DNX vs DNXC: Which DENCO Capacitor Fuse Fits?

The two series side by side

DENCO lists both series in its C-rated capacitor fuse family, on catalog pages 72 to 74. Both are current-limiting capacitor fuses intended to protect capacitor units and banks against case rupture and dielectric failure. They differ in mounting, voltage span, the way the current rating is expressed, the fuse class the catalog states, and what the catalog says about ambient temperature.

Everything in the table is taken from the DENCO catalog as printed. Where the catalog does not state a value, the cell says so instead of filling the gap. Dimensions and weights are left out on purpose: get them from DENCO for the exact rating you are considering.

Start with the mounting you already have

In a retrofit, the mounting usually decides the series before any rating does. A DNX is a clip-style fuse: its ends seat in fuse clips, and the catalog notes threaded end studs as an option. A DNXC is built for bus bar to lead mounting, with one end bolted to the bank bus and the other connected to the capacitor unit's lead.

Changing from one style to the other means changing the mounting hardware, the clearances and possibly the bus layout. If the bank or enclosure was designed around clips, a DNX is the natural starting point. If each unit is fused from a bus bar, start with the DNXC. The clip vs bus mounted article in this guide covers the mechanical side.

For new equipment you have a free choice. The decision then rests on voltage class, fuse class, the current rating the sizing work calls for, and the enclosure temperature, which the next sections take in turn.

Voltage class: where each series is listed

Fuse voltage class follows the capacitor unit voltage. Published manufacturer tables for current-limiting capacitor fuses map common unit voltages to classes as shown below. The tables agree on most rows but not all: one maker's outdoor table puts 8320 V units in the 8.3 kV class, while two other tables put them in the 15.5 kV class. Treat 8320 V as a case to confirm with DENCO, not a rule.

The DNXC is listed only in the 8.3, 15.5 and 23 kV classes. If the units need a 2.8, 4.3 or 5.5 kV fuse, or a 27 or 38 kV fuse, the DNX is the DENCO series that covers it. The voltage rating article explains how the bank connection affects the voltage a fuse sees when it clears.

The bank connection decides which unit voltage you start from. One capacitor maker's installation instructions state that units connected in delta must be rated at the line-to-line system voltage and units connected in wye at the line-to-neutral voltage. So a 12.47 kV grounded wye bank normally uses 7200 V units and lands in the 8.3 kV class, while a 4160 V delta bank uses 4160 V units. For 8.3, 15.5 and 23 kV you can then choose either series; below and above that range the choice is made for you.

C numbers and amperes are not the same scale

DNX fuses are rated by C number. No standard defines a C rating for capacitor fuses; it is a manufacturer catalog convention. DENCO's catalog defines it for the DNX: the fuse melts in 1000 seconds at a current between 1.7 and 2.4 times its C number. A DNX of 40C therefore melts in 1000 seconds somewhere between 68 A and 96 A (40 x 1.7 and 40 x 2.4).

DNXC fuses are rated in plain amperes, from 10 to 65 A, and their curves are on catalog p.73. A DNXC8-40 and a 40C DNX share a number, but nothing in the catalog says their melting or clearing curves match. When moving between series, compare the time-current curves for the specific ratings against the capacitor's case rupture curve and the bank's inrush and outrush duty. Ask DENCO for the curve that matches any DNX rating you are considering.

Full-range vs back-up, and why it matters in a bank

The DENCO catalog describes the DNXC as a full-range current-limiting fuse. The IEEE C37.48.1 tutorial describes a full-range fuse as one that can interrupt any continuous current between the minimum current that can cause melting, at the maximum application temperature, and its rated maximum interrupting current, without another device to protect against overloads or high-impedance faults. A back-up fuse, by the same tutorial, interrupts only between its rated minimum and maximum interrupting currents and is often applied in series with another interrupting device.

