Spacing Requirements for Power Distribution and Terminal BlocksSpacing Requirements for Power Distribution and Terminal Blocks New requirements in the inspector checks, among other things, to make sure there is 2005 NEC for a marked adequate spacing for the OEM application in which they are short-circuit current rating on being applied. industrial control panels, industrial machinery, and Power Distribution blocks are evaluated to UL1953, the Power commercial/industrial HVAC Distribution Block standard and are listed for general equipment have resulted in installation, meaning they have adequate spacing for most considerable attention being OEM and field applications. These Power Distribution Blocks placed upon the proper are marked with a listing mark, which means that the selection and application of inspector does not need to check conditions of acceptability Power Distribution Blocks (PDBs) and Terminal Blocks. It is as would be needed with recognized products. As listed fairly well understood that if an assembly short-circuit current products, they are suitable for use in the field. For example, rating above 10,000 amperes is desired, a Power Distribution an electrical contractor may install them in a wireway, Block or a Terminal Block with a high short-circuit current something that could not be done with a recognized product rating must be utilized. However, there is great confusion such as a terminal block. concerning the spacing requirements of these devices. This paper focuses on proper applications, based upon these Industrial Control Panels (UL508A): spacing requirements. UL508A contains two important requirements to consider when applying power distribution blocks. Proper Application Spacing of 1 through air, 2 over surface (at 600V) is When applying Power Distribution Blocks (PDBs), there are required when used in a feeder circuit (thats everything various requirements that shall be satisfied, based upon ahead of or on the line side of the final branch circuit different UL Standards, the NEC , and the specific overcurrent protective device). application. Some of the requirements and ratings include: An assumed short-circuit current rating of 10kA for blocks voltage, continuous current, wire range (load and line side), not marked with a higher short-circuit current rating, per short-circuit current rating or withstand rating (SCCR), type of UL508A Table SB4.1. upstream overcurrent protective device (fuse or circuit breaker) and spacing (between uninsulated live part and The following details the requirements regarding the use of uninsulated grounded part or uninsulated live parts of power distribution blocks in industrial control panels. opposite polarity). The requirements vary based on the application for industrial control panels (feeder circuit or Use of Power Distribution Blocks in Industrial Control branch circuit), field applications, or HVAC applications. Panel Feeder Circuits A Listed PDB (UL1953) can be used as is since it PDB Spacing Requirements for Equipment: meets the 2 and 1 spacing requirements for feeder Depending upon the specific application and the standard to circuits in UL508A section 10.2 (see table 10.2) & 28.2.4. which the equipment is being designed, there are certain A Recognized Terminal Blocks (UL1059) can only be PDBs that can meet these spacing requirements, some with a used if it meets the spacing requirements in 10.2 & high SCCR. The table below details the spacing 28.2.1 (see Table 10.1) and it is suitable for field wiring requirements at 600V based upon the equipment standards. (use Group A, C or D of UL 1059). Use Group A: Service including dead-front Spacing between Spacing between switchboards, panelboards, service equipment, and live parts of live parts and the like. UL Standard opposite polarity grounded parts Use Group B: Commercial appliances, including or enclosure Through air Over surface business equipment, electronic data processing 600V 600V 600V equipment, and the like. 508A Branch 3 1 1 8 2 2 Use Group C: Industrial, general. Circuits Use Group D: Industrial, devices having limited ratings. 508A Feeder 1 2 1 Circuits Such as where the load on any single circuit of the 3 1 1 1995 HVAC 8 2 2 terminal block does not exceed 15 amperes at 51 150 volts, 10 amperes at 151 300 volts, 5 amperes Note: Refer to Specific UL standards for complete spacing details. at 301 600 volts, or the maximum ampere rating, UL1953 vs. UL1059 whichever is less. Most power distribution blocks available today are actually terminal blocks, and are recognized to UL 1059, the Terminal Listed PDBs have adequate spacing for feeder circuit Block standard. Terminal blocks may or may not meet the applications but most recognized terminal blocks dont spacing needed for OEM applications. Terminal blocks are have the spacing required for use in feeder circuits marked with a UL recognition mark to ensure that the UL 2011 Cooper Bussman, Inc. 2