I have a single-phase, 240V, 40A circuit (on a 50A breaker) connected to a NEMA 14-50 plug (these are words my electrician used when I asked him). I use this to charge my electric car. My HOA has asked me to meter my charging usage, for which it seems there are few good options and almost zero inexpensive options.

So I bought an Efergy Engage Hub Kit. Here's a link if you're interested but basically, it's a meter that provides a set of "clamps" that clamp around wires that need to be metered. Unfortunately there are no instructions so I took a stab in the dark. Of the three wires running to my NEMA outlet (red, black, beige) I clamped my meter around the red wire. The readings look ...reasonable but the kit came with an additional clamp (I bought the "single phase" version of the kit) so now I'm wondering if I need to use it.

Specific Question: For someone familiar with this kind of meter, do I need to also have a clamp around the black wire running to my NEMA outlet or is it sufficient to have it on the red wire only?

  • Right off the bat, the Efergy units appear to not be UL listed, so they should not be installed in North America. UL approves the instructions as part of the listing, so they must exist. Nov 17, 2020 at 9:27
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    The unit does not make an electrical connection it is an inductive pickup. The sensor appears to be battery operated. Nov 17, 2020 at 9:32
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    @Harper-ReinstateMonica Yes there technically are "instructions" but they are hilariously vague.
    – medley56
    Nov 17, 2020 at 15:02
  • @TedMittelstaedt Understood. I'm trying to figure out if I need to clamp around both hot wires or if one is sufficient. It sounds like I need to put a sensor on both the red and the black wires.
    – medley56
    Nov 17, 2020 at 15:03
  • Search the device itself for a logo that says UL, CSA, or ETL all in a circle, and a 5-8 digit file number next to that. They may also be a C and/or US. Does it have that? If it does not, and especially if the biggest marking is CE, then send it back. It's illegal to sell or use in North America. Even if it came via Prime :( Nov 17, 2020 at 17:02

2 Answers 2


The manual is absolutely nuts! There is absolutely no explanation of the wires to be monitored except a series of simplified diagrams of different panel types. And what color do they use to indicate the wires to be monitored? Black? Red? No! Green! I'm sure they picked green to match their logo and because "environmental", but green is ground, the world round. If someone decided to actually put a monitor on their ground wire they would (hopefully) measure nothing. Argh!!!!

That being said, it is clear enough that you need to monitor 1 wire if there are 2 (the second would either be neutral or a second hot, and either way the same current as the first wire) or 2 wires if there are 3 (i.e., the 2 hots but not the neutral). Your red, black, beige is really red, black, white, with red & black being the hot wires and white the neutral wire.

Put clamps on the red & black.

In reality, since this is a car charger, you should have nearly all usage at 240V. The only thing running at 120V (if anything) would be some minimal electronics controlling the charging. But if there were anything significant on white, which there normally would be if these three wires were going into a breaker panel rather than into a single receptacle, the current on white (neutral) would be the difference between the black & red (hots) and the monitoring system would handle it all correctly.

The last step is that the monitoring system needs to "know" what type of circuit you are using. If the system is smart enough (can't tell from the minimalist manual) then it should actually be able to figure out that the two clamp sensors are two halves of a 240V circuit. But I would suggest seeing what the system shows when you are actually charging. There will be some minor differences, but if you charge up your car 50 kWh, you should see very close to that amount (slightly higher due to conversion losses) recorded on the monitoring system.

From a deleted-answer-that-should-have-been-a-comment:

I'm curious why the HOA is asking this. What business is it of theirs?

That is actually pretty straightforward to me. If the charger is powered from a shared garage or parking lot, the power is paid for as part of the condo. fee (or equivalent - terminology varies). Everyone needs the lights and the access system, so the electric bill for the shared areas gets split evenly across all the units served, or by square footage of individual units or some other reasonable method.

