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Run, Start & Dual Capacitors Explained

How motor run, start and dual-run capacitors differ, how to read their ratings and terminals, and how to match a replacement safely. Capacitors store energy and must be discharged before handling.

Ref HV-0003EIA-456-AUL 810

Safety-critical sheet, not yet signed off by an expert reviewer. Use it to understand the topic, then take every value and decision from the governing standard, the manufacturer’s instructions and a qualified person.

What it is

HVAC motors that run on single-phase power use capacitors to create a phase shift so the motor starts and runs efficiently. A run capacitor stays in the circuit continuously. A start capacitor is switched in for a moment to boost starting torque and then removed by a relay or a device such as a PTC. A dual run capacitor combines two run capacitors in one can with a shared common terminal, one section for the compressor and one for the outdoor fan.

How to identify

Case: run capacitors are usually metal (oval or round) and oil- or film-filled. Start capacitors are usually black plastic, electrolytic and smaller for their microfarad value.
Label: shows the capacitance in microfarads (µF or MFD) with a tolerance, the AC voltage rating, the frequency, and sometimes "protected" (an internal pressure interrupter).
Dual terminals: C (common), HERM (compressor) and FAN. The label shows two values, for example "45 + 5 µF", with the larger value on HERM.
Start capacitors usually show a µF range and may have a bleed resistor across the terminals.

Capacitor types

Type	Duty	Typical construction	How it is rated	Replacement rule
Run (single)	Continuous	Metallized film in oil or dry, metal can	µF ± tolerance, VAC	Same µF within tolerance; equal or higher voltage
Dual run	Continuous, two loads	Two film sections, shared common	Two µF values, VAC	Both sections must match, or use two singles wired correctly
Start	Intermittent, seconds only	Electrolytic, plastic case	µF range, VAC	Same µF range; equal or higher voltage; matched to its start relay
Hard-start kit	Intermittent	Start capacitor plus relay or PTC	Matched to a compressor size range	Use only where the compressor manufacturer allows

Key points

Microfarads must match. Changing µF changes motor current and torque, and can overheat windings.
Voltage can go up, never down. A higher voltage rating at the same µF is acceptable. A lower voltage rating is not.
Never use a start capacitor as a run capacitor. Electrolytic start capacitors fail if left energized.
A swollen or domed top, leaking oil, or a reading outside tolerance means replace it.
Universal and multi-tap replacement capacitors are common. Make sure the technician wires the right taps.

Safety

Capacitors can hold a dangerous charge after power is removed. Technicians should lock out power, wait, and discharge the capacitor with a proper resistor-type tool before touching the terminals. Never short the terminals with a screwdriver. Never test with power applied unless the meter and procedure are rated for it.

Common mistakes

Swapping HERM and FAN leads on a dual capacitor.
Substituting a "close enough" µF value.
Selling a lower-voltage capacitor because it is the only one in stock.

Governing standards: EIA-456-A (metallized film motor-run capacitors); UL 810 (capacitors); motor and equipment manufacturer data; NFPA 70E and 29 CFR 1910.333 for safe work on de-energized equipment.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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