New order for 400 amp service

Hello! I’m in the process of revamping my home automation stuff and am looking to add IoTaWatt to the mix. I have a 400amp utility feed which breaks apart at the meter to two 200amp panels that are side by side. What I was trying to discern is for my mains would I use 100amp CT’s or 200amp CT’s? Given I’m in the US and using split phase 240 my initial thought was 100amp CT’s since each leg is 100amps but wanted to verify my logic was correct.

Also, I have a few sizable circuits. 100amp going to a car charger; 80amp feeding a garage sub-panel which also has car chargers; and 100amp going to a generator sub-panel/transfer switch. Am I right that I can loop both legs on one CT for that?


You would need 2x200A CTs for the breakers in each panel. A 200A panel is 200A each leg.

If a circuit has only two wires (no white/neutral) you can use just one CT with a rating equal to or greater than the breaker.

If it is a three-wire circuit, you would use one CT on each of the red and black wires. If you use a single CT with both wires running through in opposite directions, the CT must be equal to or greater than the breaker amps x 2.

Thanks! I’m glad I asked!