SCCR of an enclosure air conditioner with no marked rating

Yes: power the air conditioner through a control transformer with current-limiting fuses on its primary, and a 65 kA panel can carry a unit that has no short-circuit current rating of its own. Two 8 A dual-element time-delay Class CC fuses ahead of a 2 kVA transformer put a 200 kA interrupting rating on the branch. The transformer then drops the fault current that reaches the unit to about 145 A. The branch stops dragging the panel mark down, and the panel keeps the SCCR the other components earn. The worked example below assumes those sizes and a 6 % transformer impedance; where your nameplate differs, put in your own figures and keep the method.

Why the transformer fixes the branch, not the unit

A panel's short-circuit current rating is the lowest rating on any power path that can see the supply fault current, the same weakest-link logic that governs panelboard SCCR and series ratings. Every component in this panel is rated at the bus figure except the air conditioner. UL 508A assigns an unmarked power-circuit component a default of 5 kA, so the unmarked unit alone would pull the whole panel down to a 5 kA mark. You cannot raise a factory rating by adding parts. You can change what the unit ever sees.

The transformer splits the branch in two. The primary side keeps the full fault current, so its protection carries the burden: Class CC fuses are catalog-rated 200 kA interrupting at 600 V AC, in sizes from 0.1 A to 30 A, and 600 V covers the 380 V supply. The secondary side delivers only what the winding's impedance lets through, a few hundred amps on a transformer this size. The unit itself hangs outside the enclosure, but the branch that feeds it starts inside, and that branch is what the calculation covers.

Panel builders use the same move wherever a transformer sits inside a rated enclosure; the arithmetic behind panel SCCR with a transformer inside the enclosure follows the same rules.

What you need

Read the impedance off the transformer nameplate before you lean on any secondary number; the example that follows assumes 6 %.

Steps

1. Ask the air conditioner maker for a short-circuit current rating first. Some makers publish combination ratings, the way a controller is labeled 100 kA SCCR when protected by Class J fuses of 60 A or less. A published combination retires the transformer entirely.
2. Confirm the available fault current at the supply and the mark the panel needs: 65 kA in this example.
3. Size the transformer from the load in VA and round up to the next standard size.
4. Fit one fuse in each hot line of the single-phase primary, in rejection holders. A line-to-line feed at the supply voltage makes both lines hot, so the pair is not optional.
5. Compute the secondary fault current: secondary full-load current divided by the per-unit impedance from the nameplate.
6. Fit the secondary fuse at the transformer. The primary pair already provides short-circuit protection; the secondary fuse adds overload protection for the winding and lets you clear a unit fault without opening the panel.
7. Run the branch to the cutout in the side wall and mount the unit through it, the way a side-mounted cabinet air conditioner sits on any enclosure. Put the opening on the drawing when you order a custom enclosure cut to your layout.
8. Energize and start the compressor several times. The primary fuses must hold through every start.

The worked example: the full bus at the door, 5 A at the unit

The example panel is 380 V, three-phase, 800 A, with every marked component rated 65 kA. The side unit is 230 V, single-phase, 5 A, 8,000 BTU/h, with no rating on its nameplate.

• Load: 230 V × 5 A = 1,150 VA.
• Transformer: next standard size up, 2 kVA, about 1.7 times the load.
• Primary full-load current: 2,000 VA ÷ 380 V = 5.3 A.
• Primary fuses: 5.3 A × 1.5 = 8.0 A, so an 8 A dual-element time-delay Class CC pair.
• Secondary full-load current: 2,000 VA ÷ 230 V = 8.7 A.
• Secondary fault current at 6 % impedance: 8.7 A ÷ 0.06 = 145 A, on the infinite-bus assumption, which is the conservative case.
• Sensitivity: the same formula gives 290 A at 3 % impedance and 87 A at 10 %.
• Share of the bus: 145 A is about 0.2 % of the 65,000 A the supply can deliver.
• Branch contribution: the primary fuses stand at 200 kA, above the 65 kA supply, so the branch never becomes the weak link and the panel holds its full mark.

The transformer also adds its own loss inside the enclosure: dry-type units waste 1 to 3 % of rating as heat, so 20 to 60 W here, worth carrying into the transformer heat loss inside a sealed enclosure before you close the door.

Checks when done

• The nameplate carries the short-circuit current rating, 65 kA here, because NEC Article 409 requires the finished panel to be marked with it.
• The fuses you installed read dual-element time-delay Class CC and sit in the rejection clips.
• The impedance on the transformer nameplate matches the figure in your calculation.
• Ten compressor starts, primary fuses intact.
• The calculation sheet showing the transformer branch travels with the panel documents.

Mistakes that cost the rating

• One fuse instead of two leaves the second hot line unprotected until the upstream feeder clears.
• A plain fast-acting fuse swapped into a Class CC clip opens on the first start; the rejection clip exists because that swap keeps happening.
• A direct feed from an ordinary branch breaker inherits the 5 kA unmarked default and pulls the panel mark down with it. The fuse or breaker protection for an enclosure exhaust fan trade-off is its own decision for accessories like this.
• Let-through tables do not lift an unmarked unit by themselves. UL 508A accepts that method only with a combination the maker has tested and published; arithmetic for machine SCCR with current-limiting fuses covers the cases where it applies.
• Every penetration in the wall, cutout, sleeve or fastener, has to preserve the enclosure's own rating. What a wall penetration does to an enclosure's rating is its own subject, and what carries over between an enclosure certification and the panel's is another.

The decision by case

• The maker publishes a rating with a named fuse class: feed the unit through exactly that fuse class and mark to the published figure; the transformer stays out of the panel.
• The unit carries no rating and the panel carries an SCCR mark: build the branch as worked above, a 2 kVA transformer, an 8 A dual-element time-delay Class CC pair and a 10 A secondary fuse, and the panel holds its rating.
• The supply delivers 5 kA or less: UL 508A's unmarked default already covers the unit, so a direct feed stands with the calculation on file.
• The panel is built to IEC 61439-1 on a 50 Hz site: the same fuse physics apply, and the assembly carries a conditional short-circuit current rating declared together with its protective device.

On a 65 kA bus with an unmarked unit, the transformer-and-fuse branch answers the marking requirement with catalog parts and holds the number.

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.

Questions