How to Choose the Right Bus Plug Fuses for Safe Electrical Distribution

To choose a fuse for a fusible bus plug, match four things to the plug and the circuit: a fuse class the plug accepts, a voltage rating that matches the plug, an amperage that suits the circuit and does not exceed the plug rating, and a time-delay or fast-acting type that suits the load. The plug nameplate and its catalogue record give you the first three. The BRAH Electric fusible bus plugs listed below are 30A, 60A and 100A plugs in 240V and 600V versions, and each record names the fuse classes it accepts.

Quick answer: four checks for a bus plug fuse

• Class: use a class named on the plug label and in its record, such as Class R.
• Voltage: a 240V plug and a 600V plug are different parts and take different fuses.
• Amperage: size the fuse for the circuit, never above the amperage of the plug.
• Type: time-delay for loads with starting inrush, fast-acting for loads without it.

What the fuses in a bus plug protect

A fusible bus plug taps power from a plug-in busway through a switch and a set of fuses. The switch isolates the load. The fuses protect the conductors and equipment fed from the plug by opening on an overload or a short circuit. The busway can deliver a large fault current, so the fuses must be able to interrupt it and must fit the clips the plug was built with.

A fuse is not a universal part. Two fuses with the same amperage can differ in class, voltage rating and opening characteristic. Work through the four steps below in order, because each one narrows the choice for the next.

Step 1: Find the fuse class the plug accepts

Start with the class, because it decides whether the fuse fits at all. Read it from the plug nameplate or door label. If the label is gone, look up the plug by its catalogue number: every fusible bus plug record in the BRAH catalogue states the classes the plug accepts. The table lists them by busway family, with the 600V, 3-wire plug at each amperage linked.

The same families are listed in 240V versions too, and the fuse classes are the same across a family. Variants include 4-wire, grounded and weather-strip plugs. What the class letters mean, and which UL standard covers each one, is explained in the guide to fuse classes in bus plugs. For anything not in the table, call (855) 355-2724 with the part number.

Step 2: Match the voltage rating

The voltage rating of a fuse must be at least the voltage of the circuit. In a bus plug there is a second point: the plug itself is built as a 240V or a 600V part, and fuses for the two voltage levels are different physical sizes in the common cartridge classes. Fit the fuse voltage the plug is built for.

On BRAH fusible bus plugs the part number shows the voltage and the amperage. Taking BEB3203GNW as the example:

• BEB: the BRAH plug type, here the substitute for GE Spectra type SB.
• 3: three phase.
• 2: 240V. A 6 in this position is a 600V plug.
• 03: 30A. 06 is 60A and 10 is 100A.
• G: with ground.
• N: 4-wire, with neutral.
• W: with weather stripping for drip protection.

OEM catalogue numbers follow each maker’s own scheme, so read the voltage from the nameplate when the plug is an original.

Step 3: Choose the amperage

Two limits apply. The first is the plug: its amperage is the largest fuse it is built to carry. A 60A plug must not be fitted with fuses above 60A, whatever the load.

The second is the circuit. The fuse rating is chosen for the conductors leaving the plug and the equipment they feed, following the National Electrical Code and the equipment nameplate. For a motor or a transformer the code has its own sizing rules, so use the values on the machine nameplate or the engineer’s drawings instead of copying the rating of the fuses that came out.

Cartridge fuse bodies also get larger in steps as the amperage rises. A fuse well below the plug rating can be a smaller body that will not seat in the clips. Check the body size before ordering a rating that differs from the one fitted.

• If fuses open often, look for the cause: an overloaded circuit, a failing motor or a loose connection.
• Do not fit a larger fuse to stop nuisance opening. It removes the protection the circuit was designed with.
• Replace all three fuses with the same class, type and rating.

Step 4: Time-delay or fast-acting

This choice is about how the fuse responds to a short overload. A time-delay fuse rides through a brief inrush, such as a motor starting or a transformer energizing, and still opens on a sustained overload or a short circuit. A fast-acting fuse has no intentional delay and opens quickly on an overload.

