How to Size a Capacitor Fuse

Step 1: find the capacitor unit current

For a single-phase capacitor unit, the current at rated voltage is the unit's kvar rating divided by its kV rating. USDA Rural Utilities Service Bulletin 1724D-112 gives the rule as capacitor current equals kvar per phase divided by line-to-neutral kV, and the example that each 100 kvar capacitor draws 13.9 A on a 7.2 kV system. The same arithmetic gives 200 / 7.2 = 27.8 A for a 200 kvar, 7,200 V unit, and 100 / 14.4 = 6.94 A for a 100 kvar unit at 14,400 V: twice the voltage, half the current.

Use the unit's own voltage rating, not the bus voltage. In a wye bank the units are rated line-to-neutral; in a delta bank they are rated line-to-line. For a three-phase capacitor, one manufacturer's fuse application guide gives line current as kvar x 1000 / (1.732 x V).

If the unit runs at a voltage other than its rating, the current changes in proportion and the kvar changes with the square of the ratio. One manufacturer's medium-voltage bank manual works an example: a 900 kvar bank rated 13.2 kV on a 12.47 kV system delivers 900 x (12.47 / 13.2)^2 = 802 kvar, draws 37.2 A, and its equipment must be sized for at least 37.2 x 1.35 = 50.2 A. The reverse matters more for fuses: at 110% voltage a unit draws 110% current and 121% of its rated kvar.

Step 2: what the protective margin has to cover

Rated current is not the highest continuous current the unit will carry. IEEE's capacitor subcommittee, interpreting IEEE 18-2002 in 2005, listed the contingency limits within which a capacitor must be able to operate continuously: 110% of rated rms voltage, 120% of rated peak voltage including harmonics, 135% of nominal rms current and 135% of rated kvar. It noted that earlier editions allowed 180% current and that IEEE 18-2002 changed this to 135%, and that because units are expected to stay in service up to 110% rated voltage, the fuses should be rated to withstand that overvoltage as well. IEEE 18 was revised in 2012 and 2025; check the edition your specification calls up.

The margin also has to absorb manufacturing tolerance, because a unit's actual kvar can sit above nameplate. Published tolerance figures differ between sources and none of those reviewed for this guide is the IEEE 18 text, so take the tolerance for your units from the capacitor manufacturer.

PJM's shunt capacitor design guideline breaks the margin into parts: overvoltage 1.1, tolerance 1.15 and harmonics 1.1. Added together the allowances come to 1.35; PJM recommends a multiplier of 1.25 rather than that sum, and its unit fuse table assumes 1.25. Multiplied together, overvoltage and tolerance alone give 1.1 x 1.15 = 1.265 before any harmonic current is counted. The point is not which figure is right but that every multiplier is a judgment by its publisher.

Published multipliers, side by side

Each figure below is the published practice of the source named, for its own products or system. None is an IEEE or IEC rule. Pick one method, use the whole of it, and record it in the design file.

Watch for limits that pull the other way. One relay maker's instruction sheet says its unbalance relay coordinates with unit fuses only if a fusing ratio of 1.25 or less is used, so a bank with that relay cannot simply adopt a 1.65 margin.

Step 3: account for harmonics

A capacitor's reactance is 1 / (2 pi f C), so it falls as frequency rises. RUS Bulletin 1724D-112 notes that this is why harmonic currents flow into capacitors. A bank near drives, rectifiers or arc loads can therefore carry more rms current than its kvar rating suggests, and that current flows through the fuse.

One manufacturer's selection guide says that if the application is known to have a large harmonic content, the increase in current should be included in fuse selection rather than left to the standard margin. A plant engineer's guide from another manufacturer gives a field test: if the true-rms capacitor current exceeds the rated current at system voltage by more than about 5%, harmonic voltage distortion is likely present.

Resonance is the larger risk. The same plant guide estimates the parallel resonance order as h = sqrt(kVAsys / kvar) and works an example in which a 350 kvar bank on a system with 17,391 kVA of short-circuit capacity resonates at h = 7.0, right on the 7th harmonic. Where an estimate like that lands near a harmonic the plant produces, the answer is a harmonic study and possibly a filter, not a bigger fuse. IEEE 1531-2020 is the application guide for harmonic filters.

