Understanding Fuse Classes in Bus Plugs for Safe Power Distribution

A fuse class is a standard category, defined in the UL 248 series of fuse standards, that fixes a fuse’s physical size and mounting, its voltage rating and its interrupting rating. A fusible bus plug is built around certain classes, so the class decides which fuses will seat in the plug’s clips and suit the circuit. The BRAH Electric fusible bus plugs covered in this guide accept Class H, R and J fuses, Class R, H and K fuses, or Class R fuses, depending on the busway family.

Fuse classes in bus plugs at a glance

• The class is the letter printed on the fuse label: H, J, K, R and others.
• Each class has its own UL 248 part. The class sets body size, voltage rating and interrupting rating.
• A plug accepts only the classes its record and nameplate name. Check them before you buy fuses.
• Class and amperage are separate. A 60A Class R fuse and a 60A Class J fuse are different parts.
• BRAH fuse clips in the table below are Class R rated and accept Class H, R and K fuses.

What a fuse class tells you

Fuse makers build to class standards so that a fuse from one brand fits a holder made for the same class by another brand. The class letter covers several things at once: the outline and length of the fuse body, how it mounts (ferrule ends or blades), the voltage it is rated for, the fault current it is rated to interrupt, and limits on how it performs when it opens.

That is why the class matters more than how a fuse looks. Two fuses of the same amperage can be close in size and still belong to different classes with different interrupting ratings. The class is printed on the fuse label together with the amperage, the voltage and, where it applies, the words time delay. Read the label and do not judge by size or color.

The class does not tell you the amperage to use or whether a time-delay fuse suits the load. Those are separate choices, covered step by step in the guide on how to choose bus plug fuses.

The four fuse classes named in BRAH fusible bus plug records

Four class letters appear in the fuse class field and descriptions of the fusible bus plugs in the BRAH catalogue. Each one has its own part in the UL 248 series, linked here so you can see what the standard covers.

Class R is the one class named in every family. Class H, K and R cartridge fuses of the same amperage and voltage share a body size, which is why a single fuse clip can hold all three. Class R fuses also carry a rejection feature that holders made only for Class R use to keep other classes out. Class J fuses are a different physical size from Class H and R fuses of the same amperage. If you plan to fit Class J fuses in a plug whose record names Class J, confirm the clip arrangement for that plug when you order: call (855) 355-2724 with the part number.

The interrupting rating differs from class to class. Take it from the fuse label or the fuse maker’s data sheet, and compare it with the fault current available at the plug. Do not assume that two classes that fit the same clip give the same protection.

Which BRAH fusible bus plugs accept which fuse classes

The table groups the fusible plugs by busway family. The fuse classes are taken from each family’s catalogue records. Every family in the table is listed as a three-phase plug at 30A, 60A and 100A, in 240V and 600V versions. The example column links to the 30A, 240V, 3-wire plug of each family.

Three patterns show up. The General Electric, Square D, Siemens Sentron and POW-R-WAY III plugs are listed for Class H, R and J fuses. The ITE / Siemens BD Series, XL-U Series and XL-X Series plugs are listed for Class R, H and K fuses. The POW-R-WAY I & II plugs, BRAH type BIP, are listed for Class R fuses.

The catalogue describes the Armor Clad (BEC), Spectra (BEB), Spectra Low-Amp (BEL) and BD Series (BES) plugs as UL Listed assemblies, complete with a UL Recognized internal switch and components. Each plug record also carries an AIC rating. Read it on the product page and compare it with the available fault current at the point where the plug connects to the busway. For anything not in the table, call (855) 355-2724 with the part number.

BRAH fuse clips by class and amperage

Fuse clips are the spring contacts that hold each end of the fuse. A clip that has lost tension or is burned makes a hot joint, so clips are often replaced when fuses are changed. The parts below are in the bus plug fuse clips sub-category. Every one is described as Class R rated and as accepting Class H, R and K fuses. They are field installable, mount with a single center screw (the screw is not included) and are described as substitutes for clips in bus plugs, disconnects, motor control centers and fusible panel mount switches.

Pick the clip by the amperage and voltage of the fuse position it serves, then by the mounting style. The two parts ending in BOS use a bottom-screw mount and are listed for BD Series type BOS plugs, at 100A and 200A. For anything not in the table, call (855) 355-2724 with the part number.

