Short answer: mark 10 kA. You rate the panel section by section under UL 508A Supplement SB, and the section downstream of an internal 120 V transformer is judged against the fault current that transformer can deliver — about 400 A — not against the 3 kA at the supply. GFCI receptacles rated 2 kA sit behind 400 A of let-through and pass with five times margin, so the mains side sets the panel at its weakest link. The method, the arithmetic and the marks follow.
What does the marked SCCR tell the installer?
It is a ceiling, not a goal: a panel with that mark may be connected only where the supply delivers 10,000 A or less, so a 3 kA service leaves 7 kA of headroom.
UL 508A requires the finished panel to carry the SCCR on its nameplate, and NEC 409.2 puts your assembly in scope once it holds two or more power or control circuit components. The NEC also draws the installation line: the fault current available at the connection point must stay at or below the marked rating. One more mark rides along — NEC 409.110 requires the enclosure type number on the panel, and that one comes from the box, not the wiring. The empty enclosure carries no SCCR; the panel you build inside it earns one. For what a type rating is worth next to a listing, read the UL 50E and NEMA 250 listing guide.
Do you mark one SCCR or a rating for each section?
One number goes on the nameplate, and you earn it section by section: the marked value is the lowest section rating left after current limiting, which is the mains-side value in the case above.
Supplement SB sets out the walk in four steps:
| Step | What you do |
|---|---|
| 1 | Walk each branch circuit and record its lowest component rating, or the default UL 508A gives that part type |
| 2 | Apply current-limiting devices — an internal transformer or a current-limiting fuse — wherever their let-through stays below every downstream rating |
| 3 | Take the lowest result across the feeder and branch circuits as the panel rating |
| 4 | Mark that value on the nameplate |
Step 2 is where panels are won. A 5 kA contactor by itself makes a 5 kA branch; put a current-limiting fuse ahead of it that holds the peak under 5 kA, and the branch climbs to the fuse's rating. Calculating panel SCCR with the UL 508A Supplement SB method works the same steps with fuller examples.
Does a transformer inside the enclosure split the analysis?
Yes, it splits the panel: everything on the secondary is checked against the fault current the transformer itself can deliver, so 400 A of secondary capability covers every component rated 2 kA or more.
Supplement SB treats a transformer as a current-limiting device. Its let-through is the available short-circuit current at the secondary terminals, and the pass condition is one comparison: that current below the lowest SCCR of anything fed from the secondary. The 24 V DC supply, the relays and your outlets behind a control transformer stop limiting the panel the moment the transformer outrates them. The transformer brings heat as well, which is its own calculation — see transformer enclosure cooling assessment for panel builders.
How do you calculate the fault current at the transformer secondary?
Divide the secondary full-load current by the transformer impedance: a 1 kVA unit with a 120 V secondary and 2 % impedance works out to 1000 / 120 = 8.3 A, then 8.3 / 0.02 = 415 A.
Worked example, with the numbers a reviewer would trace:
| Item | Value | Basis |
|---|---|---|
| Available at the supply | 3 kA | utility figure at the enclosure lugs |
| Lowest mains-side component | 10 kA | nameplates of disconnect, fuses, contactors, terminals |
| Internal transformer | 1 kVA, 480/120 V, 2 % Z | 1000 VA / 120 V = 8.3 A full load |
| Secondary fault current | 415 A | 8.3 A / 0.02 |
| GFCI receptacles on the secondary | 2 kA (2000 A) | marked on the device |
| Panel SCCR to mark | 10 kA | lowest section after limiting |
Two comparisons close the job. On the secondary side you set 415 A against a 2000 A rating, a margin of nearly five. At the installation you set 3 kA available against the mark, with 7 kA to spare.
Impedance moves the secondary number in a straight line. Halve it to 1 % and the same 1 kVA unit delivers 830 A, still under the outlets. Scale up to 5 kVA at 120 V and the full-load current is 41.7 A. At 5 % impedance the secondary delivers 41.7 / 0.05 = 833 A, still fine; at 2 % it delivers 2080 A, past the 2000 A rating, and the section fails. The 480 V source impedance barely registers for a small transformer, because its own 2 % dominates.
