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Old 04-22-2013, 10:07 AM   #1
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Measuring Voltage Drop


Yesterday while yanking down one side of a wall...I noticed my wazu Fluke meter was in the room (Fluke 179)....it has a Max/Min feature....and since we have been seeing a few posts regarding testing of outlets...I thought..."Hey, good time to do it".

So...using my data center (space under the stairs) as my test, it made some measurements. Now, it's important to note that this spot represents and ideal location. I have one 12g NM cable going to it. No other loads except for the electronics for router, modem, etc. For now I'm also using it to power up some tools during construction. But those are short term loads. There is maybe 25' of NM cable going to an outlet which then feeds a good surge protector.

I plugged the meter leads into the surge protector and measured the voltage.

119.6 Vac...about what I expect...at most I have maybe .2a of load on it.



I then enabled the max/min feature on the meter and turned on the miter saw.



This is the minimum voltage measured....114 Vac...understand that this is the voltage drop at initial in rush which is quite a few amps more than 20a. But this is expected of inductive loads.



After the initial startup, the voltage went up to about 117 Vac.

Then I plugged in our iron...typical 1200w iron (10a).



I need to redo this test....I just noticed from the 1st pic that I made the mistake of plugging it into the surge protector instead of the outlet next to it. I'm betting that the surge protector is 14g at best...if I plug in the iron to the outlet, I bet I'll be in the 18.5 Vac range....

I'll check it tonight.

Anyway...for anyone wanting to test their outlets....an iron is a good way...

Plug a voltmeter into one side...measure the voltage...plug in the iron and turn it on...the voltage drop will be a good indication of the health up to that point.

As you work your way down your ckts...as you get further from the load center, you should see a small drop....I would expect that drop to be less than 0.1 to 0.2 Vac from outlet to outlet....if you start seeing differences of 0.5 or more...I think you have a connection problem.

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Old 04-22-2013, 01:14 PM   #2
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Measuring Voltage Drop


You can also do a voltage drop test across an electrical connection.

A good safe way to learn this (and a likely source of dirty poor connections) is on a car. (And of course DC instead of AC on the multimeter.) Here is a primer on that...
http://www.aa1car.com/library/voltage_drop_testing.htm

P.S. Take a cheap (smaller gauge) long 50 or 100 ft. extension cord and plug in a higher amperage/wattage device and watch the voltage drop! Some things like compressors will not even work.

Here is a voltage drop calculator for those situations...
http://www.electrician2.com/calculat...alculator.html


Last edited by Billy_Bob; 04-22-2013 at 01:17 PM.
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Old 04-22-2013, 04:20 PM   #3
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Measuring Voltage Drop


Quote:
Originally Posted by ddawg16 View Post



This is the minimum voltage measured....114 Vac...understand that this is the voltage drop at initial in rush which is quite a few amps more than 20a. But this is expected of inductive loads.
Not sure what you mean by this. You are not measuring amps.





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Originally Posted by ddawg16 View Post
I need to redo this test....I just noticed from the 1st pic that I made the mistake of plugging it into the surge protector instead of the outlet next to it. I'm betting that the surge protector is 14g at best...if I plug in the iron to the outlet, I bet I'll be in the 18.5 Vac range....
Do you mean you will see this much of a voltage drop, or will the voltage drop to this?


Just currious
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Old 04-22-2013, 04:34 PM   #4
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Measuring Voltage Drop


why does anyone care since the utility company has a +/- % change all day long?

What you read one day, could be different in an hour, day, month, etc....
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Old 04-22-2013, 07:36 PM   #5
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Measuring Voltage Drop


Ok....I guess the intent behind this was sort of lost or I did a poor job of explaining.

Doing a load test is a good way to test the overall quality of an AC line. If you have good connections, the voltage drop under load will be minimal.

The actual voltage measured is not as important as the change in voltage. In my case, my NO load voltage was 119.6 Vac...with the 10a load, it dropped to 117.7, or a 1.9 Vac drop. I'm going to repeat that test without the surge protector....I'm willing to bet that the voltage drop will be less.

