8.3 kV Capacitor Fuses (DNX8 and DNXC8)

The units and systems behind the 8.3 kV class

The 8.3 kV class is where the three published manufacturer tables used in this guide agree most. A full-range current-limiting table, an outdoor combination table and an indoor combination table all place 6640 V, 7200 V, 7620 V and 7960 V capacitor units in the 8.3 kV class. The full-range table, whose range starts at 8.3 kV, also puts the low-voltage units from 2400 V to 4800 V here.

Those unit voltages are the line-to-neutral voltages of the common distribution systems. ANSI C84.1 nominal voltages, as adopted by one state regulator, include 12470Y/7200 V, 13200Y/7620 V and 13800Y/7970 V. One capacitor maker's table pairs 6640 V units with an 11.5 kV wye system, 7200 V with 12.47 kV, 7620 V with 13.2 kV and 7960 V with 13.8 kV. Another fuse maker works the same chain in its catalog: a 13.8 kV grounded wye bank puts 13.8 / 1.732 = 7.9 kV across each unit, so it selects an 8.3 kV fuse.

Delta banks on a 7.2 kV system put line-to-line voltage on the units, so they use 7200 V units. One utility's unit specification calls for single-bushing units for its 14.4 kV wye system and two-bushing units for its 7.2 kV delta system.

The 8320 V disagreement

8320 V units sit on the boundary. The outdoor combination table puts them in its 8.3 kV class. The full-range table and the indoor combination table put them in 15.5 kV. Do not pick one silently: the tables describe different fuse designs, and a unit voltage of 8320 V is above the 8300 V class figure.

Run the IEEE contingency number as well. IEEE's 2005 interpretation of IEEE 18-2002 says fuses should withstand the 110% rated voltage that units are expected to ride through: 1.1 x 8320 V is 9152 V. One maker's indoor table lists its 8.3 kV class with a rated maximum of 9.13 kV, 110% of the class figure, which is just under that number.

8320 V units are not rare. One utility uses them on its 14.4 kV grounded wye system. If your bank uses them, send DENCO the unit data and let DENCO engineering state the class. The choice changes the part: a 200 kvar, 8320 V unit needs 32.5 A after a 1.35 margin, which is DNXC8-40 in the 8.3 kV class or DNXC15-35 in the 15.5 kV class at 25 °C.

DNX8 and DNXC8 catalog numbers

DENCO offers two 8.3 kV capacitor fuse designs. The DNX8 is clip style, with threaded end studs available, listed in three C ranges on catalog p.72. The DNXC8 is a full-range current-limiting capacitor fuse for bus bar to lead mounting, listed by ampere rating on p.74 and tested at IPH to IEC 60282-1: 2009-10 and IEC 60282-2: 2008-04 in the catalog's own words.

For the DNX, DENCO's catalog defines the C number by melting time: 1000 seconds at 1.7 to 2.4 times the C number. No standard defines a C rating. The DNXC uses plain amperes, so its rating compares directly with the arithmetic below.

Unit current for common 8.3 kV class units

Unit current is kvar divided by unit kV. A rural utility bulletin gives the field version: each 100 kvar capacitor draws 13.9 A on a 7.2 kV system. Published manufacturer guides for full-range and combination current-limiting unit fuses then multiply by 1.35, a protective margin one maker says covers normal overvoltages, harmonics, capacitor tolerances and a 25 °C ambient. Each cell shows unit current, then current x 1.35.

• Smallest units: at 50 kvar the margin points to DNXC8-10, but 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 exponentially with fuse rating. Ask DENCO about outrush for 50 kvar units.
• Largest units: at 400 kvar every 8.3 kV unit voltage needs more than 65 A after the margin, above the top DNXC8 rating. DENCO decides whether a DNX8 in the 50C to 140C range or another arrangement fits.

Temperature derating for the DNXC8

DENCO's DNXC catalog p.74 requires derating when the fuse runs in an ambient above 25 °C: A_T = A_25 x [1 - 0.0065 x (T - 25)], where T is the maximum ambient the fuse will see. At 40 °C the factor is 0.9025; at 55 °C it is 0.805. The table applies those factors to each DNXC8 rating.

The DENCO catalog prints this formula for the DNXC only. For a DNX8, give DENCO the ambient and ask.