A car charger, used by one unit out of many, drastically changes things. While an electric car should cost less in the long run than a gasoline powered car, the energy isn't free. Tesla started with free supercharging to get people to buy their cars. Once sales picked up, they dropped the free supercharging because people would buy cars anyway, knowing that a stop at a supercharger would be comparable to buying a tank of gas. A 50 kWh charge (1/2 to 2/3 of a typical Tesla battery capacity) typically costs $0.10 to $0.20 per kWh = $5.00 to $10.00. Not a big deal if it is once in a while, but if someone charges that much 3 times a week, it could be as much as the entire rest of the electric bill for lights, security system, etc. The HOA is right to be concerned and to want to manage the costs equitably.

If there is only one car charger then it is actually still pretty simple - subtract last year's usage from this year's usage (seasonally adjusted by matching the month) and you have a reasonable approximation. But once a second car charger is added - and one user charges daily for 100 mile round trip, but the other charges less frequently because they have a short commute, etc. - it becomes impossible to guess.

However, in addition to the complications that OP has found trying to DIY an electric meter, there are potentially regulatory issues involved in submetering. That becomes a legal issue for the HOA. I am sure we'll be hearing more of such things over the coming years as car chargers become more common.

A fantastic technical solution would be for the charger itself to monitor usage and report it. I suspect that some indeed do that, but of course that is a feature not needed for a charger at a single family home or provided by an employer for convenience of their employees, so manufacturers will likely not include such a feature unless/until there is a demand. (Which gets into peak demand pricing, but I digress.)

Use At Your Own Risk

To follow up on @Harper's excellent (as usual) rant answer, there are serious legal and possibly safety issues with this product. From the FAQ page:


There is no need for an electrician to do this power consumption meter install, as we are not disconnecting any wires. Please take care when removing any covers in your fuse circuit box. There are live cables here, either 110V or 240V or if using 3-phase power possible 415V. This is dangerous and electricity can kill so be careful. It’s a great idea to isolate all power beforehand from the main circuit breaker.

This has numerous red flags:

  • I suspect they recommend against having an electrician install because a licensed electrician is likely to tell you to return the product!
  • Nothing should ever say "110V or 240V" - be consistent. Either "120V or 240V" or "110V or 220V".
  • It's a great idea to isolate all power beforehand from the main circuit breaker. Your average homeowner will have no clue what that means. It is CYA-ish language, at best.
  • fuse circuit box is very old terminology, at best. In fact, the diagrams I saw in the manual all appear to (rightly so) show modern-style circuit breaker panels. There are still plenty of fuse boxes around (typically in houses more than 50 years old that haven't had major electrical renovations) but that is poor terminology choice.

In fact, aside from the legal issue, there are real safety issues. If you know what you are doing, putting clamps on the wires is a trivial process. But when doing so in the panel you are literally inches away from unprotected (or overcurrent but not GFCI-protected) wires. So this is an inherently dangerous activity. In fact, the specific type of activity where I would, personally, recommend that most people call an electrician rather than DIY - as opposed to replacing switches, receptacles, etc. where DIY can be done quite safely with minimal training.

My description about (clamp red & black, why the HOA wants it monitored, etc.) all still apply 100%. But beware of any product that not only isn't properly listed but appears to care very little about safety. It is possible to have a product, especially one-off or niche products, that are designed well and 100% safe, just not listed/certified due to the expense involved. This does not appear to be one of those products.

  • Thanks for the HOA explanation. I hadn't thought about a shared-use charger, my assumption was one in the homeowner's garage.
    – FreeMan
    Nov 17, 2020 at 15:50
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    Your comments about HOA concern are spot-on. One annoying aspect is that we have people running full on woodshops out of their garage units, which use a significant amount of electricity but their costs get spread between all units. I'm happy to do it because it's like $10/mo for us (honestly it's going to cost the HOA more to back bill us). but we have infrastructure work to do to make this work on a large scale.
    – medley56
    Nov 17, 2020 at 15:52
  • The problem is (and I actually have some professional experience with this, not just random musing based on stuff I read on the internet) that most governments+utilities (utilities in most places are pretty strictly regulated, which benefits users in some ways, but not all ways) put some pretty strict rules on both how to measure (essentially, high-end meters are certified for accuracy and anti-tampering) and how to bill. The result is that while the technology to nominally submeter has gotten incredibly cheap, relatively speaking, in the past few years, actually using that technology for... Nov 17, 2020 at 16:03
  • submetering is not, legally, so simple. So for now (legally) it comes down to whether the HOA (or landlord of a multi-tenant rental unit, etc.) wants to invest the upfront cost - significant labor typically + far-from-the-cheapest meters - to do it "right" or not. Nov 17, 2020 at 16:05
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    Understood. I was thinking of your first definition and not of the second.
    – FreeMan
    Nov 17, 2020 at 16:17

