Technical information

FEEDER AND BRANCH CIRCUITS

3Feeder overcurrent protection

The devices protecting the panel from over currents are explicitly required only

in the case of Industrial Machinery

) by NFPA 79 (and relative section of the UL

508A), but its presence is also provided for in NEC and CEC in which sizing rules

are given.

Obviously the protection of the lines powering the equipment is not the

responsibility of the machine manufacturer.

It is noted that in the case of circuit breaker (devices suitable for switching and

protecting) calculations must be made for sizing both for overcurrent protection

and for disconnection means.

The sizing of the overcurrent protection follows the same rules in all the standard

texts.The size/calibration should not be more than one of the following two values:

a) the protection size/calibration of biggest branch (BCP) plus the rated current

(FLA) of all the other motors plus the rated currents of all the other loads (f

machinery , this is 125% of the larg est motor – from tab le 50.1, +125% of all

heater loads; + FLA – from tab le 50.1 of all other motors, + FLA of all other

loads;

b) the capacity of the feeder conductor (conductors or bars down line of the

overcurrent protection)

)

It often happens, especially when using automatic s witches, that the calculation

pro vided f or in case a) g ives r esults that are incompatible with the sizing of the

s witching device: in the case of circuit breaker this makes an o v ersizing of the

f eeder conductor obligatory to enter into case b)

This solution should be applied, though not r equired b y the CEC f or equipment

destined f or Canada as w ell: in fact value b) can be modified with a simple over

sizing while the modification of value a) often implies a complete revision of the

panel.

) The following are considered industrial machinery: metal working including machinery for deformation or cutting: wood; plastic,

assembly machines; robots, transfer and test machinery.

) The CEC for Canada does not provide for case b) but only the value calculated in a).

140U 1489 1494V 1491 140F