Technical information

The NFPA 79 standard for Industrial Machinery (and the relative UL508A

section) allows a group installation in accordance with points a) and b), but

point c) changes in the definition of the lowest value for the choice of the BCP:

the one linked to the markings remains the same while the other is no longer

obtained by a calculation but by a table (tab. 66.2 of UL508A) that binds the size

of the BCP to the conductor with the smallest cross section downstream of the

protection itself.

1.5.2 Sizing of conductors to the single motor.

In the case of group installation the circuit generally consists of conductors

(called branch circuits) leaving the BCP protection to arrive at a distribution

system (e.g. taps) from which the switching conductors powering the motors

leave again, normally with a lower cross-section.

The conductors of each tap that power a single motor do not need to have an

individual branch circuit protection from the short circuit provide the following

conditions are respected.

1) no conductor to the motor must have a lower capacity than the feeder

conductors capacity; or

2) no conductor to the motor must have a capacity lower than one third of that

of the i branch circuit conductors (with a minimum value not less than the

usual 125% of the motor current);the overload protection conductors must

be the same as or shorter than 7.62 m and protected manually.

3) the conductors that go from the branch circuit protection to a “manual

motor controller,” also approved as “Suitable for Tap Conductor Protection in

Group Installations”, can have a capacity of no less than 1/10 of the BCP

calibration (rule included in the NEC until 2002) and in the new NEC ed.

2005 can have a capacity not lower than the size – calibration of the BCP

(fuse or motor protector).

4) The conductors from the controller to

the motor must have a capacity of no

less than the usual 125% of the motor

current.

The conductors from the BCP to the

controller must be adequately protected

mechanically, segregated and they must

be in lengths of not more than 3 m, or

they must have a capacity of not less than

that of the “branch circuit” conductors.

A particular case of group installation is represented by the “common bus” in

direct current: these systems consist of power rectifier that powers a short circuit

that in its turn powers one or more inverters (and relative motors downline).

The UL508A standard gives specific instructions for these components:

BRANCH CIRCUIT

140M