Worked examples at 7200 V and 7620 V

Example 1, a 200 kvar, 7200 V unit in a 12470Y/7200 V bank. Unit current is 200 / 7.2 = 27.8 A, and 37.5 A with the 1.35 margin. At 25 °C the smallest DNXC8 at or above 37.5 A is DNXC8-40. If the fuse sits in an enclosure that reaches 40 °C, DNXC8-40 derates to 36.1 A, which is short, and DNXC8-45 derates to 40.6 A, which clears it.

Example 2, a 300 kvar, 7620 V unit in a 13200Y/7620 V bank. Unit current is 39.4 A, and 53.1 A with the margin. DNXC8-65 covers it at 25 °C and at 40 °C (58.7 A). At 55 °C the same fuse derates to 52.3 A, below 53.1 A, and there is no larger DNXC8. That is a case to take to DENCO rather than to round off.

Neither example checks outrush I2t, parallel energy or the capacitor's case rupture curve. Those checks belong to DENCO and the protection study.

Pole banks, fault current and the case for current-limiting fuses

Much of the 8.3 kV class is pole-mounted on 12.47 kV feeders. One utility's standard builds 300 to 1350 kvar grounded wye pole banks on 12,470 V from 7.2 kV, 95 kV BIL units. A rural utility bulletin notes that pole units are not normally fused individually but by phase, which is group fusing; one utility's 600 kvar bank of three 200 kvar, 7.2 kV units uses an 8.3 kV, 30 A combination fuse in its cutouts. A DNXC8 is a unit fuse for bus bar to lead mounting, so it fits banks that fuse each unit.

Fault level is what pushes a bank toward current-limiting fuses. One capacitor maker's group fusing note says the largest expulsion links that coordinate with its standard and heavy duty units do so only up to 10,000 A symmetrical at X/R 0, falling to 6,500 A at X/R 15. Above that it lists current-limiting fuses, ungrounded wye operation or moving the rack. In a single series group grounded wye bank, the fault current at a failed unit is limited only by the system, per one manufacturer's guide. DENCO prints 50 kA for the DNXC and 50 kA asymmetrical for the DNX.

Unit fusing versus group fusing on 12.47 kV

It helps to know which kind of fuse a specification means before comparing numbers. A group fuse protects all the units on one phase of a bank and carries the phase current. A rural utility bulletin's T link table for 12,470/7,200 V grounded wye banks lists 150 kvar at 6.9 A on an 8T link, 300 kvar at 13.9 A on a 15T, 600 kvar at 27.8 A on a 25T and 900 kvar at 41.7 A on a 40T, and says group protection should have a nominal rating of 135% to 140% of capacitor line current. One capacitor maker's group fusing note asks for at least 135% continuous capability on grounded wye banks and 125% on ungrounded wye banks.

A unit fuse such as the DNXC8 protects one unit and carries only that unit's current. In a 900 kvar bank built from 300 kvar units, each unit fuse sees 41.7 A, the same as the group link in this case, because there is one unit per phase. Build the same bank from three 100 kvar units per phase and each unit fuse sees 13.9 A while the phase still carries 41.7 A. When a request mixes the two, DENCO will ask which one is meant.

What to send DENCO for an 8.3 kV quote

• Unit voltage and kvar, especially if the units are 8320 V.
• System voltage and connection: grounded wye, ungrounded wye or delta.
• Units per phase and series groups, so DENCO can judge parallel energy.
• Available fault current and X/R at the bank.
• Maximum ambient at the fuse, so DENCO can apply the DNXC derating.
• DNX8 clip mounting or DNXC8 bus bar to lead mounting, indicator option and quantity.
• Any existing fuse part number. DENCO engineering will confirm a replacement from it.

Common mistakes at 8.3 kV

• Using the system voltage instead of the unit voltage. A 12.47 kV wye bank uses 7200 V units and an 8.3 kV fuse; a 12.47 kV delta bank uses 12470 V units and a 15.5 kV fuse.
• Assuming 8320 V units belong in 8.3 kV because the digits look close. Two of three published tables say 15.5 kV.
• Picking a DNXC8 at 25 °C ratings for an enclosed bank. At 40 °C a DNXC8-40 carries 36.1 A, not 40 A.
• Rounding past the top of the range. If the derated DNXC8-65 is still short, the answer is a call to DENCO.
• Fusing 50 kvar units on current alone, without checking outrush I2t.

Frequently asked questions