UL 508A - Ampacities of Conductors

Ampacity of Power conductors and control circuit conductors according to UL 508A

Last edit: 19/07/2023

The size of insulated conductors inside an industrial control panel cannot be smaller than 14 AWG. Actually, for Industrial Machineries, for low rated power motors, conductors smaller than AWG 14 can be used inside the control panel: please refer to Table 66.2.

In order to calculate a conductor size, inside control panels, the motor FLC must be increased by 25%: that becomes the minimum ampacity the conductor must have.

In general, conductor ampacities of power circuits, inside control panels, are stated in Table 28.1 of UL 508A, while for control circuits the table to be used is Table 38.1.

Going back to the power circuits, it is also mandatory to use conductors that can withstand 90°C, but they cannot exceed 75°C in temperatura: that means their ampacity will be lower, since we cannot bring them up to 90°C, like we do in Europe or, in general, following for example IEC 60204-1. But when table 28.1 is consulted you may have a doubt about which column to choose: the 60°C or the 75°C column.

For example, if a motor has an FLC of 50 A, I have to use a cable with an ampacity of at least 62,5 A. Shall I than use an AWG 4 (@60°C the ampacity is 70 A) or an AWG 6 (@75°C the ampacity is 65 A). That has always been implicit, since it is clearly explained in the NEC. However that has often generated doubts and confusion.

In the 2022 update of the standard, it is explained when to use the 60°C colum and when the 75°C can be used. Hereafter the key language.

[UL 508A: 2022] 29.6 Sizing
The wire temperature rating(s) marked on the component or in the instructions shall be used to select the appropriate column(s) from Table 28.1 as follows :
1)    […]
4)    If the component markings or instructions do not specify a wire temperature rating, the conductor size shall be determined using the ampacities in the 60°C column in Table 28.1 when the component is rated 100 amps or less, and the 75°C column in Table 28.1 when the component is rated greater than 100 amps.

Going back to our example, let's assume to use a SIEMENS 3RV2042-4JA10 with setting 45-63A. Since nothing is indicated on its E47705 certificate on the maximum temperature of its terminals, since the component has a nominal current of less than 100 A, we must take the 60°C column and therefore select an AWG 4 cable.