TLDR: Go to the last section.

NEC 110.2: Everything installed in US power must be approved.
NEC 110.3(B): It must be installed according to its labeling and instructions.
(which are approved by UL as part of getting a UL Listing).

Well, I can see why they're cagey about how to install it.

There's no legal way to install that kit

All AC mains power must be contained within "Class 1" wiring methods. Low-voltage circuits can follow a relaxed standard called "Class 2". However, a low voltage circuit can't enter or exit Class 1 wiring methods. If it interacts with AC mains power at all, then the entire low-voltage system must be treated as Class 1 and be inside Class 1 wiring methods.

A low-voltage system can't be "one foot in, one foot out" of a 120V/240V box.

That's why there's no such thing as a split wall socket that gives 120V + ethernet.

Now there are commercial home energy monitor products like Sense, Curb or TED. But if you look closely at them, they go in backflips to avoid the "low voltage cable crossing the membrane" problem. Some of them live inside the panel and stick a UL-approved WiFi antenna out a knockout. TED has a module that lives inside the panel that communicates with a module outside the panel. I'm surprised nobody uses fiber-optic.

So we've seen several products pop up on the market that are obviously hobby/amateur/cheap/knockoff takes, that break all the rules (it's cheaper). Needless to say, they do not get UL-approved. As such, US and Canada law prohibit their sale. So retail stores like ACE, Menards or Greybar can't sell them, nor can mail-order shops like GalCo or Grainger. So they use loopholes on the ragged fringe of the consumer protection system, like direct ship from China, Amazon warehouse, or tiny 1-person shops who don't know better.

Even worse, there's an open-source project called "open energy monitor", which has solved the easy parts of the problem by ignoring the safety parts. Many of these Code-violating cheapies are just implementations of that.

Some of them sell the "kit" as components, sometimes with a component approval. That's not the same thing at all. There must be a UL listing (not RU), and it must have a file number and instructions for installations the way you plan to do it.

ETL and CSA are acceptable substitutes for UL, but again must be a file number. If it's not there, send that crud back with extreme prejudice. The seller is selling unsafe goods.

CE (China Export, we joke) is the mark of trash. It's widely faked, since there are no consequences to faking it unless the maker is inside the European Union.

Do it old-school.

Power companies are installing smart meters as fast as they can, because it's cheaper than sending a human to read a mechanical meter. As such, the market is glutted with cheap used mechanical spinny-disc residential electric meters.

"Meter pans" are a commodity part sold at reasonable cost at any home store.

Put em together and you have a proven, undebatable, quality solution for about $60. Code compliance isn't even a question lol.

  • Powerful Chernobyl movie clip! Thank you. Nov 17, 2020 at 18:45
  • Yeah, a standard Form 2S meter in a 4-jaw 100A socket is going to be as good as anything else you can buy, easier to replace or get serviced if it breaks, and with a "smart meter" in there, can be made with a bit of legwork (pulse counting) to be auto-readable even Nov 18, 2020 at 0:11
  • Yeah, no reason you couldn't hook up a microcontroller (Pi or Arduino or whatever) to count the pulses. Nov 18, 2020 at 2:03
  • So, legality notwithstanding, I got it installed and it works surprisingly well. I now have an automated pipeline that processes data from the Efergy dashboard and calculates the cumulative usage for my HOA backbilling. Good to know it's not up to code though, for future reference.
    – medley56
    Dec 2, 2020 at 23:12

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