A Class RK5 fuse is only as dependable as the hardware that holds it. Specifiers usually spend their time on the fuse itself, with its ampere rating, time-delay behavior, and interrupting rating, while the holder, fuse block, or disconnect box is treated as an afterthought. That is where many field problems begin.
At HIITIO, we manufacture UL Listed HCNR Class RK5 fuses alongside HVAC disconnect boxes, so we see both sides of the same installation. This guide is written for engineers, installers, and buyers who need to pair RK5 fuses with the right hardware. It explains what a rejection feature does, how to choose hardware for a Class R fuse, and which installation mistakes are worth catching before they reach the job site.
Why the Holder Matters as Much as the Fuse
Class R fuses (UL 248-12) are current-limiting and non-renewable, with a 200,000 A interrupting rating, according to a Mersen technical tip sheet on Class H fuses and safety disconnect switches. Class H fuses, by contrast, are not current-limiting and can safely interrupt only up to 10,000 A. Mersen also notes that Class K and Class R fuses share the physical dimensions of Class H, which makes upgrades easy but also makes substitution easy.
That shared size is the reason “R” stands for rejection. As described in a rejection fuse holder patent, a Class R fuse has a groove around one ferrule, and a rejection-type holder has an element that fits into that groove. A Class H fuse has no groove, so the same element blocks it. In other words, the fuse carries the key and the holder carries the lock. Without a rejection-type holder, nothing stops a lower-rated fuse from being installed at the next replacement.
Standards exist for the hardware side too. UL 4248-12 covers fuseholders intended for Class R fuses, and its general-requirements counterpart, CSA C22.2 No. 4248.1:22, addresses fuseholders for the UL 248 fuse classes along with accessories such as covers, indicators, and adapters.
Class H, RK5, and RK1 at a Glance
Table 1. Fuse classes that fit the same clip footprint
| Characteristic | Class H (non-renewable) | Class R, RK5 | Class R, RK1 |
|---|---|---|---|
| UL 248 part | 248-6 | 248-12 | 248-12 |
| Current-limiting | No | Yes | Yes |
| Interrupting rating | Up to 10,000 A | 200,000 A | 200,000 A |
| Same physical size as Class H | n/a | Yes | Yes |
| Grooved ferrule for rejection holders | No | Yes | Yes |
Sources: Mersen tip sheet; UL 248 part numbers as listed in HIITIO’s published fuse classification table; patent US4775338 for the grooved-ferrule description.
Mersen reports that RK5 and RK1 fuses share the same voltage and interrupting ratings and are physically identical, but RK1 is roughly 80% more current-limiting. For equipment protection and arc-flash energy, that difference can matter, so the two classes should not be treated as interchangeable unless the equipment manufacturer allows it.
Fuse Holder, Fuse Block, or Disconnect? Sorting Out the Terms
The vocabulary overlaps in catalogs, so it helps to separate the roles:
- Fuse holder or fuse block: the clips and base that physically retain the fuse. For Class R fuses, look for a rejection-type version. Littelfuse’s LFR fuse blocks illustrate the point: the manufacturer states that its Class H and Class R blocks are dimensionally the same, but the Class R versions add a rejection feature that admits only Class R fuses.
- Fusible disconnect or safety switch: a switch with fuse clips built in. Many of these ship accepting Class H-dimension fuses by default, and manufacturers offer kits to convert them to accept Class R or Class J only, as Mersen’s tip sheet explains.
- Pull-out disconnect: a visible disconnecting means common in HVAC work. The user removes a pull-out block to break the circuit, and fused versions carry fuses in that block.
VIEW HIITIO PRODUCT SERIES
A Practical Selection Checklist
Work through these points in order when specifying RK5 hardware:
- Confirm the class. Decide between RK5 and RK1 based on the let-through you need and what the equipment manufacturer permits.
- Choose a rejection-type holder marked for Class R fuses. UL 4248-12 is the relevant holder standard. Where a switch ships in a Class H configuration, use the manufacturer’s Class R conversion kit.
- Match voltage and ampere size. The HCNR series uses two body sizes: 10–30 A fuses measure about 50.8 mm × 14.3 mm, and 35–60 A fuses measure about 76.2 mm × 20.6 mm. A holder must match the ampere size, not just the class.
- Check the DC rating if DC is present. HCNR fuses are rated 250 Vac and 150 Vdc, with a 50 kA DC interrupting rating. Confirm that the circuit voltage falls inside those limits.
- Look at the whole assembly. The short-circuit capability of an installation is limited by its lowest-rated component, so check the holder, switch, and enclosure ratings as well as the fuse.
- Verify terminals and torque. Conductor size and tightening torque are published for a reason. For example, the HIITIO HDS1 disconnect lists 35 lb-in for #14–10 AWG conductors and higher values for larger wire.
- Match the enclosure to the environment. Outdoor air-conditioning disconnects need a suitable weather rating, such as the NEMA 3R enclosure on the HDS1.
Six Common Installation Mistakes
1. Leaving a disconnect in its default Class H configuration. If the switch accepts any Class H-dimension fuse, a future replacement can quietly downgrade the circuit from a 200,000 A current-limiting fuse to a 10,000 A one. A rejection kit or a rejection-type block closes that gap.
2. Using renewable elements or stacking them. Mersen cautions that stacking two elements can double the effective rating of the fuse, which means more current is needed to open it under fault. NEC 240.60(D), as summarized by Mersen, limits renewable Class H fuses to replacement in existing installations with no sign of overfusing or tampering.
