16.3 Capacitors, Transformers Over 1000 V & Medium-Voltage Equipment (Articles 460, 450 & 495)

Capacitors (Article 460)

Power-factor correction capacitors reduce the reactive current a motor or facility draws, which lowers line current and voltage drop. DLI's knowledge areas include ampacity of capacitor circuit conductors and motor circuits including a capacitor.

Conductors (460.8(A))

• Capacitor circuit conductors must have an ampacity of at least 135% of the capacitor's rated current.
• Conductors connecting a capacitor to the terminals of a motor, or to motor circuit conductors, must have an ampacity of at least one-third of the ampacity of the motor circuit conductors, and in no case less than 135% of the capacitor's rated current.

The 135% factor allows for overvoltage, harmonic currents, and manufacturing tolerances that make a capacitor draw more than its nameplate current.

Overcurrent protection and disconnecting means (460.8(B) and (C))

• An OCPD is required in each ungrounded conductor for each capacitor bank, rated or set as low as practicable. It is not required for a capacitor connected on the load side of a motor overload device.
• A disconnecting means is required in each ungrounded conductor for each capacitor bank. It must open all ungrounded conductors simultaneously, have a continuous rating of at least 135% of the capacitor's rated current, and is permitted to disconnect the capacitor as a regular operating procedure. It is not required where the capacitor is connected on the load side of a motor controller.

Stored energy (460.6)

Capacitors operating at 1000 V or less must have a means to discharge the stored energy to 50 V or less within 1 minute after the capacitor is disconnected. The discharge circuit must be permanently connected to the capacitor terminals or arranged to connect automatically when the capacitor is disconnected; a manual means of switching or connecting the discharge circuit is not permitted. A capacitor connected directly to motor windings without a disconnecting means discharges through the windings.

Motor overload with a capacitor (460.9)

When a capacitor is connected on the load side of the motor overload device, the line current through the overload relay drops. The overload rating or setting must therefore be based on the improved power factor of the motor circuit, not the motor's full nameplate current. The 430.22 conductor sizing still uses the motor full-load current.

Example: a motor draws 40 A at 0.80 power factor. A capacitor on the load side of the overload raises the power factor of the circuit to 0.95. The current the overload sees becomes about $40 \times (0.80 \div 0.95) = 33.7$ A, so the overload must be selected for about 33.7 A, applying the 430.32 percentages to that value.

Grounding and marking

Capacitor cases must be connected to an EGC (460.10), and each capacitor must have a nameplate showing the manufacturer, rated voltage, frequency, kilovar or ampere rating, number of phases, and amount of liquid if filled with a combustible liquid (460.12).

Transformers over 1000 V (450.3(A))

Table 450.3(A) sets the maximum OCPD ratings for transformers over 1000 V, as a percentage of rated current:

Where the required rating does not match a standard size, the next higher standard rating or setting is generally permitted under the table notes. The secondary 125% column for 1000 V or less matches the rule for low-voltage transformers in Table 450.3(B).

Indoor dry-type transformers rated over 35,000 V must be installed in a vault, and those over 112½ kVA generally need a transformer room of fire-resistant construction. Oil-insulated transformers installed indoors generally require a vault, with alternatives for less-flammable and nonflammable liquid-insulated units (450.21 to 450.27).

Medium-voltage wiring and equipment

Where the rules are in 2026

The 2026 NEC places over-1000-volt rules in Articles 245 (overcurrent protection), 265 to 268 (branch circuits, feeders, outside circuits, services), 270 (grounding and bonding), 305 (wiring methods), 315 (Type MV cable), and 495 (equipment). Article 495 replaced former Article 490.

Underground cover over 1000 V

The over-1000-volt cover table (Table 300.50 in the 2023 NEC, now in the Article 305 rules) requires, for circuits over 1000 V through 22 kV, 30 in. for direct-buried cables, 18 in. for nonmetallic raceways listed for direct burial, and 6 in. for RMC or IMC. Direct-buried cable over 22 kV through 40 kV needs 36 in. A warning ribbon at least 12 in. above direct-buried cables or nonmetallic raceways is required.

Type MV cable (Article 315)

Type MV cables are rated 2001 V to 35,000 V ac. Shielded MV cable has a semiconducting layer and a metallic shield that must be grounded, and terminations need stress-relief devices, such as stress cones or preformed terminations, to control the electric field where the shield ends. Splices and terminations must be made by qualified persons following the manufacturer's instructions.

Circuit-interrupting devices (Article 495)

Listing (495.2, new in 2026): medium-voltage equipment rated up to 15,000 V must be listed beginning January 1, 2029, and equipment over 15,000 V up to 52,000 V beginning January 1, 2032. The delayed dates reflect the lack of testing laboratories for this equipment.

Worked example: a motor capacitor

A 50 hp, 460 V motor has a Table 430.250 full-load current of 65 A, so its branch-circuit conductors are rated at least $65 \times 1.25 = 81.25$ A (4 AWG copper, 85 A at 75°C). A capacitor rated 12 A is connected to the motor circuit conductors.

1. 135% of the capacitor current: $12 \times 1.35 = 16.2$ A.
2. One-third of the motor circuit conductor ampacity: $85 \div 3 = 28.3$ A.
3. The capacitor conductors must carry the larger value, 28.3 A, so 10 AWG copper (35 A at 75°C) is required.