• Motors, transformers and other loads with starting inrush: time-delay fuses are the usual choice.
• Resistive loads such as heating, and feeders with no inrush: fast-acting fuses can be used.
• A fast-acting fuse on a motor circuit tends to open on starting, which tempts people to oversize it.

Time delay is a characteristic within a class, not a class of its own. The words time delay, or a marking to that effect, are printed on the fuse label next to the class letter. If the equipment maker specifies a fuse type, follow it.

Check the fuse clips while the fuses are out

A new fuse in a weak clip will run hot. With the fuses removed, look at each clip for discoloration, pitting and loss of spring tension. A fuse should need firm pressure to seat. BRAH lists replacement fuse clips for bus plugs, grouped here by the fuse position they serve. All are described as Class R rated and as accepting Class H, R and K fuses.

The clips mount with a single screw, which is not included, and are field installable. Match the clip to the amperage and voltage of the fuse it holds. For anything not in the table, call (855) 355-2724 with the part number.

Common mistakes when replacing bus plug fuses

• Copying the old fuse. The fuse that came out may already have been wrong. Check the plug label and the load.
• Buying on amperage alone. Class and voltage decide whether the fuse fits and what fault current it can interrupt.
• Mixing fuses. Three different fuses in one plug give three different responses to the same fault.
• Ignoring heat marks. Discolored clips, fuse ends or insulation point to a poor connection that a new fuse will not cure.
• Bridging or modifying a fuse position. Never bypass a fuse or alter clips to take a fuse the plug was not built for.

What to check before you order

• Plug type and catalogue number from the nameplate, and the fuse classes marked on it.
• Plug voltage (240V or 600V) and plug amperage (30A, 60A or 100A for the plugs in the table).
• The load: motor, transformer, heating or mixed, and any fuse type the equipment maker specifies.
• Available fault current at the plug, compared with the interrupting rating on the fuse label and the AIC rating in the plug record.
• Condition of the clips, switch and stabs. If the plug body is damaged, compare the cost of parts with a replacement plug.

BRAH Electric is an aftermarket manufacturer, not an authorized distributor of the OEM brands. Its bus plugs and parts carry a 2-year warranty from the date of invoice, and in-stock orders placed before 3:00 PM Pacific ship the same business day. If you are replacing a Square D plug, the Square D bus plug cross reference lists the BPQ substitutes. The phone line, (855) 355-2724, is open Monday to Friday, 6AM to 5PM Pacific.

Safety when changing bus plug fuses

Opening the plug switch does not make the whole plug dead: the line side stays energized from the busway. Before removing or fitting fuses, de-energize the circuit, apply lockout/tagout and verify absence of voltage with a tester proved on a known source. Fuses should be changed by qualified personnel wearing the protective equipment the site’s electrical safety program requires.

Bus plug fuse FAQs

How do I choose the correct fuse rating for a bus plug?

Size the fuse for the conductors and the load on that circuit, following the National Electrical Code and the equipment nameplate, and keep it at or below the amperage of the plug.

Can I use any fuse that fits the clips?

No. The fuse must be a class the plug is marked for, with the right voltage rating and an interrupting rating that covers the available fault current. Fit alone proves none of these.

What happens if the fuse is too large?

An oversized fuse may not open on an overload that damages the conductors or the equipment. The circuit keeps running until something else fails.

Should I use time-delay fuses in a bus plug feeding a motor?

Time-delay fuses are the usual choice for motor loads because they tolerate the starting inrush. Follow the motor or machine maker’s recommendation where one is given.

How often should bus plug fuses be replaced?

A fuse has no moving parts and is replaced when it has opened or shows damage such as discolored ends or a cracked body. Inspect fuses and clips during planned maintenance, and after a fault it is good practice to replace all three.

References

• UL 248-1, Low-Voltage Fuses, Part 1: General Requirements: the general UL standard for low-voltage fuses; the fuse classes have their own parts in the same series.
• NFPA 70, National Electrical Code: the code that governs the sizing of overcurrent protection for conductors, motors and transformers.
• NFPA 70E: standard for electrical safety in the workplace.
• OSHA 29 CFR 1910.333: selection and use of electrical work practices, including de-energizing and verification.