I have a Centech 95683 Digital Clamp Meter from HF. I made up about an 18-inch 2-wire extension cord and split the conductors apart for 6 to 8 inches so that the meter could be clamped on each individual conductor.

I used it to measure the current drain of one of those electric heaters that look like hot water radiators and got reasonable results on one leg. Then I measured the other leg and the reading was different by maybe 5% or so. I re-checked the two readings and sure enough there was always a different reading for hot and neutral legs.

I looked at the manual for the meter and it says "..... position the live (hot) conductor in the center of the jaws for greater accuracy.....". Ok, why?

Since the heater is a two-wire appliance and has no ground connection there can be no difference in the current in the two sides of the line. The test was done on the second floor and there was no metal around. Something to do with the earth's magnetic field ?

Ideas?

RRM

Is there any difference between readings on either leg if you reposition the clamp 180 degrees i.e. open clamp, remove clamp, and rotate it the other way?

Not sure why it would make a difference, but maybe there is some difference in sensitivity or something else when the clamp core "sees" the induced field from the opposite axis.

Yes you can get different readings according to just how the wire is clamped. The area inside the clamp is large compared to the conductor and exact placement has a noticeable affect, which I had tried to minimize. Now that I've had more time I tried things again. Being very careful to clamp the wires in the same way as closely as possible minimizes or eliminates the difference in readings.

What puzzled me most was why the manual said to measure the hot lead for higher accuracy. I thought this odd since hot and ground must have the same current in two wire setups and that is what I was measuring when I got different readings. I though this mysterious but the real reason for this statement is that leakage in a three-wire system lowers the neutral current. This may or may not be a defect in the equipment being measured but can also be due to line filtering or internal coupling. So in order to get the most accurate reading of total current the hot line would be the one to measure. Bottom line is, the mystery isn't a mystery, just the result of clamp positioning and not having thought things through completely.

When something seemingly strange happens with something from HF the quality issue always comes up. I've come to believe it's part real and part imaginary. There are limited numbers of companies making meters and some of our favorite name brands may well come from the same factories as HF stuff but with different levels of quality control (think about going through the bin of pliers to get the one made exactly right). As for my meter, I now think it is trustworthy.

RRM

p.s. When I was in the army there was a term derived from the light 50: operator head space.

If the current is different between the hot and neutral, then "you got trouble"

Note: In a 2-wire system, you still want to use the terms "hot" and "neutral". Ground is at the service entry, and--if it goes out to a plug--should never have current on it.

I can't think of any basis for saying that measuring the hot side would be more accurate. Picture this: string some single-conductor #12 out in a straight line--maybe 6' long. Put a light bulb in the center. Now use your gadget to measure at various points along the line. What possible reason would there be for any difference?---except random errors.

One possible source of error is the location of the NON-metered conductor.
If it's close to the meter clamp it might induce some response.

- Leigh

Yabbut----any error from that would be symmetrical---ie, there would be the same error regardless of which lead the ammeter was clamped on.

A few experiments should clear this up.....

The clamp ammeter should read equal values on the hot and neutral. Even though
the currents are not the same. The currents are 180 degrees out of phase.

When the clamp meter is used, it is subject to the field in the wire, it is centered on.

It is also subject to all the induction fields surrounding the meter, from the environment.

These fields may have a phase relationship, that either add or subtract from the target.

From an electrician's point of view, watch out ! To get your reading, with the meter positioned
so you can see it, may involving shoving wires close by. In a crowded junction box, or panel.
a marginal connection may part, and cause major grief.

Many homes and apartments are sourced from 3 phase drops. Unless you are steeped
in this system, confusion will result.

Yep, I goofed and wrote "ground" instead of "neutral" in one place, correctly in the next sentence and in the tittle. The reference to better accuracy by measuring the hot lead makes sense in that ALL the current is going to be in the hot lead. Also measuring the neutral will tell you if some of it is going astray.

RRM

In a perfect world one would expect that the current into something like an electric heater would be exactly the same as the current out. But it's not as simple as that. Power is typically derived from twelve-phase alternators in power plants which are divided into three-phase branches for distribution. Neutral is not normally carried through the transmission system; it is derived locally from the substation transformer bank along with single phase power for smaller customers. As a consequence, the impedance of the "hot" wire back to the substation is not necessarily the same as that of the "neutral wire," and neutral may be carrying harmonics and out-of-phase currents that are not on the hot wire. This is more true today than in the past with lots of rooftop solar installations in many areas trying to feed power back into the power grid. So if there's a lot of "trash" riding on the neutral, you will get a more accurate reading from the hot wire.

