Short answer: the label governs, and a series rating never lifts a bus bracing number. A panelboard carries a short-circuit current rating (SCCR), not an interrupting rating, and that SCCR is the lowest of the bracing mark and every breaker installed, unless the maker's label itself lists a tested series combination. So a panel marked 65 kA stays a 65 kA panel even when its branch breakers are half of a listed 100 kA pair, and it cannot sit on a bus with 70 kA available. You compare three things: the SCCR on the panel label, the fault current the study gives at the panel line terminals, and the exact devices the maker's series table names. This article takes each in turn and ends with the decision for every case.
A panelboard has an SCCR, not an AIC
AIC stands for Amps Interrupting Capacity, the trade shorthand for interrupting rating, and it belongs to devices that open contacts: breakers and fuses. A panelboard never opens anything. Its bus, bracing and supports only have to hold the interior together for the instant a fault lasts — about one half-cycle, 8.3 ms at 60 Hz — until a breaker or an upstream device clears it. Equipment that withstands rather than interrupts gets rated in short-circuit current instead: panelboards, switchboards, motor control centers and industrial control panels all carry an SCCR. The kA figure is tested under UL 67 for panelboards and UL 508A for industrial control panels.
The figure people quote for a panelboard "enclosure" is therefore the short-circuit rating of the assembled interior: bus size, bar spacing, bracing and supports, tested inside that box. The sheet metal answers a different question entirely. Its Type rating covers water, dust, ice and corrosion, and says nothing about fault withstand.
One marking rule matters on the control-panel side. An industrial control panel containing at least one power circuit must carry its SCCR on the nameplate, and the NEC requires that marking; a panel with only 24 V control circuits needs none. What a UL 508A label covers, and when an inspector asks for it, is the subject of the UL 50E and UL 508A listing guide, and whether a shop-built panel needs the label at all is covered in the article on in-house panels and UL labels.
What the SCCR label is made of
Read a panelboard label and one of three logics applies.
Fully rated. The label states that the SCCR is limited to the lowest interrupting rating of the devices installed. Fit branch breakers a class below the bus and the panel takes their figure, whatever the bus could take. Common molded-case interrupting ratings run 10 kA, 22 kA, 42 kA and 65 kA, with mains reaching 100 kA and 150 kA at the larger frames — standard catalog values.
Bracing-limited. Some interiors are marked with a bracing figure that binds even when higher-rated devices are fitted. In practice this rarely bites: panelboard bus is continuously braced, catalog lines commonly rate it to 100 kA as standard, and one line's catalog quotes 200 kA. On most panels the devices set the number and bracing sits above them.
Series rated. The label names a tested combination — a specific line-side breaker or fused switch and specific branch devices — with a combined rating above the branch devices' own interrupting ratings. The upstream device limits the current in the first half-cycle, so a branch breaker can legally clear a fault on a bus several times its own mark. Two limits apply. The combination rating cannot exceed the line-side device's own interrupting rating, and the combination holds only for the exact device types the maker's series table names. Makers publish these tables; some sell interiors with an integrated equipment rating, the same idea marked on the equipment itself.
Why a series rating never lifts the bracing number
The series trick works on devices because devices open contacts. An upstream current-limiting breaker or fuse collapses the peak let-through in the first half-cycle — within those 8.3 ms at 60 Hz — so the downstream breaker clears a fault without ever seeing the full prospective wave. Bus bracing is passive metal. Nothing opens, nothing clears, and there is no partner device to share the stress, so its rating is a withstand figure at the full prospective current and no series column exists for it.
That makes the panel rating a minimum on one line: the SCCR is the lowest marked value among the bus bracing, every installed device, and any listed combination the label itself names. You cannot compute a higher number from a device series table plus a bracing mark. When a panel has to survive more than its label says, the fix is a new mark from the maker — normally a paperwork change, because bracing at the catalog level already covers most jobs — never arithmetic on your side.
A 70 kA bus against three labels
Example inputs, each one an assumption you replace with your own label and study: a 480Y/277 V, 400 A panelboard; interior bracing marked 65 kA RMS symmetrical; branch breakers rated 42 kA interrupting; an upstream main that is a listed series partner, combination rated 100 kA; and a short-circuit study giving 70 kA RMS symmetrical at the panel line terminals.
Label limited to the lowest installed breaker, 42 kA. The SCCR is 42 kA. The bus is 1.67 times the mark (70 ÷ 42 = 1.67), so this panel is out here, whatever the upstream pair can do together.
Interior marked with the 100 kA combination. The SCCR is 100 kA, valid with that main and those branch types. The bus uses 70 % of the mark (70 ÷ 100 = 0.70) and leaves 30 kA spare. This is the answer the series rating exists to buy, and it only exists once the label says so.
Bracing-limited label, 65 kA. The SCCR is 65 kA, 5 kA short of the bus, about 7.7 % over the mark (5 ÷ 65 = 0.077). Only an interior marked above the study figure fixes this, and most panelboard lines offer such an interior as a catalog choice.
