How to Read a DNX or DNXC Catalog Number

The pattern

Both series follow the same idea: a series prefix, digits for the voltage class, a hyphen, then the current rating. What changes is the unit of the current rating.

DNX is DENCO's clip-style current-limiting capacitor fuse. Its catalog rows are written as a prefix, voltage digits and a C range, for example DNX8-15C - 40C. DNXC is DENCO's full-range current-limiting capacitor fuse for bus bar to lead mounting. Its catalog numbers are written per rating, for example DNXC8-40.

So a DNX row tells you which range of C numbers a design covers, and you choose the C number inside it. A DNXC number already names one rating, so it is a complete identifier for the fuse itself, though not for the options around it.

Voltage digits decoded

The voltage digits are a short label for the class. Read DNX2 as the 2.8 kV class, DNX4 as 4.3 kV, DNX5 as 5.5 kV, DNX8 and DNXC8 as 8.3 kV, DNX15 and DNXC15 as 15.5 kV, DNX23 and DNXC23 as 23 kV and DNX27 as 27 kV. The catalog also lists a 38 kV DNX size, but its catalog number needs confirming with DENCO.

The class has to be at least the voltage the fuse will see. Published manufacturer tables map capacitor unit voltages to classes as in the right-hand column. They are examples, and they differ on 8320 V units, so confirm the class for your bank.

From system voltage to class digits

Most people start from the system voltage, not the unit voltage, so it helps to do the conversion before reading a catalog row. In a wye bank each unit sees the line-to-neutral voltage, which is the line-to-line voltage divided by the square root of 3. One capacitor maker's installation instructions state the rule directly: units connected in wye are rated at the line-to-neutral system voltage, and units connected in delta at the line-to-line voltage.

Applied to common system voltages, that gives 4.16 kV to 2.40 kV units, 12.47 kV to 7.20 kV, 13.2 kV to 7.62 kV, 13.8 kV to 7.97 kV (7960 V units), 14.4 kV to 8.31 kV (8320 V units), 24.94 kV to 14.4 kV and 34.5 kV to 19.92 kV. In the published tables used above, the first four land in the 8.3 kV class (digits 8), 24.94 kV wye lands in 15.5 kV (digits 15) and 34.5 kV wye lands in 23 kV (digits 23). A 14.4 kV wye bank with 8320 V units is the case the tables disagree on.

Unit voltage is not always the obvious one. A utility unit specification in this guide's sources calls for 8320 V units on a 14.4 kV grounded wye system, for example. Read the capacitor nameplate rather than inferring the unit voltage from the system.

DNX catalog numbers by voltage class

Each DNX voltage class is split into two or three C ranges. All DNX ratings carry a 50 kA asymmetrical interrupting rating in the catalog. The table lists the catalog numbers as printed on catalog p.72, except the 38 kV row, whose numbering DENCO will confirm.

Two practical points follow. A catalog row covers a range, so state the exact C number you need inside that range when you order, and DENCO will confirm the full part number. And the C ranges do not run continuously in every class: at 4.3 kV the listing starts at 18C, and at 23 kV it starts at 3C. If your sizing lands outside a listed range, ask DENCO rather than moving to another voltage class.

DNXC catalog numbers by voltage class

DNXC fuses are listed by individual ampere rating on catalog p.74, all at a 50 kA interrupting rating and all for bus bar to lead mounting. The ratings are not evenly spaced, and the three classes do not share the same list: 12 A exists at 15.5 and 23 kV but not at 8.3 kV, and 45 A and 65 A exist only at 8.3 kV.

The ampere figure is the rating at 25 °C. DENCO's catalog requires derating above 25 °C ambient with the formula AT = A25 [1 - 0.0065 (T - 25)], so a DNXC15-43 at 55 °C behaves as a 34.6 A fuse for sizing purposes (43 x 0.805).

What the C in a DNX number means

No standard defines a C rating for capacitor fuses. It is a manufacturer catalog convention, and each manufacturer that uses it states its own meaning. 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. DENCO's family listing also writes its C-rated range as 1C to 200C (1A to 200A), so a C number corresponds to an ampere value in DENCO's catalog.

The table applies that definition to a few C numbers at the edges of the DNX ranges. It shows the band of current that melts the fuse in 1000 seconds. It is not a continuous rating and not a sizing rule: sizing still starts from the capacitor current, harmonics, ambient and the inrush and outrush duty.

Worked example: from bank data to a catalog number

Take a 12.47 kV grounded wye bank built from 7200 V, 100 kvar single-phase units, in an enclosure that reaches 40 °C. The arithmetic below shows how the parts of a catalog number are filled in. It is a starting point, not a selection.

Voltage digits: 12.47 kV divided by the square root of 3 is 7.2 kV, which matches the 7200 V units. Published manufacturer tables place 7200 V units in the 8.3 kV class, so the digits are 8: DNX8 or DNXC8.

Current: unit current is kvar divided by kV, 100 / 7.2 = 13.9 A. Manufacturer application guides for full-range current-limiting capacitor fuses apply a 1.35 protective margin, giving 18.75 A at 25 °C. At 40 °C the DENCO derating factor is 0.9025, so the 25 °C rating needs to be at least 18.75 / 0.9025 = 20.8 A. The smallest 8.3 kV DNXC rating that meets it is 25 A, so the calculated DNXC number is DNXC8-25.

For a clip-mounted bank, the DNX8-15C - 40C row is the likely place to look, but the C number is chosen from the time-current curves, the outrush duty and the ambient, not from the ampere arithmetic. Send the data and DENCO will propose the C number and confirm the part.

What a catalog number does not tell you

Several order details sit outside the catalog number. Ask for each one on the quote rather than assuming it.

• Blown fuse indicator: DENCO's C-rated family is offered with or without one.
• End connections: the DNX catalog notes threaded end studs are also available as an alternative to clip ends.
• Dimensions: the DNX body diameter in the catalog is a maximum. Get confirmed dimensions from DENCO before laying out a new enclosure.
• Weights: the catalog lists an approximate weight per DNXC part. Get the value for your part from DENCO.
• Temperature: DNXC fuses need derating above 25 °C ambient. Give DENCO the maximum enclosure temperature.
• Interrupting basis: the DNX is listed at 50 kA asymmetrical; the DNXC at 50 kA without the basis stated. Match it to your fault study.

Common mistakes with DNX and DNXC numbers

Most catalog number errors on orders come from reading the digits too literally.

• Reading DNX15 as a 15 kV fuse. It is the 15.5 kV class.
• Ordering a DNX by row (for example DNX8-15C - 40C) without stating the C number inside the range.
• Assuming a DNXC rating exists in every class. DNXC8-12, DNXC15-45 and DNXC23-30 are not listed.
• Treating DNX 40C and DNXC 40 A as interchangeable because the digits match.
• Building a 38 kV DNX number by pattern instead of asking DENCO for it.

Sending a catalog number to DENCO

If you have a DENCO number, send it with the quantity, the capacitor unit voltage and kvar, and the enclosure ambient. If you have a number from another manufacturer, send it exactly as marked, with photos of the label and the mounting. DENCO engineering will confirm a replacement. No cross-reference is assumed from the number alone, and the cross-reference article in this guide explains what DENCO checks.

It also helps to say what you know the number is not. If a DNX row is written on an old drawing without a C number, say so; if a label is damaged or partly painted over, send the photo anyway. DENCO engineering would rather work from a poor photo than from a number typed from memory. Include the bank's protection study reference and the person who signs it off, so questions about curves go straight to the engineer who owns them.

Frequently asked questions