E-Rated vs R-Rated Fuses: 7 Key Differences

About this list

Each difference below is taken from a published definition: ANSI C37.40 for fuse classes, ANSI/IEEE C37.46 for the E and R melting points, and manufacturers' own catalogs where a figure is quoted. The same definitions are used across DENCO's medium-voltage fuse selection guide.

1. 1. What they are designed to protect

In short: The first question on any replacement

E-rated fuses are used on feeder circuits, power transformers and potential transformers. R-rated fuses are used for short-circuit protection of medium-voltage motor circuits, alongside a starter that provides the overload protection.

The duty drives everything else on this list. A transformer fuse has to ride through magnetizing inrush for a fraction of a second; a motor fuse has to ride through starting current for several seconds, repeatedly.

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2. 2. The melting point each rating defines

In short: Understanding why the letters are comparable between makers

ANSI/IEEE C37.46 defines both ratings by one point on the melting curve. An E-rated fuse of 100 A or less melts in 300 seconds at 200% to 240% of its rating; above 100 A, it melts in 600 seconds at 220% to 264%.

An R-rated fuse melts in 15 to 35 seconds at 100 times its R number. The E point sits in the long-time overload region; the R point sits at a high multiple of current over a time comparable to a motor start.

3. 3. General purpose versus back-up

In short: Knowing whether the fuse can stand alone

Under ANSI C37.40, a general purpose current-limiting fuse interrupts every current from its maximum rating down to the current that melts it in one hour. Mersen's catalog describes E-rated fuses as general purpose fuses.

R-rated fuses are back-up fuses by class. A back-up fuse interrupts only down to a stated minimum interrupting current; below that, another device has to clear the fault. Mersen's catalog states that back-up fuses are not designed or intended to open under overload conditions.

4. 4. What the number on the barrel means

In short: Avoiding the most common ordering mistake

The E number is tied to the fuse's current rating, which is why a 100E fuse is defined by reference to 100 A. The R number is a code for the melting point: a 2R fuse melts in 15 to 35 seconds at 200 A.

Manufacturers therefore publish a separate continuous current for each R number. Mersen's application information, for example, lists current-limiting motor starter fuses up to 36R (650 A). Mersen also notes that the continuous ampere rating defines the fuse's thermal capability and should not be the sole factor in selection.

5. 5. Who handles overloads

In short: Getting the protection scheme right, not just the fuse

An E-rated general purpose fuse can clear sustained overloads that melt it within its range, although the band below its one-hour melting current still needs another device.

An R-rated fuse leaves overloads to the motor starter. Littelfuse's R-rated page describes the split: the controller's overload relays provide overload and locked-rotor protection and interrupt low-value short circuits, which keeps the fuse away from sustained overcurrents below its minimum interrupting rating.

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6. 6. Typical voltage classes and formats

In short: Narrowing the search before you call a supplier

E-rated fuses are published across a wide voltage span. Mersen's E-rated ferrule range runs from 2.75 kV through 38 kV, and Littelfuse publishes E-rated curves from 2.75 kV to 38 kV.

R-rated fuses cluster at motor voltages. Mersen's application information lists current-limiting motor starter fuses for nominal voltages of 2.4, 4.8 and 7.2 kV, with bolt-on types to 15.5 kV on its product page. Both classes come in single, double and triple barrel constructions; DENCO's R-rated family, for example, covers all three.

7. 7. Why one cannot replace the other

In short: Emergency replacements

Put an R-rated fuse in a transformer circuit and nothing in the circuit may clear a low-level overload inside its non-clearing zone. Put an E-rated fuse in a motor starter and it may open during starts, or fail to coordinate with the contactor the starter maker designed around.

Even within one class, the letter fixes only the melting point. Voltage, interrupting rating, curve shape, barrel count and mounting must also match, which is why DENCO confirms every replacement against the original part.

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Side by side

A quick reference, using the definitions above:

• Protects: E-rated, transformers, feeders and PTs; R-rated, medium-voltage motor circuits.
• Defined point: E-rated, 300 s at 200% to 240% (100 A or less) or 600 s at 220% to 264% (above 100 A); R-rated, 15 to 35 s at 100 times R.
• Class: E-rated, general purpose; R-rated, back-up.
• Number means: E-rated, current rating; R-rated, a melting code with a separate continuous current.
• Overloads: E-rated, cleared within its range; R-rated, left to the starter's overload relay.
• Standards: both defined in ANSI/IEEE C37.46, with classes defined in ANSI C37.40.

How to tell which one you have

The letter printed after the number on the fuse barrel or label is the quickest check: 65E, 100E or 150E marks an E-rated fuse, and 2R, 9R or 24R marks an R-rated fuse. The equipment is the second check. A fuse in a transformer primary, a load-interrupter switch feeding a unit substation or a PT compartment is almost always E-rated or a PT fuse. A fuse in a medium-voltage combination motor starter, next to a vacuum contactor and an overload relay, is almost always R-rated.

If the marking is worn or missing, do not guess from the barrel size. Send a photo of the fuse, the equipment nameplate and any drawing references with your inquiry, and have the replacement confirmed before it goes in.

Where DENCO fits

DENCO makes both classes: E-rated transformer protection fuses in the 9F60/EJ-1, 9F62/EJO-1 and Clip Lock 14 series, and R-rated motor and soft-starter fuses in single, double and triple barrel construction. DENCO quotes a 30-day lead time on all products. Send the existing part number and DENCO engineering will confirm the right replacement.

Frequently asked questions