Technical information

4.1 Distribution mounts, Power Block

For panels of large capacities it is sometimes necessary to make distribution

systems to make it possible to power several users with different and/or smaller

cross-sections.

All the distribution “mounts” or “commercial” bars used should be NRTL

approved because they are particularly critical components in the case of short

circuit.

In fact power blocks are very common in North America; they are certified for the

electrical distribution inside electrical industrial control panels.

1492-PD

In case it is not possible to use a Power Block, a system of bus bars can be

constructed

Note: it is not easy to calculate the capacity of the system because short

circuit tests have not been conducted but the reference values of data

supplied by the insulation and bus bar section manufacturers can be

extrapolated. With reference for the distance in the air and surface

reference can be made to table 10.2 of UL 508A.

FEEDER AND BRANCH CIRCUITS

Voltage Involved Minimum spacing, inch (mm)

Between live parts of opposite polarity Between life parts and

Through air Over Surface grounded metal parts,

through air and over

surface

125 or less

(12.7)

(19.1)

(12.7)

126 - 250

(19.1) 1 –

(31.8)

(12.7)

251 - 600 1 (25.4) 2 (50.8) 1 (25.4)

NOTE – An isolated dead metal part, such as a screw head or a washer, interposed between un insulated parts

of opposite polarity or between an un insulated live part and grounded dead metal is evaluated as reducing the

spacing by an amount equal to the dimension of the interposed part along the path of measurement.

Table 10.2 of UL508A provides the minimum spacing also for the certified bus bar systems.