3. Upsizing to stop nuisance openings. According to HIITIO’s product literature, an RK5 overload element is designed to hold five times its rated current for at least ten seconds, which is why it tolerates normal motor starting. If a correctly sized RK5 keeps opening, look for a loose connection, a mechanical problem on the load, or a mismatched rating rather than moving to a larger fuse.
4. Treating RK5 and RK1 as interchangeable. Same size does not mean same let-through. Substitute only when the equipment listing and coordination study allow it.
5. Overlooking DC limits. An AC-rated holder or fuse should not be assumed to suit a DC circuit. Read the Vdc and DC interrupting ratings printed on the product.
6. Ignoring terminal specifications. Undersized conductors, wrong terminal types, or missed torque values create hot spots at the clips and lugs, which can look like a faulty fuse when the problem is the connection.
Where HVAC Disconnect Boxes Fit
In air-conditioning and heat-pump installations, the disconnect sits between the load center and the outdoor unit. A pull-out design gives service technicians a visible break in the circuit, and fused versions add overcurrent protection at the unit.
HIITIO’s HVAC/R range, available through the HVAC and refrigeration solution page, includes several options. The table below summarizes what the product pages state.
Table 2. HIITIO products referenced in this guide
| Product | Role | Key ratings and features | Standards listed |
|---|---|---|---|
| HCNR Class RK5 fuse | Time-delay, current-limiting fuse | 250 Vac / 150 Vdc; 10–60 A; 200 kA RMS sym. (AC), 50 kA (DC) | UL Listed, UL 248-12, Class RK-5, Guide JDDZ, File E537893 |
| HDS1 air conditioning disconnect | Outdoor AC disconnect | Fused 30 A and 60 A, non-fused 60 A; 240 V, 1-phase, 2-pole, 3-wire; NEMA 3R; padlockable door; copper-rated lugs | UL 1429 listed; RoHS compliant |
| PDS / PCDS disconnect box | Pull-out AC disconnect (PDS metal; PCDS plastic) | 30 A and 60 A, fused or non-fused; 240 VAC, 1-phase, 3-wire | UL 1429; CSA C22.2 No. 29-15 (UL certificate E255965 on the PDS page) |
| PDSB-60AN pull-out disconnect | 60 A pull-out for HVAC/R | 60 A, 10 HP max, 180 A locked rotor, 240 V, 60 Hz, 1-phase; box lugs on line side | UL E255965; UL 1429; CSA C22.2 No. 29-15 |
One practical note: the product pages for the fused disconnects do not state which fuse class they accept. Before specifying a fused model, check the downloadable specification sheet or ask us to confirm that the fuse class and clip configuration match your RK5 selection.
HIITIO HCNR Class RK5 Fuses at a Glance
The HCNR series is built for circuits that see temporary motor overloads and surge currents. The product page lists:
- Ratings: 250 Vac / 150 Vdc, with ampere options of 10, 12, 15, 20, 25, 30, 35, 40, 45, 50, and 60 A.
- Interrupting capacity: 200 kA RMS symmetrical on AC and 50 kA on DC.
- Listing: UL Listed to UL 248-12, Class RK-5.
- Typical applications: service entrance switches, motor control center mains and motor branch circuits, switchboard mains and feeders, and general-purpose circuits.
For circuits that also include contactors, our definite purpose contactors are designed for compressor and fan control in the same HVAC/R systems. If you are also reviewing fuse sizing in other systems, our article on fuse sizing mistakes in solar inverter systems covers a related set of pitfalls.
Get RK5 Protection Specified Correctly the First Time
Matching a fuse to its holder, switch, and enclosure is easier when the supplier can look at the whole circuit with you. HIITIO supplies UL Listed HCNR Class RK5 fuses from 10 to 60 A, along with HVAC disconnect boxes in 30 A and 60 A ratings, including fused and non-fused versions. Send us your voltage, current, fuse class, and enclosure requirements, and our technical team can help you check that the pieces fit together and that the specification sheets support your project. Whether you are an HVAC distributor, an equipment manufacturer, or a contractor planning a retrofit, contact HIITIO or browse the RK5 fuse range to request a quote or samples for evaluation.
FAQ
Can I install an RK5 fuse in a Class H fuse holder?
Physically, yes, because Class R fuses share the dimensions of Class H fuses. The limitation is that a non-rejection holder will also accept Class H fuses, which have a much lower interrupting rating. A rejection-type holder or a manufacturer’s conversion kit keeps the circuit locked to Class R replacements.
What is the difference between RK5 and RK1 fuses?
Both are Class R fuses under UL 248-12, with the same voltage and interrupting ratings and the same physical size. According to Mersen, RK1 is about 80% more current-limiting than RK5, which can reduce equipment stress and arc-flash energy.
Does the rejection feature sit on the fuse or the holder?
Both. The fuse has a groove around one ferrule, and the rejection-type holder has an element that engages that groove while blocking fuses without it.
Are HIITIO HCNR fuses suitable for DC circuits?
The HCNR product page lists a 150 Vdc rating and a 50 kA DC interrupting rating, alongside 250 Vac and 200 kA RMS symmetrical on AC. Confirm that your circuit voltage and available fault current fall within these values before specifying.
How do I know which fuse class a fused HVAC disconnect accepts?
Check the product marking and the manufacturer’s specification sheet. For HIITIO disconnects, use the downloadable specification on each product page or contact our team to confirm the fuse type for your selected model.
