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  • Differing amp readings from 2 meters!

    I'm trying to solve a 15A breaker puzzle and set up a test bed to check the equipment. I have a Fluke 10A digital multimeter that measures inline and an Amprobe analog clamp on. Running a heat gun I got 8.25A on the Amprobe and 10.75A on the Fluke. Who should I believe? Does the Amprobe need calibrating?

    Not really relevant, but the puzzle is a tenant in a little apartment keeps blowing the 15A "screw in" retrofit breaker in her old school "glass fuse" panel of 2 branches on one 20A line. Yes, horrifying, but it's been like that the 21 years we've owned the place, it's the last one that has not gotten a new 60A riser and panel. In that 21 years there really has been no problems, modern ACs and lighting are really efficient, and most people don't do much else that draws much power. I thought maybe these weird retrofit breakers were "breaking in" and tripping at lower than rating, but my bench test showed it not tripping at 16A on the Amprobe. But if the Amprobe is reading 20% under, than that data is off.
    Location: Jersey City NJ USA

  • #2
    The calibration may be off.

    I was going to ask about true RMS, but I see you are using a heat gun, which is generally a good resistance load, even with the fan, so that would not be an issue.

    Another point is that clamp-on meters may read differently depending on where the wire is within the loop. You might try moving it around.

    As for circuit breakers and fuses.... they often will not blow at small overloads. They will blow quickly at large overloads, more slowly at smaller overloads, and definitely are not calibrated close enough to hold at 14.5A and blow at 15.5A, or the like. Not in general, although there may be ones that will. That said, you should not expect to load a normal circuit breaker to more than 80% of it's nominal marked rating. That will take care of issues with ambient temperature, etc.

    Most breakers for household use are thermal, an element heats up and a bimetal or similar device acts to release the contacts, which open. Being in a hot place will lower the rating, being in a cold place will raise the rating. Insulating around the breaker normally will lower the rating, etc.

    As an example, fuses in the US are made to hold for a minimum time at a small overload, and open within a maximum time for a larger overload. 100% current 4 hours minimum, 60 min max @135% rating, 2 minutes max at 200% rating is typical for UL fuses.. Breakers have similar ratings.
    Last edited by J Tiers; 01-19-2019, 08:24 PM.
    CNC machines only go through the motions.

    Ideas expressed may be mine, or from anyone else in the universe.
    Not responsible for clerical errors. Or those made by lay people either.
    Number formats and units may be chosen at random depending on what day it is.
    I reserve the right to use a number system with any integer base without prior notice.
    Generalizations are understood to be "often" true, but not true in every case.

    Comment


    • #3
      Generally, I'd believe the Fluke. I have many clamp-on amp probes and rarely do they come close to "in-line".

      Comment


      • #4
        I tried to find time/current curves for the retrofit screw base breakers, but this was all I found:





        As Jerry pointed out, fuses or circuit breakers are designed to carry their rated current indefinitely, and are generally only guaranteed to trip within an hour or so at about 135% of rating. Plug fuses are available with heavy duty type T dual element construction for motor loads, and type TL with faster clearing for residential circuits. You might want to replace the screw base circuit breaker and see if it holds up better, or use a type T fuse. If they continue to blow, your tenant is probably overloading the circuit, or there is some other problem.

        Maybe you can hook up a datalogger to determine when the overloads are occurring.



        https://sense.com/ (about $300)


        eGauge home and commercial energy meters connect electricity usage and solar production to the internet for users to monitor in real time. Certified high accuracy (ANSI C12.20 0.5 percent).




        https://www.aliexpress.com/item/PR10...697027826.html ($40)

        http://pauleschoen.com/pix/PM08_P76_P54.png
        Paul , P S Technology, Inc. and MrTibbs
        USA Maryland 21030

        Comment


        • #5
          Jerry didn't say the breakers will carry the rated current indefinitely. That's not how the typical residential breaker works.
          Last edited by lakeside53; 01-19-2019, 09:38 PM.

          Comment


          • #6
            Originally posted by gellfex View Post
            ... Running a heat gun I got 8.25A on the Amprobe and 10.75A on the Fluke. Who should I believe? ...
            The heat gun should have a wattage rating on it somewhere. Is it closer to 1000 Watts (8.25A) or 1300 Watts (10.25A)?
            Location: Long Island, N.Y.

            Comment


            • #7
              Originally posted by lakeside53 View Post
              Jerry didn't say the breakers will carry the rated current indefinitely. That's not how the typical residential breaker works.
              Correct.... FUSES typically do, a 4 hour rating is near enough to indefinite, but is not "actually" indefinite. Breakers are not intended to carry full current indefinitely, and as I mentioned, should only be loaded to 80% for any long term usage. Breakers come with "typical operating curves", which any individual unit will be close to, but may not match directly.
              CNC machines only go through the motions.

              Ideas expressed may be mine, or from anyone else in the universe.
              Not responsible for clerical errors. Or those made by lay people either.
              Number formats and units may be chosen at random depending on what day it is.
              I reserve the right to use a number system with any integer base without prior notice.
              Generalizations are understood to be "often" true, but not true in every case.

              Comment


              • #8
                There are 80% and 100% rated breakers. NEC guidelines are to size a circuit for 80% continuous load, so a 20 amp service should have no more than 16 amps continuous draw. However, AAUI, the breaker must handle 125% of that load, or the full 20 amps. And the industrial breakers that I have been involved in testing will handle their full continuous rating indefinitely. That does not mean that they should be used on circuits having that current continuously.





                Typical late 50's early 60's service. Stock NM had cloth like sheath. Some with small gounds, some with no grounds. Lots of stuff added over the years including AC, pool equipment and about 6 20A 120V circuits. Drove the gnd rods @ 7AM before POCO disconnected power (for the driver)...


