Factory charge and additional charge
Split-system air conditioners and heat pumps ship with a factory refrigerant charge sized for a standard line set length, typically 15 to 25 feet depending on the manufacturer. When the installed line set is longer, additional refrigerant must be added at a rate specified in the installation manual, measured in ounces per foot of liquid line.
The additional charge depends on the liquid line diameter (usually 3/8 inch or 1/4 inch) and the refrigerant type. R-410A systems commonly require about 0.6 ounces per foot of 3/8-inch liquid line.
Additional charge = (Actual length - Factory length) x Charge per foot
Charge per foot is specified by the equipment manufacturer in ounces per foot of liquid line.