You're right, the wording can be a bit confusing. Let's break it down: "Wireways shall be supported where run horizontally at each end..." This is straightforward: always support at each end. "...and at intervals not to exceed 1.5 m (5 ft)..." This is your primary support interval. You MUST support the wireway at least every 5 feet. "...or for individual lengths longer than 1.5 m (5 ft) at each end or joint..." This reinforces the end/joint support requirement. If a single section of wireway is longer than 5 feet, you MUST support it at the ends (or at a joint). "...unless listed for other support intervals." This is an exception. If the wireway is listed (meaning a testing lab has approved it for a different support spacing), then follow the listing instructions. "The distance between supports shall not exceed 3 m (10 ft)." This is a maximum allowable span. You can't exceed 10 feet between supports, even if you could theoretically go longer based on the 5-foot rule.In summary, you must follow the stricter requirement

Support at each end. Support at least every 5 feet.

Never exceed 10 feet between any two supports.

Think of it this way: 5 feet is the typical spacing, and 10 feet is the absolute maximum spacing. You must always meet the 5-foot requirement, but in no case should the distance between supports be longer than 10 feet.

Example

If you have a 12-foot run of wireway, you would need at least three supports: one at each end and one in the middle. This gives you three supports for the entire 12 foot span, at intervals of 6 feet, which is acceptable because it does not exceed 10 feet.

Important Note: Always consult the manufacturer's instructions and any applicable local codes, as they may have more stringent requirements. If the manufacturer's instructions specify even closer support intervals, you must follow those instructions. Flag for review