Capacitor Fuse Temperature Derating in Enclosures

Why fuses are derated for temperature

A fuse is a thermal device. Its element melts when the heat generated by current exceeds what the body can shed, and catalog ratings and curves are measured at a stated reference ambient. Put the same fuse somewhere warmer and it has less thermal headroom, so it melts at a lower current and runs closer to its limit in normal service.

Running close to the limit also shortens fuse life. One manufacturer's capacitor fuse selection guide notes that each time the temperature of a fuse element is raised near its melting point, the fuse deteriorates slightly. A fuse in a hot enclosure that also sees frequent switching is exposed on both counts.

Capacitor fuses are often in exactly that situation. DENCO's catalog lists its C-rated fuses for indoor use or outdoor use within an enclosure, and many banks are metal-enclosed, sit in direct sun and share space with reactors and other heat sources. The standards recognize the issue: IEC 60282-1:2020, the current IEC current-limiting fuse standard, was revised in part to clarify requirements for fuse-links used above 40°C.

The DNXC derating formula

DENCO's DNXC catalog (p.74) requires temperature derating when a DNXC fuse runs in ambient conditions above 25°C and prints the formula below. Every 1°C above 25°C removes 0.65% of the catalog rating.

• AT = A25 [1 - 0.0065 (T - 25)]
• AT: the fuse's ampere rating at temperature T
• A25: the fuse's ampere rating at 25°C, the catalog rating
• T: the maximum ambient temperature, in °C, the fuse will be subjected to at any time

Derating factors from 25°C to 55°C

The factor in brackets is the fraction of the catalog rating available at temperature T. The table applies the formula in 5°C steps over the range most enclosed banks fall in. If your maximum ambient is above 55°C, ask DENCO to confirm that the formula covers it before relying on it.

DNXC ratings after derating

The next table applies the factors to each DNXC rating DENCO catalogs. DNXC8 ratings are 10, 18, 25, 30, 40, 45 and 65 A; DNXC15 ratings are 10, 12, 18, 25, 30, 35 and 43 A; DNXC23 ratings are 12, 18 and 25 A. Read across to find what a rating can carry at your enclosure maximum, and compare that with the margined capacitor current.

Working backwards is often quicker. Divide the margined current by the factor to get the smallest catalog rating that will do, then take the next rating up. A margined current of 37.5 A at 55°C needs at least 37.5 / 0.805 = 46.6 A of catalog rating, which in the DNXC8 series means 65 A.

Worked example 1: a 300 kvar, 13,800 V unit at 55°C

Published manufacturer tables place 13,800 V units on 15.5 kV class fuses, so the candidates are DNXC15 ratings: 10, 12, 18, 25, 30, 35 and 43 A. The protective margin of 1.35 is from published manufacturer application guides for medium-voltage unit fuses.

Unit current is 300 / 13.8 = 21.7 A and the margined current is 21.7 x 1.35 = 29.3 A. At 25°C the 30 A rating covers it. At a 55°C enclosure maximum, 30 A derates to 30 x 0.805 = 24.2 A and 35 A to 35 x 0.805 = 28.2 A, both below 29.3 A. The 43 A rating derates to 43 x 0.805 = 34.6 A and passes, so the calculated starting point is DNXC15-43.

Two ratings disappear because of 30°C of enclosure temperature, and the result lands on the largest DNXC15 rating there is. The larger fuse then has to be rechecked against the capacitor's tank rupture curve and against outrush, because a bigger fuse clears an internal fault more slowly. If it fails that check, the fix is lower enclosure temperature or a different arrangement, not a smaller fuse that runs too hot.

Worked example 2: a 300 kvar, 19,920 V unit

Published manufacturer tables place 19,920 V units on 23 kV class fuses, and DENCO's DNXC23 ratings are 12, 18 and 25 A. Unit current is 300 / 19.92 = 15.1 A, and the margined current is 15.1 x 1.35 = 20.3 A.

