Technical information

Sizing of protections

Protection from the short circuit (BCPD) remains the same and is calculated over

the entire FLA of the motor.

The thermal protection m

ust be installed downline from the line contactor(M1)

and with calibration of no more than 1.15 times the 57.7% of the motor's FLA.

Attention must be paid to this requirement because very often a single overload

relay is used as protection both thermally and magnetic but it is not accepted in

this circuit schematic since it does not guarantee protection while the motor is

starting up.

Sizing of conductors

The conductors, internal and external that bring the current to a wye-delta

starting must be scaled on the basis of a factor of 1.25 (as already described); the

reference current however changes, it is reduced to 57,7% of the motor current.

1.5 Group Installation

One of the main characteristics of the North American standard is the fact that

they provide for “group installation”, i.e. group installations that are regulated by

specific instructions.

In general, these are installations in groups when there is a circuit where a single

protection against short circuits (BCP) protects all the loads at once: the motor

group installation (several motors down line from the same BCP) will be

analysed later on. Note that each motor must have its own protection from

overloads.

1.5.1 Dimensioning of branch circuit protections

The UL508A standard and the NEC provide for two or more motors (but also one

or more motors and other loads) being able to be protected by a single branch

circuit protection if they respect at least one of the listed cases:

1) When the BCP does not exceed 20 A at 125 V or 15 A at 600 V or less and

•the full load current of each motor,FLA, does not exceed 6 A; and

•the calibration and the type of protection of the branch circuit are co-

ordinated with the restrictions of the components down line (i.e. indicated

on the marking of the component);

2) When the calibration is the BCP type they respect the scaling parameters

(previously defined) for each motor in the assembly.The start up of the largest

motor must not cause untimely cut ins i;

3) When all the power components down line from the BCP protection are

approved for the group installation, as indicated by the marking (of the

NRTL certifier) on the component or on the instructions supplied with the

components.

BRANCH CIRCUIT