4.3 kV Capacitor Fuses (DNX4)

How a capacitor ends up on a 4.3 kV fuse

The 4.3 kV class is unusual among DENCO's DNX voltages because the published unit fuse tables used across this guide skip it. The outdoor combination table jumps from 2.8 kV to 5.5 kV, the indoor combination table starts at 5.5 kV, and the full-range table starts at 8.3 kV. So the case for a 4.3 kV fuse rests on voltage rules rather than on a lookup.

One maker's low-voltage capacitor fuse guidance gives those rules in three sentences. The voltage of the capacitor being protected should be less than or equal to the voltage of the fuse. The nearest available fuse should be used, so that the voltage the fuse develops during interruption does not damage the system. On three-phase capacitors, the fuse should be rated at or above the line-to-line voltage.

Apply those rules to the 4160 V system, which is on the ANSI C84.1 nominal list both as 4160 V and as 4160Y/2400 V. In a delta bank the units are rated at line-to-line voltage, so each unit is a 4160 V unit and sits just under 4300 V. A 4160 V three-phase capacitor meets the line-to-line rule the same way. In a 4160Y/2400 V wye bank the units are 2400 V, and the nearest class below 4.3 kV is 2.8 kV.

The 4800 V problem and the 4576 V question

A 4800 V unit is 500 V above the class, and one maker's own guidance confirms the limit from the other direction: it rates a low-voltage fuse at 4.3/2.5 kV for 2500 V single-phase or 4300 V three-phase applications, and says that to protect a 4800 V single-phase capacitor you use two of those fuses in series. A single 4.3 kV fuse on a 4800 V unit is outside that guidance.

The tighter question is at 4160 V. IEEE's 2005 interpretation of IEEE 18-2002 says units are expected to stay in service for bank contingencies up to 110% of rated voltage and that the fuses should withstand this overvoltage as well. 1.1 x 4160 V is 4576 V. DENCO's 4.3 kV capacitor protection Types are catalogued by maximum AC voltage, 4300 V AC, so ask DENCO how the DNX4 rating and that maximum relate to a 4576 V contingency before you commit. If the answer is that the margin is too thin, the 5.5 kV class is the next step up.

DNX4 catalog numbers

DENCO's catalog p.72 lists the 4.3 kV DNX in two C ranges. Like every DNX, it is a clip-style current-limiting capacitor fuse, with threaded end studs available. The listed body diameter is a maximum; ask DENCO for dimensions with the quote.

The 4.3 kV listing starts at 18C rather than 1C. If the current your bank needs maps to a C number below 18, the DNX4 rows do not cover it. DENCO's catalog defines the C number by melting behavior: the DNX melts in 1000 seconds at 1.7 to 2.4 times its C number. No standard defines a C rating, so read it as DENCO's convention and let DENCO set the number.

• The Types 7, 8 and 9 are catalogued in amperes, by maximum AC voltage and Type number, and are designed to ANSI C37-41 specs in the catalog's own words.
• The two ratings use different bases. Do not compare 50 kA asymmetrical with 65 kA symmetrical as if they were the same measure; ask DENCO.

Unit and line current at 4160 V

Each unit fuse carries the unit current: kvar divided by unit kV. In a delta bank that current flows in the delta leg, and the current in each line is higher. One manufacturer's formula sheet gives line current as kvar x 1000 divided by the square root of 3 times line voltage for three-phase loads. A fuse in the line, which IEC 60549 calls a line fuse, sees that line current; a unit fuse sees only its own unit.

One maker's guide says fuses are usually applied with a continuous current margin in the range of 1.3 to 1.65 per unit, which it calls the fusing factor. The table shows both ends so you can see the spread. The kvar range stops at 200 because one capacitor maker lists 200 kvar as its largest standard unit from 2400 V to 4800 V.

Worked example: a 450 kvar delta bank at 4160 V

Take a 450 kvar delta bank on a 4160 V system built from three 150 kvar, 4160 V units, one per delta leg, each with its own fuse. Each unit draws 150 / 4.16 = 36.1 A. With a 1.35 margin the unit fuse needs at least 48.7 A of continuous capability; at 1.65 it is 59.5 A.

The line current is different: 450 x 1000 / (1.732 x 4160) = 62.5 A. That figure matters for the feeder, the switch and any line fuse ahead of the bank, not for the unit fuses. Mixing the two up is a common way to oversize unit fuses at this voltage.

Because a delta bank has a single series group, one relay maker's paper notes that losing a unit leaves no overvoltage on the remaining units, so unbalance detection is not required. The flip side is fault current: one manufacturer's guide says faults in delta banks are inductively limited, so the available system fault current reaches the fuse. Give DENCO that number.

Interrupting duty on a 4160 V bus

DENCO prints the DNX series at 50 kA asymmetrical and its 4.3 kV Types 7, 8 and 9 at 65 kA symmetrical. DENCO's family listing shows interrupting ratings from 40 kA to 80 kA across its C-rated fuses, so the right part depends on the fault level at your bus as well as on voltage and current.

The IEEE fuse working group tutorial explains why the basis matters: a symmetrical crest is 1.414 times the rms value, and a fully asymmetrical crest can approach 2.8 times the rms symmetrical value depending on X/R. Send DENCO the fault study figure with its basis rather than converting it yourself.

Applications and code points at 4.3 kV

DENCO lists its C-rated fuses for power factor correction equipment, harmonic filtering equipment, induction heating systems, high power drives and welders and capacitor bank protection, indoors or outdoors within an enclosure. On 4160 V systems that usually means plant switchgear rooms and equipment enclosures, and one maker's manual notes its 5 kV class metal-enclosed banks are normally delta connected.

For installations under NEC Article 460, the 2023-cycle text loaded for this guide has Part III for capacitors over 1000 V. It requires a means to detect and interrupt fault current likely to cause dangerous pressure within an individual capacitor, lets capacitors be protected individually or in groups, and requires protective devices to operate within the limits of the safe zone for individual capacitors. It also requires residual voltage to fall to 50 volts or less within 5 minutes after disconnection. Confirm the wording against the edition your jurisdiction adopts.

DENCO prints a temperature derating formula for its DNXC only. For a DNX4 in a hot switchgear room, give DENCO the maximum ambient.

What DENCO needs for a 4.3 kV quote

• Capacitor voltage and kvar, and whether it is a single-phase unit or a three-phase capacitor.
• Bank connection: delta, grounded wye or ungrounded wye, and the system voltage.
• Whether the fuse protects one unit or a line (unit fuse or line fuse).
• Available fault current and its basis.
• Harmonic content, filter tuning, switching frequency and any motor connection.
• Maximum ambient at the fuse location.
• Mounting style, blown fuse indicator yes or no, quantity.
• Any existing fuse part number. DENCO engineering will confirm a replacement from it.

Common mistakes at 4.3 kV

• Putting a 4.3 kV fuse on a 4800 V unit. The unit voltage is above the class.
• Assuming 4160 V fits with room to spare. The 110% contingency is 4576 V, so ask DENCO.
• Sizing unit fuses on line current. In a delta bank the unit fuse carries the unit current, which is lower.
• Looking for a C number below 18C in the DNX4 rows. They start at 18C; smaller currents point to DENCO's 4.3 kV Types.
• Asking for a DNXC at 4.3 kV. The DNXC series starts at 8.3 kV.
• Using the wye unit voltage for a delta bank. A 4160Y/2400 V wye bank uses 2400 V units; a 4160 V delta bank uses 4160 V units.

Frequently asked questions