Capacitor Fuse RFQ Checklist: What DENCO Needs to Quote

Why a complete request saves a round trip

A capacitor fuse is sized against the capacitor, not against a load. The same 200 kvar unit can need different fuses depending on how the bank is connected, how much fault current is available, how many units sit in parallel and how hot the enclosure gets. If any of that is missing, the first reply to an RFQ is a list of questions.

The table below lists what to send and why each item matters. You do not need every row to start a conversation, but each missing row is something DENCO engineering has to ask about before a firm quote.

It also matters who is asking. A utility replacing fuses in a pole or substation bank, an OEM building metal-enclosed equipment, and a plant engineer adding power factor correction each hold different parts of the data. Send what you hold, name who holds the rest, such as the capacitor supplier or the engineer who owns the protection study, and DENCO engineering can ask the right person directly.

The checklist

Send as much of this as you have with your request.

The electrical data in more detail

Capacitor current is the base of every sizing method. For a single-phase unit it is the unit kvar divided by the unit kV; the USDA Rural Utilities Service bulletin gives the example that each 100 kvar capacitor draws 13.9 A on a 7.2 kV system. Fuses are then applied with a continuous current margin. One manufacturer's capacitor fuse selection guide calls this the fusing factor and puts it typically in the range of 1.3 to 1.65 per unit, to cover kvar tolerance, harmonics and continuous overvoltage. Application guides for full-range current-limiting capacitor fuses use 1.35, and other published guidance runs to 1.65 or 2 times for low-voltage and IEC-market fuses.

Because those figures are practices of particular manufacturers and regions, not a standard, DENCO asks for the raw data rather than a target amp rating. If you have done your own calculation, send it with the inputs behind it.

Voltage matters twice. The unit voltage sets the fuse voltage class, and any operation above nameplate voltage raises the current: one manufacturer's formula sheet gives actual kvar as rated kvar times the square of actual over rated voltage. Tell DENCO if the bank runs at a voltage different from the unit rating.

Bank connection, fault current and stored energy

One manufacturer's selection guide explains that in ungrounded or multi-series-group banks, faults are limited by the capacitors, with fault current typically less than 2 or 3 times the bank load current, while in single series group grounded wye or delta banks the fault current is limited only by the available system fault current. The first case asks the fuse to clear low currents; the second asks it to interrupt high ones.

Parallel units matter for energy. When a unit fails, the energy stored in its series group is available to dump into the failed unit and its fuse. The same guide calculates that energy at 3.19 joules per kvar, from the parallel capacitance charged to 1.1 times the crest of rated voltage. For a group of 12 units of 200 kvar, the 11 healthy units hold about 11 x 200 x 3.19 = 7,018 joules. Adjacent banks on a common bus add to it, as a regional transmission organization's capacitor guideline points out.

Telling DENCO how many units are in parallel, and whether other banks share the bus, lets that energy be checked against the fuse and against the capacitor's case rupture limits.

Switching and harmonics

Back-to-back switching produces high-frequency inrush. One switchgear manufacturer's technical FAQ puts the frequency for back-to-back banks in the range of 2 kHz to 30 kHz, depending on the banks' size and configuration and on any limiting impedance. If the bank has inrush or outrush reactors, include their values. A university design standard in this guide's sources asks bank submittals to state peak inrush, rms inrush, inrush frequency and duration, which is the same information a fuse reviewer wants.

For harmonic filter banks, send the tuning and the expected harmonic currents. For plain power factor correction banks in plants with large drive or rectifier loads, send the supply transformer kVA and impedance and the available short-circuit kVA: one manufacturer's plant guide estimates the parallel resonance order as the square root of system short-circuit kVA divided by bank kvar, and a resonance near a characteristic harmonic changes the fuse duty. If you only have a power quality survey, send that.

Mechanical and environmental details

The DNX is clip style, with threaded end studs available. The DNXC mounts from a bus bar to the capacitor lead. Photos of the existing mounting and a note of the clearances answer most mechanical questions at once.

If the enclosure is new or being rearranged, say so and give the space available per fuse. The DNX body diameter printed in the DENCO catalog is a maximum, and DNX and DNXC dimensions should be confirmed with DENCO before the layout is fixed. If the bank's inspection routine relies on seeing which fuse has operated, ask for a blown fuse indicator; DENCO's C-rated fuses are offered with or without one.

Give the maximum ambient the fuse will see inside the enclosure, not the outdoor design temperature. The DNXC derating in the DENCO catalog is AT = A25 [1 - 0.0065 (T - 25)], based on the maximum ambient the fuse will see, so a 45 A DNXC carries the equivalent of 36.2 A at 55 °C. One capacitor maker's manual for metal-enclosed banks recommends shading where daytime temperatures exceed 40 °C and has fans start at about 85 °F, which shows how warm these enclosures are expected to run.

Example: a complete request

The request below would let DENCO engineering propose a rating without a round of questions. The figures are illustrative.

• Application: new metal-enclosed power factor correction bank, 13.8 kV, ungrounded wye, one series group, 7960 V 200 kvar units, 3 units per phase.
• Unit current: 200 / 7.96 = 25.1 A per unit; our calculation used a 1.35 margin, 33.9 A at 25 °C.
• Fault current at the bank: 18 kA symmetrical, X/R 12, from the attached study.
• Switching: single bank, vacuum switch, inrush reactor 50 microhenries per phase, no other banks on the bus.
• Harmonics: drive load about 10% of transformer capacity; power quality survey attached.
• Enclosure: maximum internal ambient 50 °C; bus bar to lead mounting preferred; blown fuse indicator required.
• Attached: capacitor data sheet with case rupture curve, single-line drawing, protection study reference.
• Commercial: 9 fuses plus 3 spares, required on site in 6 weeks, no third-party testing.

Common mistakes in capacitor fuse RFQs

These are the gaps that most often delay a quote.

• Sending only a target amp rating, without the capacitor voltage, kvar and connection behind it.
• Giving system voltage but not the unit voltage, which leaves the fuse voltage class open.
• Quoting fault current without saying whether it is symmetrical or asymmetrical.
• Giving outdoor air temperature instead of the maximum temperature inside the enclosure.
• Leaving out adjacent banks on the same bus, which add parallel energy and back-to-back inrush.
• Sending a part number typed from memory instead of a photo of the label.

What DENCO sends back

With a complete request, the reply is a proposal rather than a questionnaire. Expect the proposed series and rating, or a note that the catalog does not cover the duty and a custom design is needed. Where a DNXC is proposed in a warm enclosure, the reply should show the derated rating against your calculated minimum, so you can see the margin.

Ask for the time-current curve of the proposed rating if you need to plot it against the capacitor's case rupture curve or upstream devices. DNXC curves are on DENCO catalog p.73; for DNX ratings, ask for the curve that matches the C number proposed.

Commercial terms follow the same request: quantity, spares, the 30-day lead time DENCO quotes on all products, and any custom design or third-party testing scope. If something in the proposal does not match your study, reply with the study page; DENCO engineering can work directly with the engineer who signs it off.

How to send it

Use the quote form on the contact page, or call DENCO at 1-800-305-FUSE (3873) or (305) 273-6226. Attach drawings, nameplate photos and studies to the request. If the data is incomplete, send what you have and say what is missing; DENCO engineering will ask for the rest.

Frequently asked questions