Can a current-limiting fuse raise the rating instead?
Yes: put one on the feeder and the downstream section climbs to the fuse's rating whenever its peak let-through at your available fault current, read from the maker's chart, stays below every downstream rating.
Class J and Class CC fuses in the main disconnect are the usual move, and these classes interrupt far more fault current than any service you will meet. What decides the comparison is peak let-through at your available fault current set against the weakest downstream rating. That peak rises with the ampere rating of the fuse — a 3 A Class CC limits far harder than a 60 A — so the small sizes do the rescuing. When even the smallest fuse's chart cannot get under a very low component rating, put a small transformer ahead of that branch instead. Lock out and prove dead before you change fuses or read labels inside a live panel.
Do receptacles on the transformer secondary count against the panel?
They count through their supply path: fed from the internal transformer, the 2 kA receptacles ride about 400 A and never limit the panel, while the same receptacles wired to the line side would cap the assembly at 2 kA.
The method follows how a branch is fed, not what the component is. Receptacles inside a control enclosure bring their own wiring rules for GFCI protection and neutral landing — wiring a 120 V receptacle inside a control enclosure to code covers them. When one feed serves everything, one power input for multiple devices inside an enclosure shows the distribution layout that keeps your sections straight.
What goes on the nameplate and the enclosure order?
Two marks share the panel: the SCCR under UL 508A and the enclosure type number that NEC 409.110 puts there, so give the box a nameplate area big enough for both.
The enclosure never sets the SCCR. Our boxes are rated NEMA 4X / IP66 by the manufacturer and not UL listed; that rating lives in the components you mount and the fuses ahead of them. Order the box around that panel: cutouts for the disconnect handle and any viewing window, a gland plate or hub for the mains, backplate room for the transformer, and wire bending space at the big terminals — the tables sit in the how to size an electrical enclosure guide. Lay the openings out on the custom stainless enclosure page or start from the enclosure selector; custom work is delivered in 10 business days, counted from your approval of the drawing.
Enclosures for this job
Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 60 × 36 × 16 in | 304 stainless | 60 x 36 x 16 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 48 × 36 × 16 in | 304 stainless | 48 x 36 x 16 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 60 × 36 × 12 in | 304 stainless | 60 x 36 x 12 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Does the marked SCCR have to be higher than the available fault current?
Yes. The mark is a ceiling for the site: a panel marked 10 kA connects anywhere the supply delivers that value or less. At 3 kA available you hold 7 kA of margin, so a service upgrade that adds fault current still has room to grow before the panel marking fails.
Can a Class CC fuse lift a branch circuit above 5 kA?
Only above its current-limiting threshold, and only when the maker's let-through chart puts the peak, at your available fault current, below the weakest downstream rating. A 3 A Class CC limits far harder than a 60 A, so small sizes do the rescuing. The fuse itself interrupts 200 kA, so surviving the service is never the question.
How do you feed a component rated only 0.2 kA?
Put a small transformer in front of it. A 100 VA transformer with a 120 V secondary and 5 % impedance delivers 0.83 / 0.05, about 17 A of fault current, ten times under a 200 A rating. That is simpler than hunting a fuse whose let-through chart clears 200 A at a useful ampere size.
What SCCR does an unmarked component get?
The UL 508A default table assigns low values by component type: about 5 kA for contactors, overloads and disconnect switches, 10 kA for terminal blocks and fuse holders. A rating marked on the device replaces the default, so a breaker marked 65 kA at 480 V counts as 65 kA; the unmarked parts are what quietly cap a panel at 5 kA.
Can one panel carry two SCCR values for two voltages?
Yes, and dual-voltage panels do it routinely: the nameplate can read 10 kA at 480 V and 5 kA at 240 V when the component ratings or fuse let-through differ with voltage. The installer applies the line that matches the connected supply, and each line stands on its own analysis.