How to confirm the fuse class on an installed bus plug

• Read the plug label. The nameplate or the label inside the door states the plug type, voltage, amperage and the fuse classes it is marked for.
• Read the fuses that are fitted. The class letter is on each fuse label. Check all three; mixed classes in one plug are a sign that someone fitted what was on the shelf.
• Look up the catalogue number. Search the OEM number on the BRAH site. The record of the substitute plug states the fuse classes it accepts. For example, the record for BEB3610 lists Class H, R and J, and the record for BIP3610 lists Class R.
• Look at the clips. Clip size and spacing follow the fuse body. If the clips have been changed or adapted, treat the label as the reference and put the plug back to what it is marked for.

Mistakes that come from mixing fuse classes

Choosing by appearance

Classes that share a body size are not equal in interrupting rating. A fuse that drops into the clips can still be the wrong class for the fault current at that point of the busway.

Forcing or adapting a fuse

Bending clips, adding makeshift spacers or leaving a fuse loose in the clips gives a poor contact that heats up under load. Use a fuse of a class and size the plug is built to hold, and replace clips that no longer grip.

Different fuses on each phase

Use the same class, type and amperage on all three phases of a plug. A mixed set makes the plug behave differently from phase to phase during a fault or an overload.

Raising the amperage to stop fuses opening

Class and amperage are separate checks. A larger fuse of the right class still removes protection from the conductors and the load. Find the reason the fuses open before changing the rating.

What to check before you order a plug or clips

• Busway brand and family, and the plug type on the nameplate (AC, SB, SL, FVK, PQ, ITAP, P3F, BOS, UV, RV or a Sentron type).
• The fuse class you stock or plan to fit, compared with the classes in the plug record.
• Plug voltage, 240V or 600V, and plug amperage.
• 3-wire or 4-wire, and whether a ground is needed.
• For clips: amperage, voltage and mounting style, and whether the UL Listed version is required for your job.

BRAH Electric is an aftermarket manufacturer and is not an authorized distributor of the OEM brands named here. Parts carry a 2-year warranty from the date of invoice. In-stock orders placed before 3:00 PM Pacific ship the same business day. The full range is under bus plugs and parts, and the guide on fusible and non-fusible bus plugs explains the plug styles.

Safety when checking or changing fuses

Open the plug switch, then de-energize, lock out and tag out the circuit and test for absence of voltage before touching fuses or clips. The line side of a bus plug stays live while the busway is energized, even with the switch open. Fuse and clip work is for qualified electrical personnel following the site safety program.

Fuse class FAQs

What does fuse class mean?

It is the standard category a fuse is built and tested to. The class sets the fuse’s size and mounting, its voltage rating and its interrupting rating, so a fuse of a given class fits holders made for that class.

Which fuse classes do BRAH fusible bus plugs accept?

It depends on the busway family. The catalogue lists Class H, R and J for the GE, Square D, Siemens Sentron and POW-R-WAY III plugs, Class R, H and K for the ITE / Siemens BD, XL-U and XL-X plugs, and Class R for the POW-R-WAY I & II plugs. The table above gives each family.

Can I fit a Class H fuse where the plug is marked for Class R?

Only if the plug is also marked for Class H. Fit the classes named on the plug label and in its record, and check that the fuse’s interrupting rating covers the available fault current.

Do BRAH fuse clips take Class J fuses?

The fuse clips in the table are described as Class R rated and as accepting Class H, R and K fuses. For a Class J application, call (855) 355-2724 with the plug part number.

Where is the fuse class marked?

On the fuse label, next to the amperage and voltage, and on the plug nameplate or door label. If the plug label is unreadable, look up the catalogue number or send photos of the plug and the busway label to BRAH Electric.

References

• UL 248-1, Low-Voltage Fuses, Part 1: General Requirements: the general standard that the class-specific parts build on.
• UL 857, Busways: the UL standard for busways and their fittings, including plug-in devices.
• NFPA 70, National Electrical Code: the installation code that sets the rules for overcurrent protection.
• OSHA 29 CFR 1910.147: the control of hazardous energy (lockout/tagout).