Capacitor Conductor & OCPD Calculator

The 460.8 135 percent rule

A capacitor draws a current fixed by its reactive rating and the applied voltage, but its actual current runs above the nameplate value. Capacitors are built to a tolerance of 0 to plus 15 percent, so a unit rated for one current can legitimately draw more, and any harmonic content or overvoltage on the system raises the current further because a capacitor's impedance falls as frequency rises. NEC 460.8(A) bounds that combination by requiring the circuit conductors to carry at least 135 percent of the rated capacitor current.

The 135 percent figure is not the 125 percent continuous-load rule used for feeders and branch circuits, and it is not tied to an overcurrent device rating. It is a conductor-ampacity floor specific to capacitors.

Worked example: 50 kVAR, 480 V, three-phase

1. Rated current is kVAR times 1000 divided by the square root of 3 times the voltage: 50000 / (1.732 x 480) = 60.1 A.
2. Required conductor ampacity is 135 percent of that: 60.1 x 1.35 = 81.2 A.
3. In the 75 degrees C column of Table 310.16, 6 AWG copper is 65 A, which falls short of 81.2 A.
4. 4 AWG copper is 85 A, which meets it, so the minimum conductor is 4 AWG copper.
5. Add an overcurrent device per 460.8(B) set as low as practicable, and a disconnecting means per 460.8(C) rated at least 135 percent of the rated current.

Rated current versus kVAR

Capacitors are usually rated in reactive power, kVAR, rather than amperes. To get the current, divide the reactive volt-amperes by the voltage for single-phase, or by the square root of 3 times the voltage for three-phase. Because a capacitor is a reactive device, this is simply its line current at the rated voltage; there is no power factor to apply. If the nameplate lists the current directly, use that value and skip the conversion.

Overcurrent device and disconnect

NEC 460.8(B) requires an overcurrent device in each ungrounded conductor, rated or set as low as practicable, but unlike the motor and continuous-load rules it gives no fixed percentage. Practice sets the device above the 135 percent conductor figure by enough to ride through the capacitor's inrush without nuisance tripping, which is an engineering judgment for the specific bank, so this calculator reports the conductor only and does not print a breaker or fuse rating. NEC 460.8(C) then requires a disconnecting means rated at least 135 percent of the rated capacitor current and able to carry the conductor ampacity.

Capacitors on the load side of a motor

This tool sizes the standalone case at 135 percent of the rated current. Where a capacitor is connected on the load side of a motor running-overload device, NEC 460.8 permits its conductors to instead be sized at one-third of the ampacity of the motor-circuit conductors, since the motor overload also protects the capacitor. That motor-connected case is outside this tool's scope; size those conductors from the motor circuit. Capacitors operating over 1000 volts fall under Article 460 Part II and are also out of scope.

Why this tool may size one step larger than others

This calculator holds the 135 percent figure to its exact value and never rounds it down to the nearest standard conductor. When the required ampacity lands just above a conductor's Table 310.16 rating, the tool steps up to the next size rather than accepting a conductor a fraction of an amp short. That is the never-undersize choice of the accuracy principle this site is built on, not a disagreement about the NEC method.

Frequently asked questions

What size wire do I need for a power-factor capacitor?

Take 135 percent of the rated capacitor current, then pick the smallest conductor whose Table 310.16 ampacity meets it. A 60 A capacitor needs 81 A, which is 4 AWG copper at 75 degrees C.

What is the capacitor 135 percent rule?

NEC 460.8(A): capacitor circuit conductors must have an ampacity of at least 135 percent of the rated capacitor current. The margin covers the 0 to 15 percent manufacturing tolerance plus the extra current under harmonics or overvoltage.

How do I size a capacitor conductor from kVAR?

Convert kVAR to a current (kVAR times 1000 divided by the voltage, or by the square root of 3 times the voltage for three-phase), then apply the 135 percent rule and read the conductor from Table 310.16.

What size overcurrent device does a capacitor need?

460.8(B) requires an overcurrent device in each ungrounded conductor, set as low as practicable, but assigns no fixed percentage, so this tool sizes the conductor only and leaves the device rating to the designer.

Does the capacitor disconnect have to be 135 percent?

Yes. 460.8(C) requires the disconnecting means to be rated at least 135 percent of the rated capacitor current and to carry the conductor ampacity. Size it separately from the conductors.

Related calculators

• Wire Ampacity Calculator: the Table 310.16 ampacity of a chosen conductor, with ambient and fill derating.
• Wire Size Calculator: size a conductor for a load by ampacity and voltage drop together.
• Motor Breaker Sizing Calculator: branch-circuit conductor and short-circuit protection for a single motor per Article 430.
• Transformer Sizing Calculator: rated current and maximum overcurrent device for a transformer per Table 450.3(B).