Perhaps a diagram is needed. The idea of current in, and current out, of a resistor
(heater) is a bit fuzzy.

not fuzzy to me....

Connect wire from hot side of plug to heater, then another wire from the other heater connection back to the neutral on the plug.

If the current is not the same everywhere on that wire, then some of the electronics founding fathers are rolling in their graves.

Same answer at the service entrance? If the 2 hot wires have the same current, then there is 0 current in the neutral---or is there? In principle, there can be current into (or out of) the ground rod.

Greetings to the Forum:

Here's a check: Clamp around BOTH wires.... add some extra turns for greater sensitivity if necessary. If the currents are equal, then the clamp-on will read zero. If not, it will give some sort of reading.

If it does, it is either weirdness on the neutral lead as described by Chris108 or there is some sort of leakage to ground in the device under test.

Regards,

With the heater being a non-inductive load, the AC current measurement in and out should be pretty straightforward and equal.

I am still betting on a metering error due to position of the clamp on pick-up. A second, better quality meter would quickly answer the question.

The fact that the load has no direct ground connection back to the panel should also remove the possibility of current leakage from the discussion, unless there is some other accidental ground connection, like the heater touching a grounded metal surface.

Leakage can be easily ruled out by measuring the metal heater enclosure for a voltage potential with respect to ground.

Three phases, one neutral. The neutral current is only zero if the loads on the three phases are equal in magnitude and phase. Even in residential power, which is single phase centertapped (really bi-phase) in most places, the neutral current can be substantial unless the loads on each side of the line are perfectly balanced. Thus the current in the neutral and its consequent voltage drop depends on what is going on with the other phases or the other side of the line, not just the side your heater is plugged into. If there happen to be harmonics on the neutral, the meter may respond to them in ways that would tend to decrease its accuracy.

Whatever the impedance of the hot and neutral lines, they have to be orders of magnitude less than the resistance of the heater. Otherwise the wires would get hot and the heater wouldn't. Consequently it is only reasonable to expect that the resistance of the heater would isolate you somewhat from "junk" on the neutral and you'd get a better reading on the hot wire.

All things considered, the power in a resistor, is I^2 * R. That in itself should be
sufficient.

There are available some nifty simple tools to make the sensitivity of clamp
ammeters greater. They cost about $15.

Two windows: One measures the load plugged in at X1. The other measures the load
as X 10. That means, a 1/2 amp load, now reads upscale as 5 amps.

Also there are two handy pin jacks, for multimeter probes to measure the wall plug
voltage, without jamming them into the outlet itself.

I do have two of these gadgets to use in my electricity class.

What if the heater is SCR or triac controlled?

If the heater is controlled by a SCR or Triac control, a more
elaborate clamp ammeter may be used to give you more information.

I have a Fluke 33. This meter has several menu options. Google
the manual. This meter was used when I did a lot of work on
variable frequency drives.



However, using a more common type clamp meter, in general
the meter will tell you the average AC current at any heat setting.

AC ammeters found on inexpensive multi meters may have a limited
frequency response.


On the more expensive models, they will tell you the accuracy on
important power frequencies, i.e 60 to 400 Hz.

Just a thought , Watts in, May not agree with the thevens or kirchoffs theory. Not what I'm saying is a fact, but measuring current before a load, may not be the same as some current is transformed into heat. That's getting into energy changing states, without compensating for types of measurement used.

I have an electricians Fluke meter that is designed to correct it's input load for ghost voltages.

We're talking about the current in the hot and neutral lines to a single appliance.

The "neutral" current is only related to current in the service neutral by name.
Whether the service neutral current is zero or 100 amps, the current induced in the appliance neutral will be zero.

- Leigh

The cheapo clamp meter that I had was next to useless, except that it looked good sticking out of my tool bag. Simply not accurate at low current measurements. A good clamp meter will accurately measure miliamps.

We are discussing AC here, so you want to picture a pulse moving back and forth through the heater. Equal strength in either direction.

Not flowing from the hot side to the neutral (or vice versa), as that would be DC.