Sure way to know if the 240

How is a sure way of knowing if the 240 (double breaker) needs both side measured or just double one side?


There are a couple of ways to determine this:

  • If the cable leading out of the load center does not have a neutral connection, that is to say there are only two conductors (excluding the bare copper ground), then you only need to measure one conductor with “double” checked.

  • You can put separate CTs on each of the two conductors and monitor them for long enough to see the appliance run in all modes, then look to see if the two plots are the same on the graph. If they are the same, then you can use on CT with “double” checked.

When in doubt, I prefer to pass both conductors through one CT, reversing the orientation of one. This not only takes all the guesswork out of it, but increases the current sensed by the CT making it potentially more sensitive when measuring low power. The only downside to this is that you need a CT that is rated for the combined amperage of the two conductors where monitoring a single conductor can get by with half the capacity.

Some examples of appliances that are typically true 240V:

  • water heaters
  • well pumps
  • central A/C compressors

And some that are typically not:

  • branch panels
  • ranges
  • clothes dryers
  • air handlers
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