                My point is that a breaker (or fuse) will handle (not trip) on the continuous rating of the device. It is only guaranteed to trip at typically 135% of rating, and may take several hours to do so. At 300% of rating, a protective device will typically trip within 30-90 seconds, and at some point, perhaps 8 times rating, it will trip instantaneously (6 cycles or 100 mSec or less). Some (mostly larger industrial) breakers have complex and adjustable trip characteristics. Residential and small commercial branch circuit breakers are usually thermal-magnetic, while some smaller inexpensive equipment breakers may be thermal only, or magnetic only.

                A time current curve plots the interrupting time of an overcurrent device based on a given current level. Time current curves are part of the product acceptance testing required by Underwriters Laboratories and other rating agencies.







                http://pauleschoen.com/pix/PM08_P76_P54.png
                Paul , P S Technology, Inc. and MrTibbs
                USA Maryland 21030

                Comment


                • #9
                  Thermal and thermal-magnetic breakers are sensitive to ambient temperature, which is partly the reason for the 80% rule.

                  The difference between a 4 hour and a 40 hour rating for blow time at a specific current is tiny.... trying to make breakers or fuses which adhere to that closely through thermal design and mechanical means only would be very hard, and very dependent on exact temperature. It would be fair to call that job basically impossible given a reasonably wide range of temperatures over which it has to work.
                  CNC machines only go through the motions.

                  Ideas expressed may be mine, or from anyone else in the universe.
                  Not responsible for clerical errors. Or those made by lay people either.
                  Number formats and units may be chosen at random depending on what day it is.
                  I reserve the right to use a number system with any integer base without prior notice.
                  Generalizations are understood to be "often" true, but not true in every case.

                  Comment


                  • #10
                    Originally posted by gellfex View Post
                    I'm trying to solve a 15A breaker puzzle and set up a test bed to check the equipment. I have a Fluke 10A digital multimeter that measures inline and an Amprobe analog clamp on. Running a heat gun I got 8.25A on the Amprobe and 10.75A on the Fluke. Who should I believe? Does the Amprobe need calibrating?.
                    I have both a fluke and amprobe. Like them both.

                    IMO you are going to get better (more precise) with an in line reading VS an induction reading. So the clamp amp meter mighht be a lil off. JR

                    Comment


                    • #11
                      Originally posted by J Tiers View Post
                      Thermal and thermal-magnetic breakers are sensitive to ambient temperature, which is partly the reason for the 80% rule.

                      The difference between a 4 hour and a 40 hour rating for blow time at a specific current is tiny.... trying to make breakers or fuses which adhere to that closely through thermal design and mechanical means only would be very hard, and very dependent on exact temperature. It would be fair to call that job basically impossible given a reasonably wide range of temperatures over which it has to work.
                      Yes the trip point does vary with temperature but that variance is quite small. A common square D 20amp rated breaker varies from that 20A by only about +- 3 amps from Zero F to 140F, its nominal rating being at 104 degrees F. There is a UL spec regulating this.

                      So, yes they vary with temp but not as much as you are making it sound like.

                      Here is the UL spec and the curves for common square D breakers.


                      Comment


                      • #12
                        It is sensitive to the wire position. Older amprobe units often have been tossed around in toolboxes and may have issues, or may just need calibration.

                        Agree, in-line gets all the current, clamp-on can miss some of the magnetic flux in certain wire positions. Try turning the meter at an angle so the wire lays on both sides of the loop.

                        If you can do a bench test, then can't you just compare them there? If the Fluke is newer and in-cal, set the Amprobe to match it with a "typical" wire position.

                        Originally posted by Sparky_NY View Post
                        Yes the trip point does vary with temperature but that variance is quite small. A common square D 20amp rated breaker varies from that 20A by only about +- 3 amps from Zero F to 140F, its nominal rating being at 104 degrees F. There is a UL spec regulating this.

                        So, yes they vary with temp but not as much as you are making it sound like.
                        .....
                        Your +- 3 amps aligns pretty well with a derating to 80% (4A less) for continuous current in a 20A breaker....... I never stated a number, but since you have, there's one reason for it right there.
                        Last edited by J Tiers; 01-19-2019, 11:03 PM.
                        CNC machines only go through the motions.

                        Ideas expressed may be mine, or from anyone else in the universe.
                        Not responsible for clerical errors. Or those made by lay people either.
                        Number formats and units may be chosen at random depending on what day it is.
                        I reserve the right to use a number system with any integer base without prior notice.
                        Generalizations are understood to be "often" true, but not true in every case.

                        Comment


                        • #13
                          Its also worth pointing out that the "screw in" fuse replacement breaker the OP mentioned may well NOT be UL listed and as such its trip points and such wouldn't conform to any UL standard. The ones I have seen are far from being a high quality piece.

                          Comment


                          • #14
                            Looks like a UL symbol on this one



                            Eaton Bussmann website shows UL Listed, File E14942

                            CNC machines only go through the motions.

                            Ideas expressed may be mine, or from anyone else in the universe.
                            Not responsible for clerical errors. Or those made by lay people either.
                            Number formats and units may be chosen at random depending on what day it is.
                            I reserve the right to use a number system with any integer base without prior notice.
                            Generalizations are understood to be "often" true, but not true in every case.

                            Comment


                            • #15
                              Switch to inches, 2 meters is too large distance for Fluke

                              OP mentioned heat gun, if it has any sort of power/temperature control it can throw off the measurements.
                              Triac control screws the RMS value. And I have one where the 50% power setting is done with diode in series with heater. That would screw off some of the clamp on meters seriously.
                              Location: Helsinki, Finland, Europe

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