Technical information

FEEDER AND BRANCH CIRCUITS

Disconnect switch (or switch with fuses)

(UL 489 or UL 98, “Enclosed and Dead-

Front Switches”

Sizing follows different rules according to the type of load.

a) one or more loads, no motor: the size in current of the switch should not be

less than 115% of the total current (sum of the rated currents of all the

branch circuits).

b) a single motor: the power size (hp) of the switch should be shown in current

(using table 50.1 of UL 508A): this value obtained shall not be less than 115%

of the FLA (also obtained from tab. 50.1)

c) one or more motors and other loads: the power size (hp) of the switch shall

be shown in current (using tab.50.1 of UL 508A): this value shall not be less

than 115% of the FLAs of all the motors plus the rated currents of all the

other loads.

EXAMPLE

230V Three-phase system

The current for the sizing of the switch is: (1.15x22) A + 4 A + 8 A = 37.3 A

Now it is necessary to choose a switch with a size in power such that the current

received as in table 50.1 is above 37.3A: at 230V a switch of 15 hp (corresponding

with 42A) is required.

Sect. 1.25 x (22 + 8 + 4) >= 42.5A

Cond. 1.25 x 22 + 1.25 x 8 + 4 > = 41.5 A

AWG 10 (40A)