Can this device be used to monitor the power used by my car charger which operates on 220 without a neutral ?

Thanks for the inquiry. Yes you can do this with 2 hots and no neutral. Wire it this way: With the Omnimeter you will use only 1 CT and not 2, since you have 2 hots (Line1 and Line2) and no neutral. Connect CT1 into terminals 1 and 2, and run Line1 through CT1. Tap off of Line1 wherever is convenient and connect this voltage reference wire to terminal 7 (L1) on the meter. Tap off of Line2 wherever is convenient and connect this wire to terminal 10 (N) on the meter. That's it and you are good to go. We do recommend 1A fuses on L1 and L2. Thanks. Seth, EKM Metering

what size wires/how many wires will pass through the donuts? I want to meter about 4 separate 20 amp circuits.

Thanks for your inquiry. We carry many different sizes of Current Transformer "donuts" to accommodate different sizes and amperages. These are all sold separately: https://www.ekmmetering.com/collections/current-transformers-cts If you want to have multiple individual readings for multiple individual circuits then you will need separate meters for each. However, if you want a total reading for all the circuits combined you can use one meter and run all the hot wires (from the same phase/source/leg/etc.) through their own CT - just make sure the diameter of the CT hole is large enough to accommodate your wires. Alternatively you can use separate CTs for each hot line and combine them into the same ports on the meter. Here is how you can pass multiple wires from the same phase through a current transformer: https://help.ekmmetering.com/support/solutions/articles/6000131301-can-i-pass-multiple-wires-through-a-current-transformer- And here is how you can use multiple current transformers on the same phase: https://help.ekmmetering.com/support/solutions/articles/6000131304-can-i-use-multiple-current-transformers-on-the-same-phase- More info you will need to know about this meter and its options: https://help.ekmmetering.com/support/solutions/articles/6000067374-ekm-omnimeter-i-v-3 Best regards, Seth EKM Metering Inc. www.ekmmetering.com www.encompass.io

Does this unit ever have to be calibrated to keep it's accuracy?

To my knowledge, no, it does not have to be recalibrated. We been using these meters for 3 or 4 years and have not had any call backs for a single unit that we've installed.