DNXC Selection by Capacitor kvar and Unit Voltage

What these tables are, and what they are not

This article computes a starting point for DENCO DNXC full-range current-limiting capacitor fuses from two pieces of nameplate data: the capacitor unit's kvar and its rated voltage. The arithmetic is simple and is shown in full so that anyone can check a cell. It is not copied from any manufacturer's fusing table, and it is not a DENCO recommendation.

A real selection also depends on things these tables cannot see: the inrush and outrush I2t the fuse must withstand, the capacitor's case rupture curve, the available fault current, the energy stored in parallel units, harmonics beyond what the margin allows for, and the bank connection. Use a cell to frame an RFQ or to sanity-check a proposal, then confirm the rating with DENCO and the bank's protection engineer.

The method, step by step

Each cell is built the same way. The only inputs are the unit's nameplate kvar and voltage, the 1.35 margin, the derating formula and the DNXC ratings DENCO lists on catalog p.74.

The 1.35 figure comes from published manufacturer application guides for current-limiting capacitor fuses, which give minimum fuse current as capacitor current times a 1.35 protective margin and say the margin accounts for normal overvoltages, harmonics, capacitor tolerances and a 25 °C ambient. Other published guidance uses factors from 1.25 up to 1.65 or more, depending on the source and the application, so 1.35 is one practice, not a standard rule. The derating formula and its 0.0065 coefficient are from DENCO's DNXC catalog, p.74.

• Unit current: I = kvar / kV, using the unit's own rated voltage (line-to-neutral for wye banks, line-to-line for delta). The USDA RUS bulletin gives the check value: a 100 kvar unit draws 13.9 A at 7.2 kV.
• Minimum fuse current at 25 °C: I x 1.35.
• Voltage class: from the unit voltage, using the mapping in the next section.
• Derating above 25 °C: each DNXC rating A25 carries AT = A25 [1 - 0.0065 (T - 25)] at maximum ambient T.
• Selection: the smallest listed DNXC rating in the class whose AT is at or above the minimum fuse current. If none is, the cell reads Contact DENCO.

Unit voltage to DNXC class

The rows use capacitor unit voltages that appear in published manufacturer fuse tables, mapped to the three DNXC classes. Most rows are consistent across those tables. Two are not. One maker's outdoor current-limiting table puts 8320 V units in the 8.3 kV class, while two other tables put them in 15.5 kV, so 8320 V appears twice below, once in each class, and the class has to be confirmed. 22800 V units appear in only one of the tables.

Units below 2400 V, and units that need a 2.8, 4.3 or 5.5 kV fuse under a different maker's table, are outside the DNXC range; see the low-voltage and per-voltage articles in this guide for the DNX and Types 1 to 9 options.

Calculated starting points at 25 °C ambient

Each cell gives the DNXC ampere rating, with the calculated minimum fuse current (unit current x 1.35) in brackets. The catalog number is DNXC plus the class digits and the rating: a 25 in the 8.3 kV class is DNXC8-25, a 43 in the 15.5 kV class is DNXC15-43.

Read the cells as arithmetic, not as advice. Two cells sit exactly on a rating: 21600 V at 400 kvar needs 25.0 A and lands on DNXC23-25 with no headroom, and 4160 V at 200 kvar needs 64.9 A against a 65 A rating. Results like these deserve a closer look at harmonics and ambient before anyone relies on them.

Calculated starting points at 40 °C ambient

At 40 °C, DENCO's formula gives a factor of 1 - 0.0065 x 15 = 0.9025, so each DNXC rating carries 90.25% of its 25 °C value. Equivalently, the 25 °C rating has to be at least the minimum fuse current divided by 0.9025. Cells change wherever the derated rating drops below the requirement: a 7200 V, 50 kvar unit needs 9.4 A, a 10 A DNXC derates to 9.0 A, so the result moves to 18 A.

Calculated starting points at 55 °C ambient

At 55 °C the factor is 1 - 0.0065 x 30 = 0.805. That is the kind of temperature an enclosed bank can reach in sun; one capacitor maker's metal-enclosed bank manual recommends shading where daytime temperatures exceed 40 °C. Use the maximum ambient the fuse will see at any time inside the enclosure, which is what the DENCO formula is based on, not the outdoor design temperature.

More cells read Contact DENCO at 55 °C, because the top rating in each class derates too: 65 A becomes 52.3 A at 8.3 kV, 43 A becomes 34.6 A at 15.5 kV, and 25 A becomes 20.1 A at 23 kV.

Worked examples

Example 1, 13.8 kV wye bank, 7960 V units, 300 kvar, 55 °C enclosure. Unit current = 300 / 7.96 = 37.7 A. Minimum fuse current = 37.7 x 1.35 = 50.9 A. In the 8.3 kV class at 55 °C: 45 A derates to 36.2 A (too low) and 65 A derates to 52.3 A, which clears 50.9 A. Calculated starting point: DNXC8-65, with only about 3% headroom, so this one should go to DENCO with the harmonic data.

Example 2, 24.94 kV wye bank, 14400 V units, 400 kvar, 40 °C. Unit current = 400 / 14.4 = 27.8 A. Minimum = 37.5 A. In the 15.5 kV class at 40 °C: 35 A derates to 31.6 A (too low), 43 A derates to 38.8 A. Calculated starting point: DNXC15-43. At 55 °C the same unit reads Contact DENCO, because 43 A derates to 34.6 A.

Example 3, 34.5 kV wye bank, 19920 V units, 400 kvar, 25 °C. Unit current = 400 / 19.92 = 20.1 A. Minimum = 27.1 A. The largest 23 kV DNXC is 25 A, so the cell reads Contact DENCO. That does not mean the unit cannot be fused; it means the listed DNXC ratings do not cover it and DENCO engineering needs to look at the application.

Why some cells say Contact DENCO, and what the cells leave out

A Contact DENCO cell means the calculated minimum fuse current exceeds the largest listed DNXC rating in the class at that ambient. Some of those cells also describe units that may not exist: one capacitor maker's catalog tops out at 200 kvar for its 2400 to 4800 V standard units and starts its 300 to 600 kvar units at 6640 V.

The 50 kvar column needs particular care. One manufacturer's fusing table notes that for 50 kvar capacitors it is difficult to choose reasonably sized fuses that withstand the I2t outrush, because I2t withstand falls off faster than the fuse rating. A small fuse on a small unit in a large bank may be the right size for current and still too weak for the discharge of its neighbors.

The tables also say nothing about case rupture. The fuse's total clearing curve has to fall below the capacitor's tank rupture curve up to the available fault current, and published tank rupture curves for some units are rated only to 10 kA. Nor do they check parallel energy, the interrupting rating basis, or the voltage the fuse sees when it clears in an ungrounded bank.

Common mistakes with calculated fuse tables

These are the errors that most often turn a correct calculation into a wrong fuse.

• Using system voltage instead of unit voltage, which understates the unit current in a wye bank by the square root of 3.
• Reading the 25 °C table for an enclosure that runs hotter.
• Treating a cell as a recommendation and skipping the case rupture and outrush checks.
• Choosing the 8320 V class from whichever row looks better, instead of confirming it.
• Applying a different margin (1.25, 1.65, 2) and still reading these tables, which are built on 1.35 only.
• Rounding a minimum fuse current down to fit a listed rating.

Frequently asked questions