In the case of the saw, the inrush on a motor is several times the FLA (Full Load Amps). Hence, a motor that pulls 15A a normal operating speed could pull up to 10x that on initial startup. The circuit breakers in your load center are made for this. Go look at trip curves and you will see different time constants for different types of loads. Hence, a breaker made for resistive loads will be different than one for inductive (motors).

The load test is just a good way to see if you have a good connection. If you plug in your iron and see a 15 Vac drop....you have a bad connection somewhere.

When I get home tonight...I'm going to repeat that test without the surge protector and again on the longest ckt I have (one in the kitchen at the front of the house).

Once again, it's not the actual voltage, but rather the change in voltage.

Disclaimer....if your voltage is dropping to or below 105....you have problems.....
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Old 04-22-2013, 07:47 PM   #6
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Measuring Voltage Drop


So whats your point ?
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Old 04-22-2013, 07:58 PM   #7
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Measuring Voltage Drop


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Originally Posted by dmxtothemax View Post
So whats your point ?
I assume your joking...right?
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Old 04-22-2013, 09:07 PM   #8
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Measuring Voltage Drop


Actually a meter with min / max function would be quite handy for all those A/C breaker tripping problems - Always good to learn new things is the way I look at it!
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Old 04-22-2013, 09:50 PM   #9
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Quote:
Originally Posted by ddawg16 View Post
I assume your joking...right?
Not really !
You have not really asked any questions yet !
I thought the main purpose of this forum
was for people to ask questions about something
that they might not understand ?

Is your question weather voltage drop testing is a valid
test of integraty of a electrical circuit ?
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Old 04-22-2013, 10:15 PM   #10
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Measuring Voltage Drop


STICKBOY makes a good point. The voltage measured at the mains (or any receptacle outlet) will vary widely according to many factors: time of year, health of the local/adjacent utility grids (amount of generation versus load), etc. Here in Chicago, I have seen 120V receptacle voltages range from 105-135Vrms with the low range typically occurring during summer cooling months when air conditioning loads and reactive power demand is high.

On the other hand, voltage drop, as measured at the load, is not a measure of the integrity of the circuit (unless we assume that the drop is due to high-resistance joints/connections which shouldn't apply as these issues would be due to poor installation).

Let us all remember that the 3% NEC recommendation is just that: A recommendation based on FPN. It ain't gonna kill the operation of your toaster.

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Old 04-22-2013, 11:07 PM   #11
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I'm beginning to think that the Question ???? thread was raising some very accurate points........

I'm sorry I didn't make it real obvious....this is not a 'question' thread, but an 'information' thread.

I'll try to rephrase....

One of the common electrical questions here in DIY are issues with outlets..pigtails versus back stabbing, etc.

One of the ways to test how good those connections are is to do a load test. For those that don't understand...a load test is where you put a known load on a ckt and measure the voltage drop. For a given load (current), there will be a pretty accurate voltage drop down that wire.

For example....if your using 12g wire....and you have a 10a load 100' (200' of wire) from the voltage source....then, you will have an ideal 3.096 volt drop at the load. Or, if you source is 120 Vac....the voltage at the load will be 116.904 volts.

Now....the real world is not a single length of NM wire....there are connections along the way and each one adds resistance. But it helps to know what it 'should' be....so when you get something lower...you have an idea that something is wrong.

Using my house as an example.....a 10a load 25' away (50' of wire) had a voltage drop of 1.9 Vac. But calculations say it should only be 0.77 Vac. That means I'm loosing a little over 1.1 Vac somewhere else.

I performed the same test at the front of the house where the outlets are the greatest distance from the load center. By my 'apx' calculations...about 100' (200' of wire), I should only have 3.096 volt drop. This evening the (almost) no load voltage was 119.6 Vac....with the iron plugged in it dropped to 115.2 Vac. That is a 4.4 Vac drop vs the calculated drop of 3.096.

