What Does C-Rated Mean on a Fuse?

Where the C rating comes from

North American high-voltage fuse standards are split across several documents. IEEE C37.40 held the service conditions and definitions and has since been folded into IEEE C37.41, the design test standard. IEEE C37.42-2016 held the specifications, and its scope included distribution and power class expulsion, current-limiting and combination-type external capacitor fuses rated 1 kV through 38 kV, for capacitors complying with IEEE 18 and NEMA CP 1. Before that, IEEE C37.43-2008 covered the same external capacitor fuses and was superseded by C37.42-2016.

Those scopes cover capacitor fuses, but the IEEE pages give no rating letter for them. None of the standards reviewed for this guide defines a C rating. The C on a capacitor fuse is a designation that manufacturers use in their catalogs, and its meaning comes from each manufacturer's own definition.

Catalogs often add a line such as designed to ANSI C37-41, which is how DENCO's catalog describes its Types 1 to 9 capacitor protection fuses. IEEE C37.41 sets design tests such as dielectric, interrupting, temperature-rise and time-current tests. That wording tells you the test basis the maker designed to. It is not a definition of what C means.

The IEC side works the same way. IEC 60549:2013 covers high-voltage fuses for the external protection of shunt capacitors, and an IEC-oriented fuse technical guide says such fuse-links should meet the general current-limiting fuse standard, IEC 60282-1, with additional tests from IEC 60549. IEC-market sizing guidance quoted elsewhere in this guide works from the fuse's rated current; none of it uses a C letter.

How DENCO defines the C number

DENCO's catalog gives a melting definition for its DNX capacitor fuses: the fuse melts in 1000 seconds at a current between 1.7 and 2.4 times its C number. The C-rated family range is written 1C to 200C (1A to 200A), so in DENCO's catalog the C number corresponds to an ampere value.

The definition is a band, not a single trip point. The table works the arithmetic for several C numbers that appear in DENCO's DNX ranges. Read the published time-current curve when you need the melting current for a specific rating; the band only tells you where the 1000 second point can fall.

Reading a C number against capacitor current

A worked example shows where the 1000 second point sits relative to the current a capacitor actually draws. Take a 200 kvar, 7,200 V single-phase unit. Unit current is kvar divided by kV: 200 / 7.2 = 27.8 A.

IEEE's 2005 interpretation of IEEE 18-2002 says capacitors must be capable of continuous contingency operation at up to 135% of nominal rms current. For this unit that is 27.8 x 1.35 = 37.5 A. One manufacturer's capacitor fuse selection guide arrives at the same multiplier from a different direction: its minimum fuse current is capacitor current times a 1.35 protective margin covering normal overvoltages, harmonics, capacitor tolerances and a 25°C ambient.

Now compare a 40C DNX fuse. By DENCO's definition it melts in 1000 seconds at 68 A to 96 A, which is 2.4 to 3.5 times the unit's 27.8 A and well above the 37.5 A contingency current. That gap is the point of the definition: the fuse is not meant to melt on steady-state capacitor current, even at contingency levels.

What the arithmetic does not settle is whether a given C number carries the required current continuously in your enclosure, or whether it rides through inrush and outrush and clears ahead of the capacitor's case rupture curve. Those are separate checks, covered in the sizing, derating and curves articles. For the 8.3kV class, DENCO's DNX8 is catalogued in 1C to 12C, 15C to 40C and 50C to 140C ranges; send the unit data and DENCO engineering will confirm the rating.

C vs E vs R ratings side by side

The three rating letters describe different melting definitions. The E and R definitions come from the IEEE fuse specifications (originally ANSI/IEEE C37.46, consolidated into IEEE C37.42-2016). The C definition below is DENCO's, because no standard definition was found.

