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Old 04-23-2013, 08:17 PM   #16
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Measuring Voltage Drop


Quote:
Originally Posted by ddawg16 View Post

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.
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This all makes perfect sense to me and I'm not an electrical contractor. By using that method you certainly would be testing the quality of the connections as you said, what you describe is actually a pretty nifty technique.

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Old 04-23-2013, 09:14 PM   #17
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Measuring Voltage Drop


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Originally Posted by RWolff View Post
This all makes perfect sense to me and I'm not an electrical contractor. By using that method you certainly would be testing the quality of the connections as you said, what you describe is actually a pretty nifty technique.
To many variables for any practical usage though...
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Old 04-23-2013, 11:06 PM   #18
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Measuring Voltage Drop


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Originally Posted by stickboy1375 View Post
To many variables for any practical usage though...
It's a rough and ready approach,
But it's good enough for most layman type DIY'ers.

Electricains and engineers would be a bit more
precise and thorough due to legal liablities.
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Old 04-24-2013, 10:19 AM   #19
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Measuring Voltage Drop


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Originally Posted by dmxtothemax View Post
It's a rough and ready approach,
But it's good enough for most layman type DIY'ers.

Electricains and engineers would be a bit more
precise and thorough due to legal liablities.
How? This is pretty darn precise. And thorough. How would one measure voltage drop more precisely than by applying a known load and using a voltmeter?
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Old 04-24-2013, 01:14 PM   #20
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Measuring Voltage Drop


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Originally Posted by SquishyBall View Post
How? This is pretty darn precise. And thorough. How would one measure voltage drop more precisely than by applying a known load and using a voltmeter?
Yep, that's about as accurate as one would get, what they call real world measurements, you measure the actual physical loads on the real system running them.

I have a single phase 220v, 2 HP Century repulsion-induction motor built in 1928 in my basement of my house running an organ blower, the plate has the specs and I've put a clamp meter on it years ago to measure the amps on startup but I forget what it is was, I haven't noticed any of my lights dimming, I know it has good connections.

http://i.imgur.com/FQYM3nY.jpg
http://i.imgur.com/vWoROWp.jpg

As ddawg16 said earlier but in different words- putting a known load on and measuring the drop:

Originally Posted by ddawg16

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

Last edited by RWolff; 04-24-2013 at 01:20 PM.
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Old 04-25-2013, 01:44 AM   #21
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Measuring Voltage Drop


It's nice to see that a few people 'get it'....

I put the method to work this evening....in the process of removing part of a wall over the weekend, my sawzall (me) cut a piece of existing NM cable by accident...so this evening I climbed up into the attic...pulled the damaged piece up...stuck it in a box...ran a new piece down....wire nutted everything......4.2v drop through about 70' (140') of wire at the furthest point.

So....I have a slightly higher voltage drop than I should have...it should be around 2.2 volts.....that means I'm dropping about 20w at one or more connections. It could be the outlet....it could be one of my connections. Part of my problem is that the back half of this ckt is the 60 year old part of the house....and I know that at least 2-3 of the outlets are wired using the outlet as the junction instead of using a pig tail.

20w is not much....but....if I'm using a slightly higher load...say a crock pot that is going to be on for hours...that 20w could produce enough local heating that it starts to degrade the connection....which causes more loss....and more heat....and more loss....and more heat....and....more problems.

This weekend I'll make some mid ckt measurements to isolate the location of the high loss.
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Old 04-25-2013, 05:59 AM   #22
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Measuring Voltage Drop


20w loss over 70' is not too serious,
provided its equally dropped over the entire 70'.
thats about 300 milli watts per foot.
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Old 04-25-2013, 09:29 AM   #23
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Measuring Voltage Drop


If you are using a calculated IR drop based on published numbers for a given wire size ( instead of actually measuring the resistance of the wire) and you are using the nameplate specs on the device (instead of actually measuring the current draw) then a lot of your calculations of what the "voltage drop" should be may be theoretical instead of actual. You are probably using the DC wire resistance from some chart. The exact resistance is slightly greater with AC than it is with DC. Also, the exact resistance will vary with temperature. These are the type of variables that I referred to in an earlier post.

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