OpenEnergyMonitor Community

CT hole diameter for North America?

TheSetup/Use In North America page state

Typical residential feeder conductors are AWG 2/0 Copper or AWG 4/0 Aluminum. The opening in the standard clip-on CT sensor sold in the OpenEnergyMonitor shop is 13mm, and hence can accommodate a maximum wire size of AWG 1/0.

The AWG page on wikipedia notes 4/0 has a diameter of 11.684mm. I suppose that is too tight for the 13mm constraint, but wouldn’t up through 2/0 work (2/0 has a diameter of 9.266mm)? Thank you.

I think you fail to consider a couple things - the insulation of the wire and the amperage-- the CT at openenergy are 100 amp most newer house in N.A have 200 amp service - though some still have 100 amp if no electric heat … so those CT will work if you have <=100 amp service as that is what I have a home that only uses a 100 amp so OpenEnergyMonitor CT will work fine for me. but not my neighbour who has 200 amp service it will not fit.

If the csa of 107 mm2 is correct, then a solid round bar of that area has a diameter of 11.672 mm. That is without any insulation or protective sheath. A stranded conductor will be slightly larger than this owing to small interstices between the strands. That probably accounts for the size you cite.

I strongly advise that you do not remove the insulation from a main incomer in order to fit a current transformer.

Hi Margaret,

The Outside Diameter (OD) of AWG 4/0 is approximately 0.62 inches (15.75mm).
Definitely too large to accomodate an SCT013 CT.

US homes built before the late 60s were wired with Copper and typically had 100 Amp service, which used AWG 0 copper. An SCT013 would fit Service Entrance Wires in that particular scenario.

Sometime in the late 60s to early 70s, Copper Service Entrance Wires were replaced by Aluminum. Since Aluminum has more resistance per foot than Copper, the equivalent Aluminum wire is two gauges larger than its Copper counterpart. About that same time, 200 Amp service became the norm.

The term 200 Amp service causes much confusion as it’s the rating of the Load Center, (circuit breaker box)
and the Service Entrance Wires vice the maximum amount of continuously available current.

More info on that, here.

Thank you all for the response…

Stephen - thank you for the reply. I did not realize the 100 vs. 200 amp differences. I learned something. Thank you!

Robert - I hear you about without insulation. What strikes me as inconsistent is I used my caliper to measure our 120V cable and w/ insulation it measured 0.3249" / 8.2 mm … which is spot on what the wikipedia entry said was the diameter for 1/0… i.e.: the cable I measured that had insulation had the dimensions stated for 1/0…perhaps some AWG tables list w/ insulation and some without…perhaps the wikipedia entry is wrong, perhaps my measurements are not correct. Most likely it is not a big deal.

Bill - thank you for the pointer to the article. I am not sure where our transformer is located. I haven’t seen one while walking around the neighborhood…i suppose if i am that interested i could contact our energy provider.

It is interested you note the outside diameter of 4/0 is .62" / 15.75mm … the AWG wikipedia entry says 4/0 diameter is .46" / 11.684. perhaps the wikipedia entry is wrong.

So I was thinking…the main breaker has a 90 A rating on it. This makes me think our house is 100A service? Is that correct?

Not necessarily. It means that the cabling etc downstream is protected by a 90 A breaker. Hence the cabling etc between that and the next breaker downstream has to be capable of withstanding a fault current limited by the supply fault level at the place where the fault is, for as long as it takes for this 90 A breaker to trip.

What’s upstream of the breaker is immaterial - as far as the breaker is concerned, except that it has to handle the fault safely.

It might be an underground unit, or it could be a pad mounted unit that’s been overgrown by shrubbery.

That breaker is occupies positions 2 and 4. which means it’s serving as a double-pole branch circuit breaker vice the main breaker. The main breaker should be above or below the group of breakers. Its handle will be much larger than any of the other breakers in the panel. Can you post a picture of the entire panel?

That’s the diameter of the wire itself i.e. sans insulation. Insulation thickness can vary,
but in general 4/0 wire OD is very close to 0.62 inches.

In the UK, the wire csa is standardised, but the overall diameter can vary by a small amount from manufacturer to manufacturer, depending on the combined thickness of insulation and sheath. I presume exactly the same thing happens everywhere.

Out of curiosity, I did a search for AWG 4/0 wire diameter with insulation

This came up:


Hi Bill, Thank you for your replies. I posted images of our panels here: The Wiring in our House | bitknitting in case you have a chance to look at. I am confused where the mains breaker truly is…thank you.

Hi Margaret,

Could you attach your pics to your post? That way there’s no worries if the external site becomes unavailable.
Your post loses its value if that happens.


Is there another enclosure? Perhaps on the outside of the building?

curious your fuse box- in canada the main breaker would need to be right beside the distribution panel-- or with in a few feet. but from your last web picture just follow where that conduit goes. I am guessing your building is using a cartridge fuse perchance

or perhaps your building is on a farm and you have a farm service meter and the main breaker right on the pole or where you find your meter

hope that helps you find your main breaker


Maybe everyone just already assumes this, but what I see is that your main panel, the “original” panel, has nothing connected to most of the branch circuits. The 90A breaker probably (can’t see it all in the pic) goes to the other breaker panel via the generator box which is a transfer switch (among other things). So in normal operation (generator off), the 90A breakers feed the new subpanel where your generator supported loads are. Any loads that remain on the original panel are probably not supported by the generator. Although I can’t read the rating of the remaining breakers in service, several have wires that suggest they are large loads that would potentially overload the generator.

The question remains where the main breaker is, but you should be able to monitor the whole shebang with two SCT019 (YHDC) on the feeds to the original panel… Those should be available soon from OEM with 3.5mm plugs or you can source them elsewhere and attach plugs.

As for monitoring, SCT019s are configured through the browser based menu on IotaWatt, which should also be available soon. There are 14 channels total available on IotaWatt, configurable for a variety of CTs from a WiFi browser based app. Basically plug-and-play.

I came to the same conclusion.

The SEWs enter the original panel from the rear, suggesting an integrated meter base as Stephen pictured in his post above, on the opposite side of the wall from the original load center.