Technical information

FEEDER AND BRANCH CIRCUITS

4.2 Sizing of the Feeding Conductor

In UL 508 A and CEC the current is determined according to the single loads,

such as:

125% rated current of the largest motor

added to

100% of rated currents of all the remaining loads

where the rated current of the motors is the FLA in the USA, the rating plate value

in Canada.

In the case of resistive loads, the CEC introduces a modification in the calculation

that is also present in NFPA 79 for Industrial Machinery. From the point of view

of a unified sizing it is recommended that this calculation method be

always adopted:

125% rated current of largest motor

added to

125% of rated current of resistive loads (heaters)

added to

100% of the rated currents of all the remaining loads

The capacity of the internal conductors should not be lo

w er than the calculated

load.Table 28.1 of the UL 508 A standard is used. It contains the reference

capacities for isolated thermoplastic conductors with a functioning temperature

at 90° C and in this the installation and temperature rating is considered.This

table can be taken as the basis of a unified sizing even if for Canada an

oversizing of sizes lower than AWG 10 of 30% for conductors with

temperature of 90°C and 10% for conductors of 105° is recommended.

The adoption of AW G 14 is always recommended as minimum limit at the feed

conductor section.

EXAMPLE

A feeder conductor supposedly protected by a 140U circuit breaker that powers

the following loads:

•motor with FLA of 20 A protected by 140M type 23 A combination motor

controller;

•control transformer with rated current of 4 A, protected by DC fuse of 6A in a

1492-FBxC fuse holder;

•heater resistor with 8A rated current protected by a 140U circuit breaker.

All the rated currents are called “FLA”for simplicity’s sake.

On the basis of the previous equation we find that the capacity of the feeder

conductor should not be lower than (1.25 x 22) A + (1.25 x 8) A + 4 A = 41.5 A.

From the table 28.1 of UL 508 A the AWG 6 section is obtained with a 55 A

capacity.