At 25°C the 25 A fuse has room. At a 50°C enclosure maximum it derates to 25 x 0.8375 = 20.9 A, which still exceeds 20.3 A, but only just. At 55°C it falls to 25 x 0.805 = 20.1 A, below the margined current, and no DNXC23 rating passes.

That outcome is a reason to talk to DENCO engineering and the bank designer, not to drop the margin. Options include reducing the enclosure temperature, refining the harmonic and tolerance allowances from measured data, or a different fuse arrangement confirmed by DENCO.

Putting current, harmonics and temperature in one check

Derating is the last factor in a chain, and it is easiest to apply all of them at once. The smallest catalog rating that will do is the unit current, times any measured harmonic increase, times the protective margin, divided by the derating factor. Each factor must come from a named source: the unit nameplate, a measurement, the published margin you have chosen and the DNXC formula.

Take a 150 kvar, 7,200 V unit whose true-rms current measures 5% above rated, in an enclosure that reaches 45°C. Unit current is 150 / 7.2 = 20.8 A. With harmonics, 20.8 x 1.05 = 21.9 A. With the 1.35 margin, 21.9 x 1.35 = 29.5 A. Divided by the 45°C factor, 29.5 / 0.87 = 33.9 A of catalog rating, so the DNXC8 choice is 40 A, which derates to 40 x 0.87 = 34.8 A. The same unit at 25°C with no harmonic allowance would have needed only 28.1 A, and a 30 A fuse.

Order matters only for rounding. Do not round the intermediate figures down, and do not apply a margin that already includes temperature and then derate again for the same degrees.

Choosing the ambient temperature T

The formula uses the maximum ambient the fuse will see at any time: the air around the fuse inside the enclosure on the hottest day at full load, not the site's design outdoor temperature. Published practice gives reference points. One manufacturer's capacitor fuse guide says its 1.35 protective margin for outdoor unit fuses already accounts for a 25°C ambient, while its indoor design's margin accounts for 40°C. Another manufacturer's worked example uses an enclosed bank with a maximum ambient of 55°C.

Temperature varies inside one enclosure. A relay maker's paper reports a study that found a 10°C variation simply from the bottom of an outdoor cabinet to the top on a sunny day, so measure or estimate at the fuse location, which is often high in the enclosure. One metal-enclosed bank maker's manual says that where daytime temperatures exceed 40°C some form of shading should be provided, and its ventilation fans turn on at about 85°F (29°C).

Where there is no measured data, ask the enclosure or bank supplier for the design internal temperature rise and add it to the maximum site ambient. Solar gain on dark enclosures, ventilation, and heat from reactors or resistors in the same enclosure all count. Capacitor units have their own limits too: one capacitor maker gives 46°C as the maximum ambient for single-tier open-rack units, reduced by 3°C per tier.

Write the chosen value of T into the design file with its basis, measured or calculated. A later engineer replacing fuses after a failure will need to know what ambient the original rating assumed.

DNX and low-voltage Types

The formula above is printed for the DNXC series. Derating methods differ by product: one manufacturer's general-purpose current-limiting fuse catalog, for example, bases its derating curves on a six-hour melting time rather than a linear coefficient. The catalog data used for this guide does not give a derating method for DNX clip-style fuses or for DENCO's 1 kV to 5.5 kV Types. For those, send the maximum ambient with your request and DENCO engineering will confirm the derated rating.

Common mistakes

Derating errors are easy to make because the catalog rating looks comfortable at first glance.

• Using the site's design outdoor temperature instead of the air temperature at the fuse inside the enclosure.
• Applying derating on top of a margin that a manufacturer already states covers 40°C, or skipping it when the margin covers only 25°C.
• Uprating a fuse for ambients below 25°C. The formula is printed for ambients above 25°C only.
• Applying the DNXC coefficient to DNX, Types 1 to 9 or another manufacturer's fuse.
• Choosing a larger fuse after derating without rechecking the tank rupture curve and outrush.
• Not recalculating when reactors, filter components or extra units are added to the enclosure.

Frequently asked questions