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Among all these very precise guidelines is one major variable: tow ball load. There’s a convention that sets tow ball load at around 10 percent of the trailer ATM, but this rule of thumb developed in the days of small trailers. It’s still the practice by many North American and Australian trailer makers to specify 10 percent loadings for heavy trailers, but in Europe, where there has been considerable testing done, the tow ball load range is specified in EC E94/20 regulation as a minimum figure of 25kg and a maximum of 100kg, for trailers up to 2000kg ATM. Even above that figure the heaviest gazetted coupling for cars, light commercials and 4x4s is rated at 120kg tow ball load.