Step 4: derate for ambient temperature

Fuse ratings are published at a reference ambient. The 1.35 margin in published guides assumes 25°C for outdoor unit fuses, and fuses inside a metal enclosure in direct sun see more than that. DENCO's DNXC catalog (p.74) requires derating when the fuse runs above 25°C and prints AT = A25 [1 - 0.0065 (T - 25)], where AT is the rating at temperature T, A25 the catalog rating at 25°C, and T the maximum ambient the fuse will be subjected to at any time.

The factor is 1 - 0.0065 x 15 = 0.9025 at 40°C and 1 - 0.0065 x 30 = 0.805 at 55°C. The derated rating, not the catalog rating, is what must meet or exceed the margined current. The temperature derating article in this guide covers choosing T.

Worked example 1: 150 kvar, 7,200 V unit in a 40°C enclosure

This uses the 1.35 margin from published manufacturer application guides and the DNXC derating formula. Published manufacturer tables place 7,200 V units on 8.3 kV class fuses, so the candidates are DENCO's DNXC8 ratings: 10, 18, 25, 30, 40, 45 and 65 A.

At 25°C the 30 A fuse would have passed. The enclosure temperature moved the selection up one step, and the 40 A choice still has to pass the outrush and case rupture checks in Step 5.

Worked example 2: 100 kvar, 14,400 V unit with harmonics

Published manufacturer tables place 14,400 V units on 15.5 kV class fuses, where DENCO's DNXC15 ratings are 10, 12, 18, 25, 30, 35 and 43 A. Unit current is 100 / 14.4 = 6.94 A. Suppose a measurement shows true-rms current 5% above rated: 6.94 x 1.05 = 7.29 A. With the 1.35 margin, the margined current is 7.29 x 1.35 = 9.84 A.

At 25°C the 10 A rating clears that by only 0.16 A. In a 40°C enclosure it derates to 10 x 0.9025 = 9.03 A, which fails, and the 12 A rating derates to 12 x 0.9025 = 10.8 A, which passes. Small units sit at the bottom of every rating table, where the I2t withstand of the fuse is lowest. One manufacturer's capacitor fuse table notes that for 50 kvar units it is difficult to choose reasonably sized fuses that withstand the I2t outrush, because I2t withstand goes down exponentially with fuse rating rather than linearly.

For clip-style DNX fuses the same margined current is matched to a C number. DENCO's catalog defines its DNX fuses as melting in 1000 seconds at 1.7 to 2.4 times the C number, so a 12C DNX melts at 1000 s somewhere between 1.7 x 12 = 20.4 A and 2.4 x 12 = 28.8 A. The C rating is DENCO's catalog convention, not a standard rating; ask DENCO to confirm the C number for your current.

Step 5: the checks that can override the current calculation

The continuous-current figure sets a minimum. Two transient checks decide whether that minimum is usable. First, inrush and outrush: one fuse maker's application note says inrush lasts less than a quarter cycle and is typically less than 25 times the capacitor's current rating, and a published protection paper requires that the fuse on healthy units must not blow when they discharge into an adjacent faulted unit. The fuse's minimum melt I2t has to sit above those events with room for repetition, because one selection guide notes that each heating near the melting point deteriorates the element slightly.

Second, case rupture: the fuse's total clearing curve must stay to the left of the capacitor's tank rupture curve up to the available fault current. One capacitor maker prints a minimum of 2,000,000 A2s on the tank rupture curves for its standard and heavy-duty units, for fault currents of 10,000 A or less. In the NEC 2023-cycle text, 460.25(D) asks for protective devices rated or adjusted to operate within the safe zone for individual capacitors; confirm the wording against the published edition you work to.

If no rating satisfies the current check, the outrush check and the rupture check together, change the bank, the fuse type or the arrangement. Do not trade one check away.

Common mistakes

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

• Dividing by the bus voltage instead of the unit voltage, which understates the current of a wye-connected unit by a factor of 1.732.
• Mixing methods, for example a 1.65 low-voltage margin on a medium-voltage unit fuse, or adding derating on top of a margin that already assumes 40°C.
• Skipping derating for an enclosed bank because the catalog rating looked comfortable at 25°C.
• Ignoring measured harmonic current, or treating a resonance problem as a fuse-size problem.
• Choosing a larger rating to stop nuisance operations without rechecking the case rupture curve.
• Copying another manufacturer's fusing table onto DENCO parts. Ratings, curves and I2t differ between products.

Frequently asked questions