I have a moderately-sized home (~2400 sq. ft.) in the United States that was constructed in 1999. I am looking to build a home energy monitor to track my electricity usage versus time. Obviously, in order to do so, I need to get some reading of the current consumption of my home. To do so, I would like to use a device like this:

enter image description here

This is a current transformer that you clip around the outside of the (insulated) conductor. It outputs a voltage proportional to the current that passes through the conductor; that will then be monitored by some electronics that I'm building. What I haven't figured out yet is how best to install the sensor; I'm very comfortable with electronics, but I don't have much experience in building electrical wiring (beyond some very simple rerouting of existing circuits).

From what I gather, my home has two 120V phases that come into my breaker box like this:

enter image description here

I've been hesitant to open things up without knowing exactly what I'm doing, but I think it will be required to install the sensors (I believe I will need one on each phase). I've verified that the run from the outdoor meter to my breaker box is all conduit (which I've read is required by modern building codes), so I don't have a point where I can access the hot phases unless I install the sensors inside the box. I would likely then just route the signal wires up through one of the box ports and plug it into my electronics package nearby the breaker box.

Apart from my hesitance due to my lack of experience with the interior of my breaker box, the other detail that I'm not sure of is what thickness of wire is typically used for the inputs to the main breaker (labeled B in the above diagram). I need to ensure that the sensor I select has an opening large enough to accommodate the insulated conductor (for instance, the probe I linked indicates that its opening is 13x13 mm). I'm sure there isn't a single answer for all cases, but is there a good rule of thumb for what my home might have based on its characteristics?

  • 1
    What type of service do you have? Look at the amp rating on the main breaker. 60, 100, 200, etc.
    – Steven
    Jan 17, 2014 at 3:22
  • Looks like the main breaker is 200 A (it has a 200 on the switch). I guess that would disqualify the sensor I linked, as it's designed for up to 100 A. Unless I could get away with it since there's one on each phase.
    – Jason R
    Jan 17, 2014 at 3:26
  • First I don't see a main breaker so there should be one at the meter that feeds this and you could turn it off there. 2nd the CT is insulated as are the wires you want to clip them around. My only caution other than not touching a metal conductor is to not touch the male connector of the CT (in my opinion this should be a female) CT's do provide a easy way to monitor power ussage but the voltage can be many times higher than the supply. Had a friend find the wrong way off a roof when messing with the CT that powered his meter.
    – Ed Beal
    Jan 3, 2018 at 9:26

3 Answers 3


In the 2011 version of the National Electrical Code, there was Table 310.15(B)(7). This table was quite handy for determining the service conductor size. In the 2014 version, it looks like they've done away with this table. However, the long code trail that has replaced it, seems to basically say the same thing.

National Electrical Code 2011

Article 310 - Conductors for General Wiring

II. Installation

Table 310.15(B)(7)

Typical services (in the US) are 100, 150, or 200 ampere (though 60 is common for older dwellings). Which means you'll be dealing with a possible range from 4 AWG copper to 2/0 copper (2 AWG al to 4/0 al). Which means you'll want a device that can accommodate conductors from about 0.3 in. diameter to about 0.7 in. diameter, if you want to cover the whole spectrum of possibilities.

  • Don't forget that the service conductors can be derated so they can be smaller than the table size.
    – Ed Beal
    Jan 3, 2018 at 9:13

Consider consulting a licensed electrician (I'm not). Iirc, the electrical code typically calls for #3 or #4 gauge wire on each phase for 100Amp service.

You'd likely need one sensor on each phase and keep in mind that you may end up double counting current on 240v appliances though with correct impact on your bill.

If installing inside the breaker box (check local elec code) keep in mind that you'll be doing it on the hot side and the wires will remain energized unless you remove the meter (re-installing meter usually requires a permit and inspection).

Finally, if you have a smart meter, you utility company may have a way for you to monitor you live usage.

  • Also if you have an external cut off switch near connected between your meter and your breaker box, you can use it to de-energize the breaker box.
    – Dan D.
    Jan 17, 2014 at 5:52
  • 1
    What code says #6 is good for 100 amp service? #6 for the neutral maybe, but not the ungrounded conductors. Can you please site your source?
    – Tester101
    Jan 17, 2014 at 11:58

You have twice confessed your hesitance regarding your electrical box in your question. This to me is a RED FLAG! Recently I replaced all the outlets and most of the light switches in my condo. But when it comes to the panel box, I instead paid an electrician $1300 to replace that 2 1/2 years ago. The worst thing that could happen with replacing the outlets and switches is I could get shocked (provided I’m dry). That’s because I was dealing with only 120 VAC. But with the panel box, you are dealing with 240 VAC, which can kill you even if you are bone dry! The rule is, NEVER (0%) mess with the panel box unless you know ABSOLUTELY (100%) what you are doing in advance, without any hesitation whatsoever.

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