So....in real world terms.....I think that's ok....I might have a little loss in a couple of splices....but nothing that means I need to go searching for the culprit.

But we also need to look at another aspect....energy loss. With that 10a load across 100' of wire....I'm loosing 31w of energy....in other words, it's like have a 31w bulb being left on. That is the wasted energy just through the wire....energy that is not doing anything for you...but heating up the wire. (now you might understand the de-rating spec in the NEC)

So...to summarize in simple terms.....

If you want to test the quality of your wiring....if you put a 10a load on an outlet....for every 10' from source, you should have a 0.31 Vac drop. By drop, I mean the voltage measurement before the load....and the measurement after the load is applied.

If this is not clear enough, feel free to PM me.
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Old 04-23-2013, 05:36 AM   #12
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Yes you can put a significant load such as a heater or hair dryer at "the end" of a circuit and measure voltage at various points (daisy chained receptacles) to see what the voltage drop is. This way you might find two locations between whcih there is a relatively large voltage drop and then look for a loose connection in that stretch.

But hardly anybody ever takes the time and trouble to do this.

More likely you will find the voltage drop to be proportional with distance from panel to your measuring point, so your loss of 31 watts in the wires while running your 900 watt hair dryer is not worth the effort to correct or recoup (by stringing heavier wire).
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Old 04-23-2013, 11:50 AM   #13
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Measuring Voltage Drop


Also, most of the "error" in your calculated voltage drop is most likely the corresponding "error" in you approximated length of the wire and the inaccuracies of the power consumption of the devices under test. There is nothing wrong with your theory, as long as you allow for the variables; however, I do not think that this is likely to become a common troubleshooting procedure.
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Old 04-23-2013, 12:58 PM   #14
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Measuring Voltage Drop


I found the post interesting from an analysis standpoint. Nice to know. Of course the real world application is minimal... for one, I'm not pulling 10A regularly, so I'm not really wasting the 30W all the time, just while plugging in a 10A draw for the 10 min it takes to iron or dry one's hair... and only if doing so at the end of a long run. And even then, if the connections were wired perfectly, the loss might be 27W instead, so the "waste" due to some imperfection is perhaps only 3W - completely insignificant compared to the 1200W you're burning in the iron.

Running a more typical load like 3-5A, the loss will be even less significant.

And those devices which do draw a lot for a consistent time - like your dryer - are upped on the wire size to compensate, plus will have a home run with no intervening connections. Still, I might do a test just to observe the .31/ 100' of 12g measurement.
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Old 04-23-2013, 05:29 PM   #15
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Quote:
Originally Posted by ddawg16 View Post
Ok....I guess the intent behind this was sort of lost or I did a poor job of explaining.

Doing a load test is a good way to test the overall quality of an AC line. If you have good connections, the voltage drop under load will be minimal.

The actual voltage measured is not as important as the change in voltage. In my case, my NO load voltage was 119.6 Vac...with the 10a load, it dropped to 117.7, or a 1.9 Vac drop. I'm going to repeat that test without the surge protector....I'm willing to bet that the voltage drop will be less.

In the case of the saw, the inrush on a motor is several times the FLA (Full Load Amps). Hence, a motor that pulls 15A a normal operating speed could pull up to 10x that on initial startup. The circuit breakers in your load center are made for this. Go look at trip curves and you will see different time constants for different types of loads. Hence, a breaker made for resistive loads will be different than one for inductive (motors).

The load test is just a good way to see if you have a good connection. If you plug in your iron and see a 15 Vac drop....you have a bad connection somewhere.

When I get home tonight...I'm going to repeat that test without the surge protector and again on the longest ckt I have (one in the kitchen at the front of the house).

Once again, it's not the actual voltage, but rather the change in voltage.

Disclaimer....if your voltage is dropping to or below 105....you have problems.....

Sorry, I didn't really understand at first where you were going with it... At least you had fun with your meter.

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