Large Number of Breakers

I have a breaker box with my mains coming in plus 28 breakers. Would I have to get 3 IoTaWatt’s to measure all of my circuits at once vs. some type of expansion module?

This question comes around every couple of months, and I usually take the opportunity to talk a little about monitoring in general.

Typical 200A US split-phase load centers frequently have a lot of circuits. Twenty-eight breakers is not a huge number. The question is how many significant loads are there? I’ve got thirty-seven breakers including the 8 in a branch panel right next to the main panel. With one IoTaWatt, I get all of the data I care to see.

The first thing to know is that once you measure the mains to get total power used, you can use an output script to subtract all of those measured circuits from the total to show the amount of power not directly measured. That is essentially a 15th measurement. If you take a hard look at those 28 circuits, you will find that some are low-use and/or low-power and don’t really warrant direct measurement, especially if constant.

Next are your 240V circuits. These are two circuit breakers but can each be monitored with one CT. The two wire (no neutral) circuits like pumps, water heaters, and AC compressors need just a 50A on one of the wires. Three-wire (with neutral) like dryer and range can be monitored by passing both of the hot conductors through the CT in opposite directions. See

One input on an IoTaWatt can be used to monitor multiple circuits. You can run multiple circuits through one CT, and you can combine multiple CTs into one input. My kitchen has two plug circuits, a microwave circuit, and a disposal circuit all on one CT labelled “Kitchen”. The wall oven and partial electric cook top, 240V circuits, are combined in one CT labelled “Oven/Stove”. Those two inputs cover 8 circuit breakers. with two CTs.

My 8 breaker branch panel has circuits for the bedrooms, bathrooms and a family room. None of them amount to much, and I can easily see in the graphs where a hair-dryer is used or when the family room is in use with a TV and lots of lighting. Otherwise, it’s negligible. Here are the last 24 hours. You can see the hair dryer spikes and that the family room ran from about 9pm-11pm, otherwise it was 175 watts pretty constant. That’s 2/3 of the load.

Experience tells me that there can be too much data. six or eight really significant loads will probably make up 80% of your consumption. That leaves 4-6 other inputs plus the “unmonitored”.

My advice would be to take a picture of your panel, and then break it down into the 240V loads, and individual groups of 120V circuits that might make sense to group together. Work out a plan for one IoTaWatt, install it, and see what it tells you over the first few weeks. I think you will be surprised at how well it tells your energy usage story.