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Old 05-01-2013, 02:50 PM   #1
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Using Clamp-On Multimeter


I'm thinking about purchasing a Yamaha generator that's rated at a continuous 16.7 amps (with a surge capability of 20 amps). My plan is to use it with a 13,500 BTU RV a/c. Yamaha says this generator can run "most" 13,500 BTU RV a/c's. Obviously, I don't want to commit to purchasing this generator unless I'm confident it can run my 13,500 BTU RV a/c.

Questions:

1. Can I accurately use a clamp-on multimeter to determine the "surge" start-up current of my 13,500 BTU RV a/c?

2. Can I reasonably assume the Yamaha generator will run my 13,500 BTU RV a/c if the clamp-on multimeter reports a surge start-up current that is less than 20 amps?

3. Are there special precautions that must be taken to ensure an accurate measurement with the clamp-on multimeter? I've been told you must make sure only the hot-side wire is inside the mulitimeter clamp.

Thank you!


Last edited by HomeGuy2; 05-01-2013 at 04:16 PM.
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Old 05-01-2013, 07:12 PM   #2
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Using Clamp-On Multimeter


1. Can I accurately use a clamp-on multimeter to determine the "surge" start-up current of my 13,500 BTU RV a/c?

Most clamp-ons don't do very well at catching a surge. There is a sampling interval which must be considered. If your surge is short, and outside the sampling window, it will never register. I would expect a current surge like the one you're looking for to be relatively long, say 50-200 milliseconds. But even if you catch it, there is also a low-pass filter to smooth out the readings, so the digital display doesn't jump around in the presence of noise. That effectively removes any hope.

You'd be better off to use a current sensor with a BNC output, and put it on an oscilloscope. If you do so, make certain you check the impedance spec and set the scope correctly, or it will ruin the measurement. You can check calibration by passing a known, steady-state (non surging) current through it, such as driving a 100W light bulb you've measured with the standard clamp meter.

2. Can I reasonably assume the Yamaha generator will run my 13,500 BTU RV a/c if the clamp-on multimeter reports a surge start-up current that is less than 20 amps?

Ehh, maybe. But I would not want to have a recurring surge at the limit of the generator spec. As generators age and fall behind on maintenance, they start to choke on big transients. It gets super annoying to have to restart the genny every 10th or 12th on-off cycle. When Yamaha says "most", I assume they allowing some margin. Perhaps some common 13,500 units will have their surge current specified, and that might give you an idea of what margin Yamaha is keeping. If you cant find any, I'd recommend a design factor of at least 1.5x. Also keep in mind the generator will not perform to full spec if you're at altitude.

3. Are there special precautions that must be taken to ensure an accurate measurement with the clamp-on multimeter? I've been told you must make sure only the hot-side wire is inside the mulitimeter clamp.

Modern test instruments are remarkably accurate these days. But current is also notoriously difficult to know accurately. If you use a sensor driving an oscilloscope, your biggest concern will be the specified accuracy of the sensor, which will likely be 5-10%. (The scope itself is measuring voltage which is likely to be better than 1%.)

Also, when using a sensor like the one described, they often need to be zero-ed. Do the zero in full-off, a/c not even plugged in because it can draw a small amount for the computer, but otherwise with the probe in the exact location it will be. After taking a reading, make sure the sensor RETURNS to zero, as they sometimes don't. If not, repeat. Also flip the sensor around and make sure you read the exact same amount but negative. That will confirm the measurement.

And after all that, buy an even bigger generator. Also, I've found the "quiet" versions from Honda and others are well worth the big expense.

Good luck. -Upton

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Old 05-02-2013, 01:41 PM   #3
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Using Clamp-On Multimeter


Upton O Good, thanks for taking the time for your very helpful reply. You bring up a good point about whether it's a good idea to use a generator that's running so close to its maximum current capability. I don't have access to a scope, but it's good to know the clamp-on multimeter isn't the most accurate device to gauge surge current. Wouldn't be fun to purchase a $1500 generator only to find out the surge reading was inaccurate and the generator wouldn't work with my 13,500 BTU a/c Thanks again.
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Old 05-02-2013, 03:24 PM   #4
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Using Clamp-On Multimeter


How big is this generator KW?
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Old 05-02-2013, 04:02 PM   #5
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Using Clamp-On Multimeter


2kw continuous, 2.4kw peak.
I think he is talking about the Yamaha EF2400ISCH.

It will probably do the job, particularly if he puts a hard start kit on the AC.
But extremely hot weather and/or high altitude might be a problem. Particularly on a restart of a hot AC.

I would either jump up to the EF3000IS, or get a two of the EF2000IS and parallel them. The 2000's are 44lbs ea vs 136lbs for the 3000. Easier to store & handle, and if you don't need AC, one would probably power the RV.
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Old 05-03-2013, 06:02 AM   #6
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forgetaboutit, up the KW or your in for nothing but trouble with that RV.
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Old 05-03-2013, 12:17 PM   #7
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Quote:
Originally Posted by Oso954 View Post
2kw continuous, 2.4kw peak.
I think he is talking about the Yamaha EF2400ISCH.

It will probably do the job, particularly if he puts a hard start kit on the AC.
But extremely hot weather and/or high altitude might be a problem. Particularly on a restart of a hot AC.

I would either jump up to the EF3000IS, or get a two of the EF2000IS and parallel them. The 2000's are 44lbs ea vs 136lbs for the 3000. Easier to store & handle, and if you don't need AC, one would probably power the RV.
I like this idea. Has lots of advantages. I've handled the Honda 2kW units, a guy can carry it with one hand. Redundancy is really nice. For example, you can run one gen while refueling the other. The a/c must be turned off at that time, but at least you have lights and music. (Its even possible the a/c would continue to run if you can refuel before it cycles.) You also have the flexibility of running only one gen when the a/c is not needed, and the 2kW units will be quieter. And if one unit develops a problem, at least you have some basic power.

