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Please see the UPDATE section below! The question still stands (with new info).
I don't feel the need to open a new one, but let me know if you think otherwise.


Is AmazonCommercial 600A AC/DC Clamp Meter (https://www.amazon.com/dp/B07VX41MPR/) making any sense with clamped current measurements? It shows weird values, and even weirder - it shows current when there is no wire inside the clamp.

I wanted to use it to catch parasitic draw from the automotive 12V battery, but don't know if I can trust the tool.

Kill-a-Watt is displaying AC current in all photos, ClampMeter is set to match current in the wire.

First 2 photos show original incorrect approach: clamp has to go around a single wire!
[I removed other extra photos of incorrect measurements]

Clamped over DC going into LED lamp (WRONG WAY - see above): 1

Then over 110V AC going into a different LED lamp (also WRONG WAY - see above): 2

UPDATE 2021-Mar-25
As suggested, I wrapped the ClampMeter in aluminum foil, and even grounded that "cage". Ordered a second unit; one shows 1.2, the other -1.28 A DC, with no wires clamped - they don't even agree next to each other (both set with fresh batteries from the same batch): 3

4

Set up properly to test AC readings: 5

0.15A is a bit (25%!) lower than 0.2A shown by other instruments: 6

But at least in AC mode with no wire in the clamp displayed reading is always 0.0A..

DC mode however is totally confusing. Tried to actually clamp around the vehicle's 12V battery feed. I'll spare you for now the many photos (unless asked), but the gist is: while in the air with clamp still closed, no wires inside, ~2ft away from battery it shows -1.7A. Getting closer to the battery raises the value up to -2A. Clamped over negative feed (single wire) shows -0.84A with shut off engine and no accessories.. Oh, I'll take it into the forest and get an ambient reading there!

There is a [Rel] button that should reset the measurement relative to ambient. But if I keep it depressed, while clamping and taking a reading, display will continue showing 0.0A. If the button is pressed momentarily display changes to 0.0A for that short interval, but comes back to weird (ambient?) reading as soon as the button is released.

Yes, I do have concerns about this $80 instrument's accuracy!
How to explain two units displaying very different ambient readings in the same spot at the same time? Which one to trust? How to make sense of it all?
And how [in theory?] should I read accurate DC current with clamp?

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    A constant DC current generates a static magnetic field. If you have some Mu-metal around you can try wrapping the meter in it, but aluminum foil certainly isn't up to the job of blocking ambient magnetic fields. The meter should be put in place to make a measurement with no current present and zeroed. Moving the meter thereafter will alter the relationship to any ambient magnetic field and likely change the reading. – HABO Mar 26 at 20:09
  • Had a thought that Al-foil may not be enough to shield from magnetic fields. As noted, the issue is resolved, but I still appreciate your comment for completing the whole picture! – Astrogator Mar 26 at 20:26
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You're not using it correctly.
You can only have a single wire running though the clamp - in all the cases you show here you have 2 wires - even the 1st pics with the adapter plugged in to the outlet powering the clip-on LED lamp has 2 wires inside that single round cable.
If you use it to measure current from your automotive 12V battery you'll probably be fine since you're only likely to clamp the meter over either the positive or the negative wire at any one point in time.

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    What the OP needs is something like this: amazon.com/Thsinde-AC-Line-Splitter/dp/B07GRQRPGW (this unit also amplifies the current 10X by looping the wire around the hole 10 times.) – DoxyLover Mar 5 at 6:47
  • Duh, I can't believe I didn't think about the single wire! :)))) Thank you! Also much appreciated @DoxyLover's tip on the splitter, didn't know about such devices! – Astrogator Mar 5 at 15:14
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    Even if you were using this correctly I'm not sure what this experiment would prove. You want to gain trust in the tool. There's no harm here and it's probably fun. But it's an $80 instrument with documented accuracy and precision characteristics, and you are using a silly $25 "lower your electric bill" gadget to compare it with. Unless you got a dud unit, which is always a possibility, there's very little trust you'll gain here. Just go wrap it around the main red wire from your car battery and it ought to work well. – jay613 Mar 5 at 18:17
  • I took an ordinary extension lead, opened the socket part and put an insulated wire loop out the side of the case - about an inch diameter. Works perfectly with my Fluke clamp meter AND it is only for my personal use... – Solar Mike Mar 5 at 21:05
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    @jay613, I tested that "$25 gadget" with Fluke and another multimeter, and it's been spot on - so I do trust it. First time working with a clamp meter - so wanted to check. Didn't have a simple way to put ammeter into the test circuits to compare against (yet! but now will :))), but Kill-a-Watt was on hand. And based on Harper's answer now I got some new question towards the "$80 instrument"'s accuracy :). – Astrogator Mar 5 at 21:13
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Code (NEC 300.3) requires that in any given cable, all currents in the cable be equal and opposite.

I.E. if you have a 3-wire+ground in your bathroom: red is 3 amps for a fan, black is 1 amp for a light, and white is 4 amps of neutral return for both of them... that is fine.

