductor must comply with the following condition:

           IZ ≥ 1,25% FLCLARGEST MOTOR + 1,25% FLCHEATING LOADS  + ∑ FLCOTHER UTILITIES

• Sizing of feeder protection (§32.3.1): it is stated that the feeder conductor protection must comply with the following condition:

             IN   ≤  Largest BCPD (Table 31.1) + 1,25∙∑ FLCALL RESIST. LOADS + ∑ FLCOTHER LOADS   or   IN≤IZ

Here below the text from Section 32.3.1:

32.3.1 The size of the overcurrent protection shall not exceed the ampere value determined from (a) and (b), whichever is larger:

a)The rating of the largest branch circuit protective device in the circuit plus 125 percent of all heater loads plus the full-load currents of all other motors or other loads in the group that could be in operation at the same time; or

b)The ampacity of the conductors or bus bars on the load side of the overcurrent device.