Fusing Industrial Power Factor Correction and Motor-Switched Capacitors

Why plants add capacitors, and where they go

USDA Rural Utilities Service Bulletin 1724D-112 lists four consequences of low power factor: voltage regulation problems, increased system losses, power factor penalties in power contracts and reduced system capacity. Power factor is the ratio of real power (kW) to total power (kVA), equal to the cosine of the angle between voltage and current. The same bulletin shows how a penalty can work: billing demand = metered demand x (target PF / measured PF), so 0.95 / 0.83 = 1.145, a 14.5 percent increase. It notes that utilities typically want 90 to 95 percent power factor.

In a plant, capacitors go in one of three places: a bank at the service or main bus (often metal-enclosed, with automatic steps), a bank on a feeder or motor control center, or a capacitor at an individual motor that switches with the motor. The location decides which parts of NEC Article 460 apply, how the capacitor is fused and how it is switched.

The bulletin also cautions that line losses at 80 percent leading power factor are just as detrimental as losses at 80 percent lagging, so overcorrection is not free. One maker's plant guide gives a separate rule of thumb for harmonics: when nonlinear load exceeds 25 percent of the main transformer capacity, filters will almost always be required. That case is covered in the harmonic filter guide.

Worked example: how many kvar

One maker's plant engineering guide gives the correction as kvar = kW x [tan phi1 - tan phi2], where phi1 is the present power factor angle and phi2 the target. Its multiplier table lists 0.316 for raising power factor from 0.87 to 0.97. The arithmetic: tan(acos 0.87) = 0.5667 and tan(acos 0.97) = 0.2506, and 0.5667 - 0.2506 = 0.316. For a 400 kW load, 400 x 0.316 = 126 kvar, and the guide's advice is to use 140 kvar, the next practical size.

The RUS bulletin works a utility-scale version: 1,200 kVA at 87 percent power factor is 1,044 kW and 592 kvar. Adding 600 kvar leaves 8 kvar leading, power factor virtually 1.00, kVA drops to 1,044.1, and current on a 7,200 V system falls from 56 to 48 A. For a motor, the same guide uses hp x 0.746 / efficiency in place of kW.

NEC Article 460, read with a caution

Source caution: the Article 460 wording below is from the 2023-cycle draft text printed in an NFPA public comment report, not from the published NEC. In that text the boundary between Part II and Part III is 1000 V; older editions and some current guides still say 600 V. Confirm every section against the edition your jurisdiction has adopted before relying on it.

With that caution, the draft text includes the following requirements relevant to fusing:

Fuse multipliers you will see, and why they differ

The 135 percent figures in Article 460 are for conductors, disconnects and switches, not fuse ratings. For the fuse itself, published guidance differs by voltage, fuse type and region, and no single figure is a standard's rule. A fuse maker's application note also points out that capacitor fuses are not overload protection: they are there for short circuits and case rupture.

The spread matters because a larger multiplier rides through inrush more easily but clears a failed capacitor more slowly, which moves the total clearing curve toward the case rupture curve. Pick the basis the fuse maker publishes curves for, then check the result against the capacitor maker's rupture data.

• Medium-voltage current-limiting unit fuses: capacitor current x 1.35, a margin covering normal overvoltages, harmonics, capacitor tolerances and a 25 °C ambient (current-limiting capacitor fuse catalogs).
• Low-voltage indoor capacitor fuses at 600 V and 1.2 to 3 kV: 1.65 times the phase current, to cover harmonics and switching currents (one manufacturer's selection guide).
• Medium-voltage capacitor fuses at 165 to 200 percent of capacitor current (one fuse maker's application note).
• Fusing between 1.65 and 2.5 times rated current to protect the case from rupture (one maker's plant engineering guide).
• Rated current about twice the capacitor current (IEC-market fuse guides).

Motor-switched capacitors

A capacitor connected at a motor switches with it, which cuts reactive current in the motor feeder but brings two risks. The first is self-excitation: with too much capacitance, a motor that is disconnected while still turning can act as a generator. One maker's plant guide says to avoid it by not exceeding the maximum kvar specified by the motor manufacturer. The RUS bulletin adds that the maximum capacitor is one not large enough to augment the motor's magnetizing current at any point on its starting and running curve, or the motor may run at subsynchronous speed and draw excessive current, and that if the table value is not a standard rating, the next lower rating should be used.