One check belongs before all three: a combination needs a line-side main itself rated at least at the combination mark, because a combination cannot outrate the upstream device.
| What the label says | Panel SCCR | Good at 70 kA available? | What you do |
|---|---|---|---|
| "SCCR limited to the lowest installed breaker", 42 kA branches | 42 kA | No, 1.67x over | Fit 65 kA branch breakers, or have the maker list and mark the interior with the series pair |
| Bracing marked 65 kA, no series mark | 65 kA | No, 5 kA short | Order an interior marked above 70 kA; most lines offer 100 kA as standard |
| Tested series combination marked on the panel | 100 kA | Yes, 30 kA spare | Record the exact device types; swapping either device voids the mark |
How to fix a panel that falls short
Ask what the label actually says before you reorder anything. Panel labels and order records drift apart, and on some lines the same interior carries a higher mark once the maker records the series combination on the order. Get the marked SCCR confirmed and, where a series rating is wanted, get the combination named on the order.
Three levers raise a short panel:
- Raise the lowest device. Swap the 42 kA branches for 65 kA frames and the label reads the higher interrupting rating — enough only where the study comes back at or below it at the terminals.
- Buy the series mark. Order the interior listed and marked with the maker's tested combination, both device types on the label; this is the only way branch breakers below the bus figure live on it.
- Lower the available current. Put impedance between the source and the panel — longer secondary conductors, or a transformer — and re-run the study. A 1000 kVA, 480 V transformer on a 5.75 % impedance nameplate puts about 21 kA at its own terminals; a 75 ft run of secondary conductor sized for the 1203 A full-load current pulls a panel well below a 70 kA switchboard bus. The arithmetic for a transformer inside the cabinet is in the article on panel SCCR with an internal transformer.
Where no listed, marked combination fits the job, the NEC still allows a series rating when the selection is made under the engineering supervision of a licensed professional engineer; NEC series-rating combinations reference walks through that path. De-energize, lock out and verify absence of voltage before swapping devices in an energized panel.
Close the loop on paper: order note, panel label and single-line diagram should all name the same SCCR, and for a series rating, the same two device types.
What the enclosure Type rating has to do with it
None of these numbers comes from the box. The Type rating — NEMA 250 and UL 50E territory — covers water, dust, ice and corrosion, and NEC Table 110.28 lists which type suits which location; NEC 409.110 puts that type number on the panel marking, right beside the SCCR. Type 4X says the panel survives washdown; the SCCR says what the bus holds together at. They travel on the same nameplate and answer different questions.
That is why a washdown stainless box rated NEMA 4X / IP66 by the manufacturer, like the 316L wall-mount enclosures we build, carries no kA figure; the fault rating belongs to the interior you fit inside it. Ratings are manufacturer ratings, not UL listed. Picking the Type for a wet or corrosive room is its own job, covered in the panelboard enclosure rating article for a pump room.
What to put on the order
Four lines on the order make the label problem disappear:
- Interior SCCR at or above the study figure, stated on the panel label rather than quoted only in catalog pages.
- Series combination by exact device type, line-side and branch, copied from the maker's series table, with the combination rating at or above available current and the line-side device rated at or above the combination.
- Type rating for the room: 4X for washdown and corrosion, chosen per NEC Table 110.28. The NEMA enclosure types guide sorts the types, and a custom enclosure carries the Type you name while the listed interior carries the kA.
- One paper trail: order, label and single-line showing the same SCCR, so the inspector and the next engineer read the same number.
Then the answer to the opening question stays short. The panel is good for exactly what its label says. A series rating rides on that label or not at all, so a bracing mark stays put in any series table, and a 70 kA bus needs a label that reads 70 kA or more.
Catalog dimensions to compare
Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Does a 100 kA series rating lift a 65 kA bus bracing limit?
No. Bracing is a withstand figure for the bus and its supports at full prospective current, and passive metal has nothing to series-rate. The panel stays at the bracing mark, so it is out on a bus with 70 kA available even when the breakers inside are half of a listed 100 kA pair. Only an interior the maker lists and marks at a higher rating moves the number.
Is AIC the same as SCCR?
No. Interrupting rating, which the shorthand AIC points to, is how much fault current a device can safely clear; common molded-case values run 10 kA, 22 kA, 42 kA and 65 kA. SCCR is how much fault current equipment that never opens, such as a panelboard, holds together until something else clears. Breakers carry interrupting ratings; panelboards carry an SCCR.
Can a series rating span two separate enclosures?
Yes, where the maker's series table lists the exact line-side and load-side devices as a tested pair and honors any conductor length it states between them; a switchboard main protecting a downstream panelboard is a common listed combination. The combination rating cannot exceed the line-side device's own interrupting rating, and the equipment carries the mark naming the combination.
What fault current does a 1000 kVA transformer deliver at 480 V?
About 21 kA at its terminals on an infinite bus, taking a 5.75 % impedance nameplate, a common value on 480 V dry-type units. Full-load current is 1000 kVA divided by 1.732 times 480 V, about 1203 A, and 1203 divided by 0.0575 gives roughly 20.9 kA RMS symmetrical. Secondary conductors and upstream impedance pull the figure down at the panel, so the study number rules.
