Short-Circuit Current Rating (SCCR)

What the rating means

When a bolted fault occurs downstream of a panel, the current that flows for the few cycles before a protective device clears it is limited only by the impedance of the utility, the transformer, and the conductors. At a small site that might be a few thousand amperes. Close to a large transformer it can be fifty thousand or more. Every component in the power path of the panel has to survive that current for those cycles without exploding, arcing to the enclosure, or welding closed.

The short-circuit current rating is the assembly-level statement of what the panel can survive. It is expressed in kiloamperes, symmetrical RMS, at a voltage. A panel marked 10 kA at 480 V has been evaluated to withstand a 10,000 A fault at that voltage. Installed where the available fault current is 22,000 A, it is a code violation and a hazard, regardless of what the main breaker is rated to interrupt.

What the code requires

NEC Article 409 covers industrial control panels. Section 409.110 requires the panel to be marked with its short-circuit current rating, based on one of the listed methods, of which UL 508A Supplement SB is the one panel shops use. Section 110.10 requires that the overcurrent protection, the impedance, and the component short-circuit current ratings be selected so that a fault can be cleared without extensive damage to the electrical equipment, which is the requirement that the panel rating be adequate for the installation.

Section 110.24 requires service equipment to be field marked with the maximum available fault current and the date it was calculated. That marking, or the calculation behind it, is where the number to compare against comes from. Industrial machinery panels are covered by NEC Article 670 and NFPA 79, which carry equivalent marking requirements.

How the rating is determined

UL 508A Supplement SB gives the procedure. Reduced to its essentials: identify every component in the power circuit, find the short-circuit current rating of each, and the assembly rating is the lowest of them. The power circuit is everything from the incoming terminals through the disconnect, the branch protection, the contactors, overload relays, drives, terminal blocks, and the feeders to the loads. Control circuit components fed through a control transformer or a limited supply are generally excluded.

1. 1

   List the power-circuit components.

   Disconnect, main overcurrent device, power distribution blocks, branch breakers or fuse holders, contactors, overload relays, soft starters, drives, power terminal blocks, and busbars.

2. 2

   Find each rating.

   From the component listing or datasheet. Many components carry a marked SCCR that depends on the type and size of the protective device ahead of them; a contactor might be 5 kA on its own and 65 kA when protected by a specified class of fuse.

3. 3

   Apply the default where nothing is marked.

   Supplement SB assigns default ratings to unmarked components by category. The defaults are low, in the range of a few kiloamperes for most categories, and a single unmarked terminal block can set the whole panel to that value.

4. 4

   Take the lowest.

   The assembly SCCR is the smallest rating in the list. It is common for a first pass to land at 5 kA because of one component.

5. 5

   Raise it where needed.

   Replace the limiting component with a higher-rated one, or protect it with a device it is listed in combination with. Current-limiting fuses and breakers let a downstream component be rated at the combination value rather than its standalone value, but only for the exact combinations that were tested.

Why the number is usually smaller than expected

People see a 65 kA breaker at the top of a panel and assume the panel is 65 kA. The breaker is; the panel is whatever the cheapest component in the power circuit is. Power distribution blocks, terminal blocks, and small drives are the usual culprits, and a general-purpose terminal block that was never evaluated for short circuit gets the default. A panel shop that does not do the calculation ships panels marked at the default, and a panel marked at the default is only legal at the small fraction of sites where the available fault current is that low.

Getting the available fault current

The panel rating is only meaningful against a number. The available fault current at the panel depends on the utility contribution, the transformer size and impedance, and the conductor length and size between the transformer and the panel. The utility will state its contribution at the service; the transformer nameplate gives the impedance; and a point-to-point calculation carries it down the feeders. Motors on the system contribute as well, briefly. The result is the number the panel rating must meet or exceed.

This calculation is engineering work and, where a stamp is required, it belongs to the engineer of record. A controls integrator should ask for it before designing the panel, because it is far cheaper to select 65 kA components at the start than to rebuild a 5 kA panel after inspection.