How many 400 watt Halide Light fixtures should I put on a 20 amp 120 volt circuit. Four?
Thanks, Guy

The 20 AMP breaker usually only rated about 80% of full capacity, therefore, the calculation is:

20 amp * 120 V * .8 = 1920 Watt

1920 /400 = 4.8

You are correct.

Need to be aware that a 400 watt Metal Halide fixture draws more than 400 watts, anywhere from 430 to 480 watts, due to the ballast load. So better to use the rated amperage draw of each fixture to calculate how many fixtures on a circuit.

With the four you have still fits on a circuit, even at 480 watts each, just be aware of the total load if more are needed.