How to Read a Run Capacitor: MFD, Voltage, and Why 'Close Enough' Isn't
4 min read · from the Cold Mountain Supply counter
The run capacitor is the most-replaced part in residential HVAC, and summer is its season. When a condenser hums but won't start, or a fan motor sits there cooking itself, the round or oval can wired to the contactor is the first suspect.
Two numbers matter: microfarads (MFD or µF) and voltage. The MFD rating has to match the motor it serves — printed on the failed cap and usually on the motor label. The tolerance is typically ±6%, which is the manufacturer's tolerance for a new part, not permission to substitute a different size. A 40 MFD cap on a motor that calls for 45 runs the winding hot and shortens its life; oversizing does the same job on the start winding.
Voltage is a floor, not a target. A 440V capacitor on a 370V application is fine — the reverse is not. Most techs stock 440V rounds for exactly this reason: one can covers both calls.
Dual capacitors serve the compressor (HERM) and the fan from one can, labeled like 45/5 — 45 MFD for the compressor, 5 for the fan. Replace like-for-like on both numbers. If the exact dual isn't on the truck, two single capacitors wired correctly do the same job and get the customer cooling tonight.
Test with the capacitor discharged and a meter that reads microfarads. Anything reading more than the tolerance below its rating is done — a cap at the edge in May is a callback in July. At wholesale prices there's no reason to leave a marginal one in the cabinet.
Everything this guide mentions is on the shelf — same-day will-call pickup, wholesale pricing with a contractor account.