On the negative side, those 2kW units tent to walk away. My boss had one disappear right of his hitch plate. The fat chain through the handle did nothing, as the body is plastic.
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Old 05-03-2013, 12:47 PM   #8
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Using Clamp-On Multimeter


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Originally Posted by Upton O Good View Post
I like this idea. Has lots of advantages. I've handled the Honda 2kW units, a guy can carry it with one hand. Redundancy is really nice. For example, you can run one gen while refueling the other. The a/c must be turned off at that time, but at least you have lights and music. (Its even possible the a/c would continue to run if you can refuel before it cycles.) You also have the flexibility of running only one gen when the a/c is not needed, and the 2kW units will be quieter. And if one unit develops a problem, at least you have some basic power.

On the negative side, those 2kW units tent to walk away. My boss had one disappear right of his hitch plate. The fat chain through the handle did nothing, as the body is plastic.
You need to use a wire rope sling with a good padlock. You can't cut that with a bolt cutter.
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Old 05-03-2013, 12:50 PM   #9
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Leave a snake beside it.
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Old 05-03-2013, 01:25 PM   #10
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You need to use a wire rope sling with a good padlock. You can't cut that with a bolt cutter.
I mean the body of the generator is plastic, including the handle. He still had the chain intact. Looking over some Yamaha images, I see some of each kind; metal and plastic.
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Old 05-03-2013, 01:59 PM   #11
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Using Clamp-On Multimeter


Quote:
Originally Posted by Oso954 View Post
2kw continuous, 2.4kw peak.
I think he is talking about the Yamaha EF2400ISCH.

It will probably do the job, particularly if he puts a hard start kit on the AC.
But extremely hot weather and/or high altitude might be a problem. Particularly on a restart of a hot AC.

I would either jump up to the EF3000IS, or get a two of the EF2000IS and parallel them. The 2000's are 44lbs ea vs 136lbs for the 3000. Easier to store & handle, and if you don't need AC, one would probably power the RV.
Appreciate everyone offering up other generator options. Iím aware that the EF3000is and/or two EF2000isí would be the ideal choice; however, these would cost me approx. $700 to $900 more (not to mention that the EF3000is is almost double the weight of the EF2400iSHC). Like a lot of folks Iím trying to squeeze as much bang out of my dollar (and my back) as possible. Some folks have been able to power their 13,500 BTU a/cís with the EF2400iSHCóeven under very adverse conditions (high temperatures and higher altitudes). IMO, $700 to $900 in savings is worth my time to research the EF2400iSHC option further.

Since I donít have the option of trying a EF2400iSHC before buying it, Iím hoping to try the next best thingówhich is to try and determine the surge current of my 13,500 BTU a/c by using a clamp-on multimeter.

Upton O Good has given me the impression a clamp-on multimeter is not that accurate. Unfortunately, I donít have access to an oíscope as Upton O Good suggested. Iím still hoping someone may have used a clamp-on multimeter that did prove accurate.

Thanks again to everybody for the helpful inputs!
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Old 05-03-2013, 02:20 PM   #12
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Using Clamp-On Multimeter


Yes, if you can find some forum postings of other people's experience with that generator, then you can save a pile of money, that's true.

I wonder if a local Yamaha dealer would let you try it at their store? I realize you can get them cheaper on the inner-webs. But the knowledge of exactly how it will behave with your load might be worth the extra $50 or so. (I have an ethical habit where if I use a business' showroom, I don't then go buy it on the webs, I pay the stupid tax.)

Hey I found this one web site which says the EF2400iSHC is "Load bank tested to hold a 25 Amp load for up to 10 seconds. Hayes Equipment Test Results: Load bank tested at our facility on 6-13-2005". Based on that I would think this gen will do your job. 10 seconds is way beyond any a/c surge.
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Old 05-03-2013, 05:31 PM   #13
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Quote:
Originally Posted by Upton O Good View Post
Yes, if you can find some forum postings of other people's experience with that generator, then you can save a pile of money, that's true.

I wonder if a local Yamaha dealer would let you try it at their store? I realize you can get them cheaper on the inner-webs. But the knowledge of exactly how it will behave with your load might be worth the extra $50 or so. (I have an ethical habit where if I use a business' showroom, I don't then go buy it on the webs, I pay the stupid tax.)

Hey I found this one web site which says the EF2400iSHC is "Load bank tested to hold a 25 Amp load for up to 10 seconds. Hayes Equipment Test Results: Load bank tested at our facility on 6-13-2005". Based on that I would think this gen will do your job. 10 seconds is way beyond any a/c surge.
Somewhere I saw a test of the newer EF2400iSHC (they made the older EF2400i in 2005) that held a 30+ amp surge load for 10 seconds. Even so, there are still reports of a small percentage of 13,500 BTU a/c's that won't work with the newer EF2400iSHC.

Anyhow, I've come to the conclusion that I need to physically try a EF2400iSHC with my 13,500 BTU a/c before I buy one. A surge load test with a clamp-on multimeter appears to be too unreliable.

So, as you suggested, I called a number of dealers and found 3 dealers that would allow me to demo an EF2400iSHC with my a/c unit before I purchase. Surprisingly, 2 of these dealers also had prices that were very, very competitive (within $30-$50, even with sales tax included) with the lowest online prices!

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