K and T: the other letters in capacitor fusing

Pole-mounted capacitor banks are often fused by phase with expulsion links, and those links carry their own letters. One capacitor maker's group fusing guide describes T and K links as the EEI-NEMA slow (T) and fast (K) expulsion link types, and notes that tin T and K links carry 150% of their rating continuously while silver links carry 100%. The letter is a speed class for a distribution fuse link. It is not a capacitor rating and says nothing about stored energy or case rupture.

USDA Rural Utilities Service Bulletin 1724D-112 prints a T-link table for grounded wye pole banks. A few of its rows for a 12,470/7,200 V system are below. The same capacitor maker states that the largest links that coordinate with its units' tank rupture curves are 100K and 65T, and only up to stated fault current levels.

Why the definitions are not interchangeable

Each letter fixes a different point on the time-current curve. Published time-current curves typically run from 0.01 seconds to 1000 seconds, so DENCO's C definition sits at the long-time end of the curve, while the E definition sits at 300 or 600 seconds and the R definition at 15 to 35 seconds. K and T describe link speed rather than a single melting point.

Two fuses with the same number and different letters will therefore melt at different currents for the same time, and their short-time behavior, which governs inrush withstand and case rupture protection, is not described by the letter at all. A 40C fuse, a 40E fuse and a 40T link are not substitutes for each other without checking each full curve against the capacitor and the bank.

Amperes, C numbers and type codes in catalogs

Capacitor fuse catalogs label ratings in more than one way. Some use C numbers, some plain amperes, and one manufacturer's selection guide rates its capacitor fuses by the maximum current the fuse can carry continuously without deterioration, including harmonics. In at least one other manufacturer's catalog numbering, the letter C simply marks the part as a capacitor fuse, with the current rating in a separate field.

In DENCO's catalog the voltage class is in the part number and the C number is the rating. The DNX ranges by class are below. DNXC fuses are listed in amperes instead: 10, 18, 25, 30, 40, 45 and 65 A at 8.3kV, and 12, 18 and 25 A at 23kV.

What the C rating does not tell you

Even with a clear melting definition, the C number is one line of a capacitor fuse specification. The other ratings decide whether the fuse can be applied at all.

Temperature is a good example of why the rating number is only a starting point. DENCO's DNXC catalog (p.74) prints a derating formula for ambients above 25°C: AT = A25 [1 - 0.0065 (T - 25)], where A25 is the rating at 25°C and T is the maximum ambient the fuse will see. Worked through at 40°C, a 40 A DNXC carries 40 x (1 - 0.0065 x 15) = 40 x 0.9025 = 36.1 A. The nameplate number stays 40; the usable current does not.

• Voltage class, which must suit the capacitor unit voltage and bank connection.
• Interrupting rating, for example 50kA asymmetrical for DNX and 50kA for DNXC.
• Whether the fuse is full-range or back-up, which decides whether it can clear low overcurrents on its own.
• I2t withstand for inrush and outrush, and total clearing I2t against the capacitor's case rupture curve.
• Temperature derating in an enclosure. DENCO's DNXC catalog prints a derating formula for ambients above 25°C.
• Mounting style: clip-style for DNX, bus bar to lead for DNXC.

Common mistakes

Most C-rating errors come from assuming the letter means the same thing everywhere.

• Writing the ANSI C rating in a specification. No standard reviewed for this guide defines one; name the manufacturer's definition instead.
• Assuming another maker's C number melts like DENCO's 1.7 to 2.4 times band. Read each maker's own definition and curve.
• Swapping C, E and K or T fuses of the same number. Each letter fixes a different curve point, and only the full curves can be compared.
• Reading the 1000 second melting band as a continuous rating. Continuous capability in a hot enclosure is a separate derating question.
• Choosing by C number alone. Voltage class, interrupting rating, I2t and mounting decide whether the fuse fits the bank.
• Ignoring derating. In a hot enclosure the usable current of a rating falls below its nameplate number; DENCO's DNXC catalog prints the formula for ambients above 25°C, and DENCO engineering can apply it to your enclosure.

Frequently asked questions