Why?? Because a single wire will throw a great deal of EMF (proportional to its current). AC power has wildly varying EMF, on purpose, so transformers will work. That is why AC power is AC.

As long as the multiple wires are tightly bound together, e.g. in a cable or conduit, their EMFs effectively cancel each other out (because that is how magnetism works).

If current goes out-and-back on different cables, then the EMFs are reacting against each other. Electro-Magnetic Force is, after all, force: it will make the wires vibrate, and anything metallic between them will be subject to eddy-current heating. (which is why transformers have laminated cores, not just a solid block of iron). If it is magnetism-sensitive, it will certainly know it's being subject to EMFs.

And that's how clamp meters work, and why any cable should always read zero.

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  • Thank you, totally makes sense! But then does a splitter tool like suggested in comment introduce a problem because it has unbalanced (unequal lengths) wires inside (see review photos @Amazon)? And another question to accuracy of the clamp meter then: why does it display non-zero current in most of my scenes? – Astrogator Mar 5 at 21:03
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    With two wires in a closed circuit, the current anywhere will be the same regardless of how long each one is. If you make one of them very very long, its resistance might decrease the overall current but that new current will still be the same even if you measure it in the other, short, cable. Things are different if there is leakage, e.g. if there are more than two cables. Which there are here ... there is a ground cable. It's possible for some current to flow through that and then the other two cables may not be balanced. But not because of unequal length. – jay613 Mar 5 at 22:24
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    Maybe it displays non-zero current because of stray EMFs you have in your room. It would have to be very sensitive, but it is right? Test this idea: wrap the clamp end in tin foil and ground the foil by touching it to a metal pipe or to the unpainted screw on a light switch cover. If the reading becomes zero, the device is reading actual stray currents. If it remains non-zero you may have a calibration issue. – jay613 Mar 5 at 22:32
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    @Astrogator -- the thing with the clamp-meter splitters is that they're temporary tools, that you only plug in when you need them -- you don't leave appliances plugged into them and running (they're also made of plastic and copper, with no ferrous metal in between the wires to cause eddy current heating issues) – ThreePhaseEel Mar 6 at 0:18
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Yes of course it reads non-zero in DC current mode with no wire through it. An inductive DC current sensor is measuring an unchanging (more or less) magnetic field. It has no way to differentiate a “real” reading from ambient magnetic fields (from the earth, nearby magnets, hunks of iron, etc). These will vary by location, orientation of the sensor, and other factors.

There should be a button to zero out the reading after selecting DC current mode. Hit the button to zero, and then put the clamp on the wire to be measured. Consult the users manual for details.

Due to the above you should not expect super accurate, precise readings from a random DC clamp meter in the field. It should get you in the ballpark, but that might be about it. If you need more accuracy (especially for small values) a meter that wires into the circuit (in series) should do better.

AC current measurement doesn’t really have this problem. The meter can automatically take the difference between the peaks of the waveform and effectively ignore any static local magnetic fields that would interfere with DC measurements.

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  • I'm not expecting high accuracy, but 25% error is not acceptable in my book. Still, if it is known and consistent, it can be accounted for - I can live with that. My problem is so far observations do not make sense. And I wrote about the Rel button which should do exactly what you describe (and I expect), but does not. – Astrogator Mar 26 at 2:22
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    An error of 0.05A on a 60 A scale is only 0.08%. It may be 25% away from the “truth” but it’s a very small difference in the measurement scale since the “true” value is so small. If you want better accuracy on small values, use something with a smaller scale. – nobody Mar 26 at 2:58
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    Also you don’t really know what the truth is. AC power is complicated, with power factors, RMS, and more. You show a CFL as your load which is not a simple resistive load. Both readings may be accurate, but of different things (such as true vs. apparent power). Try a pure resistive load. – nobody Mar 26 at 2:59
  • Ok, I'll agree with that. – Astrogator Mar 26 at 14:24
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Resistance may be futile, but persistence is not! :))

The first meter was a dud. After so many unsuccessful attempts to get a proper DC current reading with the first unit I probably didn't even try the REL button on the replacement, being confused by other things.
I swear that button did not make the relative mode stick on the 1st one. On the 2nd it does: 1

Now, that logical consistency of the Universe [and my sanity] is restored, let's verify.
Reset to zero over the 2 wire DC cable from battery charger: 2

Current on negative cable: 3

Current on positive cable: 4

Yes, values do not match precisely, but are within acceptable reasonable tolerance of each other.

Now let's accomplish my original goal. Another reset to zero near the negative battery cable: 5

And finally, DC current from the battery into the car over negative feed ('cause it's single): 6

Engine and all accessories are off. Current is 0.0A, i.e. no parasitic draw. QED.

This is not the target vehicle, but this exercise demonstrates and proves [to me] tool's usability. Now things make sense.
All your help and guidance are very much appreciated! I will mark the very first answer; it couldn't possibly deduce that the 1st meter was defective, but it was spot on about how to take clamp reading correctly.

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