Use of Power Distribution Blocks in Industrial Control

Panel Branch Circuits (600V & below)

• A Listed PDB (UL1953) can be used “as is” since it exceeds

the spacing requirements for branch circuits in UL508A

section 10.2 (see table 10.1) & 28.2.4.

• A Recognized Terminal Block (UL1059) can only be used if

it meets the spacing requirements at the required voltage in

UL 508A, section 10.2 (see Table 10.1) & 28.2.1 and it is

suitable for field wiring (Use Group A, C or D of UL 1059).

Power Distribution Block Application Examples –

UL 508A

Termination Ratings

Important considerations in the electrical and thermal

relationship for components in a circuit are the conductor

size, material, rated ampacity, conductor insulation

temperature rating and the permissible connector device

conductor temperature limits.

If a conductor terminates at two devices that have conductor

terminations rated at two different temperatures, the

conductor ampacity must correlate to the terminal with the

lowest temperature rating.

Conductors with higher temperature ratings can be used at

their rated ampacities if the terminations of the circuit devices

are rated for the higher temperature rated conductor [NEC

110.14(C)(1)(a)(3)]. However, the industry norm is that most

devices rated 100A or less, such as blocks, disconnects,

controllers and circuit breakers, have 60°C or 75°C rated

terminations. For circuits greater than 100A, the norm is

75°C terminations. UL508A has further internal wiring

restrictions requiring 90°C insulated conductors, but these

conductors are sized based on either 60ºC or 75ºC

allowable ampacities.

Terminations have a temperature rating that must be

observed and this has implications on permissible conductor

temperature rating and ampacity. Shown below are three

common termination ratings and the rules. The conductor

ampacity may also have to be derated due to ambient,

conduit fill or other reasons.

Terminal

Temp Rating

60°C, 75°C, 90°C or higher temperature rated

60°C

conductor can be used, but the ampacity of the

conductor must be based as if conductor is

rated 60°C.

75°C, 90°C or higher temperature rated conductor

75°C

can be used, but the ampacity of the conductor must

be based as if conductor is rated 75°C. A 60°C

conductor is not permitted to be used.

Dual temperature rated termination. Either 60°C

conductors at 60°C ampacity or 75°C conductors at

60°C/75°C

75°C ampacity can be used. If 90°C or higher

temperature rated conductor is used, the ampacity of

the conductor must be based as if conductor is

rated 75°C.

PDB in UL 508A, 480V Feeder Circuit must have spacing of 1˝ through air and 2˝

over suface. PDBs listed to UL 1953 meet these spacing requirements. Terminal

Blocks recognized to UL 1059 must be verified to have required spacing.

over surface. PDBs listed to UL 1953 meet these spacing as well as most

terminal blocks recognized to UL 1059.

75°C terminal

60°C terminal

Conductor size and insulation rating?

Listed PDBs have adequate spacing for branch

circuit applications and most recognized terminal

blocks are also adequate for branch

circuit applications

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SB-BU09622

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Data Sheet 1049

Application Notes