Technical information

1.9 Heater branch circuit

156

The heater branch circuits are the circuits that feed the heater r

esistors.

The present analysis contains a description of the required standards for the

process heating, i.e. linked to the functioning of the machine, and not that linked

to the heating of the environments (“fixed electric space-heating”).

The UL508A standard brings together the main NEC and CEC regulations and

imposes:

•Branch Circuit Protection: this must be calibrated at 125% of the rated current

of the element and must not be higher than 60 A: this entails the maximum

current of an element must be 48 A.

If there are heater circuits with currents higher than 48 A it is necessary to

separate them into several subcircuits that respect the norms.In the panel a

size/calibration protection above 60 A may be placed in the panel providing

the separation into subcircuits occurs in “in the field” using BCPs of a suitable

size.

•The capacity of the conductors must be higher than the rated current of the

resistive load.The capacity of the conductors must be above the nominal

current of the resistive load.

Exceptions to the 48 A limit are allowed for special applications: for example the

heaters outside to meld show and ice or the immersed type for heating water or

generating steam.

The NFPA 79 standard for Industrial Machinery (and relevant section of UL508A)

requires the oversizing of the wires to 125% of the rated resistant current. It is

advisable to unify the design by adopting the oversizing of the wires to

125% because it is required by some AHJs for all the applications

EXAMPLE

A three-phase heating resistance that absorbs 20 A is assumed.

The branch circuit protection must have sizes above 1.25 x 20 A = 24 A,

consequently protection such as 25A fuses will be chosen.

BRANCH CIRCUIT