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Old 10-05-2013, 01:25 PM   #1
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High Inrush Current Measurements?


Just purchased a new Mastech MS2108 clamp-on meter which measures true RMS and inrush current (not to be confused with the cheaper, less capable MS2108A).

Measured current and inrush current on the Dometic 13,500 BTU a/c on my travel trailer. Measured 12.2 amps (continuous) while the compressor was on which seems like a reasonable reading. However, the inrush current for the compressor start-up is another story. I measured 58 amps! For comparison purposes (using the same meter) I also measured the current and inrush current on our microwave inside the house. It measured 13.2 amps. The start-up inrush current was 48 amps.

These inrush measurements seem very high. Is this possible, or do I have a very inaccurate meter? FWIW, followed the manufacturer's directions closely for measuring inrush current. They're very simple.

Thanks!

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Old 10-05-2013, 01:38 PM   #2
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High Inrush Current Measurements?


Quote:
Originally Posted by HomeGuy2 View Post
Just purchased a new Mastech MS2108 clamp-on meter which measures true RMS and inrush current (not to be confused with the cheaper, less capable MS2108A).

Measured current and inrush current on the Dometic 13,500 BTU a/c on my travel trailer. Measured 12.2 amps (continuous) while the compressor was on which seems like a reasonable reading. However, the inrush current for the compressor start-up is another story. I measured 58 amps! For comparison purposes (using the same meter) I also measured the current and inrush current on our microwave inside the house. It measured 13.2 amps. The start-up inrush current was 48 amps.

These inrush measurements seem very high. Is this possible, or do I have a very inaccurate meter? FWIW, followed the manufacturer's directions closely for measuring inrush current. They're very simple.

Thanks!
Inrush current is generally 10 - 50 times larger than running.... this is why it is so important to size motor conductors, OCP, and ground fault and short circuit protection correctly....

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Old 10-05-2013, 02:32 PM   #3
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High Inrush Current Measurements?


On the compressor for your camper A/C, it will list its LRA. Its probably 52 to 54 amps. The rest of the inrush current was the fan motors.
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Old 10-05-2013, 02:52 PM   #4
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High Inrush Current Measurements?


Stickboy1375 and beenthere, thanks for your replies! It's good to know my clamp-on meter is working properly. Now I see why some smaller generators have trouble powering a/c units on campers/TT's. That's a lot of start-up current!
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Old 10-05-2013, 02:54 PM   #5
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High Inrush Current Measurements?


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Stickboy1375 and beenthere, thanks for your replies! It's good to know my clamp-on meter is working properly. Now I see why some smaller generators have trouble powering a/c units on campers/TT's. That's a lot of start-up current!


And people wonder why the lights dim on AC start ups....
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Old 10-05-2013, 10:59 PM   #6
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High Inrush Current Measurements?


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Inrush current is generally 10 - 50 times larger than running.... this is why it is so important to size motor conductors, OCP, and ground fault and short circuit protection correctly....
In my experience and we do a fair bit testing including inrush testing and 4-6 times is more typical, 10 times is high and 14 times is the highest I remember recording.

The 14 times was a ASCO ATS with open transition without an in-phase-monitor, very fast transfer time feeding a 75 kva transformer.

We utilize high speed line disturbance analyzers for inrush and waveform capture.
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Old 10-05-2013, 11:22 PM   #7
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High Inrush Current Measurements?


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In my experience and we do a fair bit testing including inrush testing and 4-6 times is more typical, 10 times is high and 14 times is the highest I remember recording.

The 14 times was a ASCO ATS with open transition without an in-phase-monitor, very fast transfer time feeding a 75 kva transformer.

We utilize high speed line disturbance analyzers for inrush and waveform capture.
Probably correct, i googled for an average.... I really wasn't sure on the correct percentages... although, I didn't think the OP's numbers sounded too far off....
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Old 10-06-2013, 12:25 AM   #8
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High Inrush Current Measurements?


7.5 times nameplate is my rule of thumb.
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Old 10-06-2013, 12:50 AM   #9
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High Inrush Current Measurements?


Copeland Scroll compressor ZRD36KC-PFV, 13,500 BTUs. RLA 18.6, LRA 95

Copeland CL13KQ-PFV compressor, 13,500 BTUs. RLA 9.7, LRA 49

So HomeGuy2's reading was close, but probably lower then the real inrush was.


ZRD36KC-PFV
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Old 10-06-2013, 06:50 PM   #10
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High Inrush Current Measurements?


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Probably correct, i googled for an average.... I really wasn't sure on the correct percentages... although, I didn't think the OP's numbers sounded too far off....
There are so many variables regarding inrush, you might only get the worse case inrush every 8-10 starts
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Old 10-06-2013, 07:06 PM   #11
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High Inrush Current Measurements?


As an FYI....inrush is a very difficult measurement to make. For example, where did the switch make in regard to the phase angle of the incoming ac? Was it as the 0 cross over, or was it at the peak? These variables will impact the actual inrush measurement....along with a lot of other things....like...what was the line loss...how good are the connections...etc.

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