That difference matters more in capacitor banks than in many other uses. One manufacturer's capacitor fuse selection guide explains that in ungrounded or multi-series-group banks, faults are limited by the capacitors themselves, with fault current typically less than 2 or 3 times the bank load current. A unit fuse in that kind of bank may have to clear a current only a few times its own rating.

The DNX series page describes the DNX as current-limiting and clip style but does not use the term full-range. Before relying on a DNX to clear low-magnitude faults on its own, ask DENCO which fuse class applies to the rating you are specifying.

Ambient temperature and derating

The DENCO catalog requires derating when a DNXC fuse runs above 25 °C ambient, and prints the formula on p.74: 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. At 40 °C the factor is 0.9025; at 55 °C it is 0.805.

Use the maximum temperature inside the enclosure, not the outdoor design temperature. One capacitor maker's metal-enclosed bank manual recommends shading where daytime temperatures exceed 40 °C, which gives a sense of how hot enclosures can run. The DNX series page does not state a derating rule, so ask DENCO how the DNX should be applied at your enclosure temperature.

Worked example: one bank, both series

Take a 13.8 kV grounded wye bank built from 7960 V, 200 kvar single-phase units in a metal enclosure with a 55 °C maximum ambient. The numbers below illustrate the method only; they are not a fuse selection.

Unit current is kvar divided by kV: 200 / 7.96 = 25.1 A. Manufacturer application guides for full-range current-limiting capacitor fuses apply a 1.35 protective margin, which they say covers normal overvoltages, harmonics, capacitor tolerances and a 25 °C ambient: 25.1 x 1.35 = 33.9 A minimum at 25 °C.

At 25 °C the smallest 8.3 kV DNXC rating at or above 33.9 A is 40 A (DNXC8-40). At 55 °C the DENCO formula gives 40 x 0.805 = 32.2 A, which is short of 33.9 A. The next rating, 45 A, gives 45 x 0.805 = 36.2 A, which clears it. So the calculated starting point is DNXC8-45 in this enclosure.

If the enclosure uses clip mountings instead, the DNX8 15C to 40C range is the place to look, but the C number cannot be read off the ampere arithmetic. DENCO picks it from the curves, the outrush duty and the ambient. In both cases the result still has to be checked against the capacitor's case rupture curve, the parallel energy and a protection study.

Common mistakes when choosing between DNX and DNXC

These come up most often in replacement requests and first-time specifications.

• Treating a C number and an ampere rating as the same thing because the digits match.
• Swapping series without changing the mounting hardware and clearances.
• Sizing a DNXC at 25 °C for an enclosure that runs at 40 or 55 °C.
• Comparing 50 kA asymmetrical (DNX) and 50 kA (DNXC, basis not stated) as if they were the same figure.
• Assuming every DNX rating is full-range without asking DENCO.
• Choosing a class for 8320 V units from one table without noting that published tables disagree.

What to send DENCO if you want the choice made for you

If you are not sure which series suits the bank, send the inputs rather than a guess. DENCO engineering can recommend a series and a rating from the same data the sizing work uses, and point out where the catalog does not cover the application.

The most useful items are the capacitor unit voltage and kvar, the bank connection and grounding, the number of units in parallel per group, the available fault current and whether it is symmetrical or asymmetrical, the maximum enclosure ambient, photos of the existing mounting, and whether a blown fuse indicator is required. Add any protection study and the capacitor's case rupture curve if you have them. The RFQ checklist in this guide sets the full list out as a table.

When neither listed rating fits

If the sizing work lands above the top DNXC rating in a class, between listed ratings, or needs a voltage, mounting or end connection the catalog does not show, send the application data rather than forcing a listed part. DENCO takes OEM and specialty custom design work, and DENCO engineering can review whether a listed DNX or DNXC rating fits or a custom build is needed. The RFQ checklist and the DNXC selection by kvar article in this guide show what to include and how the calculated starting points are built.

Frequently asked questions