I have 1 240v circuit on a 20a breaker and am able to run 4 L3+ (one is a ++) and it takes about 15-16 amps which keeps me at or below the 80% rule. I have a metered PDU plugged into the wall (https://www.amazon.com/gp/aw/d/B0052NM4JA) to monitor and ensure I don't go over on amperage.
I am running the stock firmware but under volted them using this tool https://bitcointalk.org/index.php?topic=3546316.0