UL Question Corner

UL Question CornerDo wire connectors used to make line side splices or taps need to have a short circuit current rating (SCCR)? If I use a power distribution block for line side conductor terminations does it need a SSCR? What is the difference?

UL Question CornerWire connectors that connect conductors of the same polarity or phase on the line side of the service are not required to have a SCCR or be specifically Certified (Listed) for use on the line side of the service. However, power distribution blocks are intended to be permanently installed and secured to an enclosure. When they are used to terminate line side circuit conductors of different polarity or phase and ground, power distribution blocks must be Certified (Listed) for use on the line side of service equipment. Additionally, Certified (Listed) power distribution blocks are assigned a SCCR.

Why do the requirements differ for these methods of connecting conductors on the line side of service equipment? Insulated wire connectors are used to join conductors of the same polarity or phase. The connectors are insulated to provide dielectric protection for the voltage rating of the connector from short circuiting to another uninsulated conductor or grounded metal. The connectors are not permanently affixed to the enclosure. Instead, they are floating free inside the enclosure, supported by the wires connected to them. As such, these connectors move with any force they may be subjected to, including those experienced under a short circuit condition, so that a conductor is not likely to pull out of the connector.

Power distribution blocks are intended to be permanently fixed within an enclosure and may be used to terminate conductors of opposite polarity or phase and ground. They utilize terminals that rely on through air and over surface spacings to maintain the proper distances between conductors based on the voltage rating of the power distribution block. These spacings are intended to prevent arcing and short circuiting between connectors of different voltage potentials.

Because power distribution blocks are not free floating in the enclosure, they must have the ability to withstand opposing forces so that conductors do not pull out of the terminals and potentially short circuit to each other or to ground during a short-circuit condition. NEC 376.56 (B)(1) requires that a power distribution block installed on the line side of the service be marked “suitable for use on the line side of service equipment.” Power distribution blocks marked as such are evaluated without an overcurrent protective device and are subjected to the short circuit current for a duration of no less than three electrical cycles.

Power distribution blocks are Certified (Listed) under the UL product category Power Distribution Blocks (QPQS). The UL guide information and Certifications (Listings) can be located on UL Product Spec at www.ul.com/productspec by entering QPQS at the Category Code search.

The guide information for Power Distribution Blocks (QPQS) states that they are intended to be used on the line or load side of service equipment. Power distribution blocks that have been investigated for use on either the line side or the load side of service equipment are marked “Suitable for Use on the Line Side of Service Equipment” (or equivalent). Power distribution blocks are considered suitable for use on circuits having available fault current not greater than 10,000 rms symmetrical amps, unless marked with a larger value. Power distribution blocks rated greater than 10,000 A are marked “Short-Circuit Current Rating” together with the value of the rating and the maximum voltage. A power distribution block only intended for use on the load side of service equipment may additionally be marked to communicate the need to use an overcurrent protective device (fuse or circuit breaker) ahead of the power distribution block. In addition, power distribution blocks are marked to indicate that they are for use in specific enclosures (identified by either catalog number or specific dimensional information).

Wire connectors are Certified (Listed) under the UL product category Soldering Lugs and Wire Connectors (ZMVV). The UL guide information and Certifications (Listings) can be located on UL Product Spec at www.ul.com/productspec and enter ZMVV at the Category Code search.