Tools
Circuit loading and breaker sizing
Fixture count per circuit at the 80 percent continuous-load limit, which is the number the electrician is actually working to.
This is the question that comes back from the field more than any other: how many of these will go on one circuit. Input watts is the figure to use, not the wattage in the fixture name and not the LED module wattage. A driver has losses, so the input watts published on the specification sheet is always the higher number, and it is the one the circuit sees.
The answer is deliberately conservative on two counts. It loads the breaker to 80 per cent because lighting normally runs three hours or more, and it ignores inrush, which is the other reason a circuit that passes on paper still trips at switch-on. If the count matters, tell me the fixture and I will get the published inrush and the manufacturer maximum per breaker before anyone commits to a panel schedule.
Before you rely on it
This is arithmetic, not a photometric calculation. It will get you to a sensible starting point and tell you when something is obviously wrong, but a real layout gets run in AGi32 against the actual IES files. Send me the plan and I will run it properly.