The second is location. The plant guide recommends connecting the capacitor on the line side of the starter for motors that are jogged, plugged or reversed, multi-speed motors, motors with frequent starts, and high inertia loads where disconnecting the motor with the capacitor can turn the motor into a self-excited generator. If the capacitor sits on the motor side of the overload device, the RUS bulletin notes that either the relay needs to be adjusted or a lower-rated fuse installed, and the Article 460 draft text bases the overload rating on the improved power factor.

Whether to fuse a motor capacitor separately is a design choice. The Article 460 draft text exempts a capacitor on the load side of a motor overload device from the separate overcurrent device requirement, but one maker's plant guide recommends fusing for all indoor applications whenever employees may be working nearby, even on the motor load side.

Worked example: a 4,160 V plant bank

Take a 300 kvar bank on a 4,160 V plant bus, built from three 100 kvar single-phase units connected in delta. One bank maker's instructions state that capacitors connected in delta must be rated at line-to-line voltage, so the units are rated 4,160 V. Unit current is 100 / 4.16 = 24.0 A. Line current is 300 / (1.732 x 4.16) = 41.6 A. The bank is over 1000 V, so the draft Part III rules apply: the switch carries at least 1.35 x 41.6 = 56.2 A and withstands the maximum inrush.

Unit fuse on the 1.35 basis: 24.0 x 1.35 = 32.4 A. Fuse voltage class for 4,160 V units differs between published tables: 8.3 kV in one maker's full-range current-limiting table, 5.5 kV in another maker's combination fuse table. DENCO offers DNX5 at 5.5 kV (6C to 18C and 20C to 75C), DNX8 at 8.3 kV, and DNXC8 at 10, 18, 25, 30, 40, 45 and 65 A. Applying DENCO's DNXC derating formula (catalog p.74), AT = A25 x [1 - 0.0065 x (T - 25)]: the 40 A size carries 40.0 A at 25 °C, 36.1 A at 40 °C and 32.2 A at 55 °C. It fits at 40 °C but falls just short of 32.4 A at 55 °C, where the 45 A size (36.2 A) is the calculated starting point.

Two more checks come from the delta connection. One maker's selection guide explains that delta banks with a single series group are inductively limited, so a failed unit sees fault current limited only by the plant's available fault current; DNX and DNXC are rated 50 kA (DNX as 50 kA asymmetrical). And one relay maker's paper notes that with only one series group, isolating a failed unit causes no overvoltage on the rest, so unbalance detection is not required. One capacitor maker warns that three-phase capacitors fused on only two terminals, applied on delta or ungrounded wye systems, can suffer case rupture from an internal fault from the unfused phase to case, so fuse every phase conductor.

Low-voltage power factor correction

IEEE 18 covers shunt power capacitors rated 216 V and up. At 1000 V and under, the Article 460 draft text calls for discharge to 50 V within 1 minute, and California's low-voltage safety orders (Title 8, Section 2534.6) set the same 50 V within one minute, with a permanently connected or automatic discharge circuit. One manufacturer's low-voltage guidance is that on three-phase capacitors the fuse should have a rating equal to or exceeding the line-to-line voltage.

Low-voltage banks see high inrush. A contactor maker's guide says fixed-bank inrush can reach 30 times nominal current and multi-step banks up to 100 times the nominal current of the step being energized, with transients of 3 to 15 kHz lasting 1 to 2 ms. A fuse maker's note puts PFC inrush at less than 1/4 cycle and typically less than 25 times capacitor current. The same note warns that in parallel low-voltage banks, the parallel capacitors discharge into a failed one and violent case rupture can result, and that individual capacitor fusing eliminates this problem.

DENCO's capacitor protection fuses from 1 kV to 5.5 kV are catalogued as Types 1 to 9, for example 1.5 kV Types 1 and 2 at 25 A to 200 A, 40 kA symmetrical, and 4.3 kV Types 7, 8 and 9 at 6 A to 100 A, 65 kA symmetrical. The catalog describes them as designed to ANSI C37-41 specs and notes they can be designed to customer specifications.

Common mistakes

These show up in plant installations more than any others.

• Treating the 135 percent conductor and switch rule as the fuse rating. It is not; the fuse basis comes from the fuse maker.
• Exceeding the motor maker's maximum kvar on a motor-switched capacitor, risking self-excitation.
• Leaving the overload relay unchanged after moving a capacitor to the motor side of it.
• Fusing only two terminals of a three-phase capacitor on a delta or ungrounded wye system.
• Ignoring enclosure temperature. A fuse that fits at 40 °C can be short at 55 °C.
• Adding fixed capacitors to a plant with heavy nonlinear load without a resonance check.

Frequently asked questions