Do I need an 100A or a 200A current transformer to measure total current draw at my home?

I would like to monitor total current draw at my home from my 200A electrical panel. I have L1, L2 & N (a typical three-wire single-phase distribution in an North American home). Assuming I have a perfectly balanced panel, I would assume that I need only a 100A CT on L1 and a 100A CT on L2. Am I correct?

Since we do not know if it is perfectly balanced, I assume it would be a good idea to put a 200A CT. Please let me now if this is incorrect.

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migrated from electronics.stackexchange.comFeb 14 '14 at 16:56

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Do you mean a traditional two-winding current transformer (a few turns on the primary which is connected in series with the conductor you are sensing) and a secondary with many turns on which you monitor a voltage proportional to the sensed current? Or, do you mean a toroid which slips over the current-carrying conductor and which is wound with many turns in a single winding which outputs a voltage proportional to the current flowing in the conductor? – FiddyOhm Feb 13 '14 at 17:00
Are you not able to access the cable feed before the transformer? – Andy aka Feb 13 '14 at 22:06
According to your title you're doing this to measure the current draw. You don't need that to measure current draw though. – Brad Gilbert Feb 15 '14 at 22:15

If you have a 200 Amp service, I would take that to mean 200 amps at 240 volts - 200 amps on each hot leg.

However, you should check the rating of the main breaker in your electrical panel, and match the current transformers to that.

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Great - thanks for the info. So does this mean that the total current draw can be up to 400A? – Trewq Feb 14 '14 at 1:03
You can draw up to 200 amps on each 120 volt leg. However, the voltage on the two legs are 180 degrees out of phase, so you can't connet them together to power a 300 amp 120 volt load. – Peter Bennett Feb 14 '14 at 5:47

A typical residential 200A service is 200A @ 240V as has been stated. Wattage wise this is the equivalent to 400A @ 120V, but it is never referred to that way. It's just simply not accurate or helpful. Also, while it is hard to get a greatly imbalanced residential panel due to the transient nature of usage, it is almost impossible to be perfectly balanced for the same reason. Simply too many 120V loads. To do what you want you'd need 200A CT's. Although, I'm not sure what you are looking to do. The POCO meter outside will tell you exactly the same thing. Especially with today's smart meters.

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