Voltage Drop at outlets on a Custom Residential Home

[kwired]

190514-1153 EDT

Loading is not a factor in the measurement. What is important is that there are no other load changes during the short time the meter is doing its test.

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Wouldn't that depend if it actually determines % change in drop during the test vs assuming 120 is normal and giving results in relation to what is supposed to be normal?

If you are already at say 113 volts before running the test you maybe failed the test before ever starting it if the latter is the case.

 

[oldsparky52]

Wouldn't that depend if it actually determines % change in drop during the test vs assuming 120 is normal and giving results in relation to what is supposed to be normal?

If you are already at say 113 volts before running the test you maybe failed the test before ever starting it if the latter is the case.

Or pass a test you should fail (based on % drop) if the voltage starts off at 124.

 

[kwired]

Or pass a test you should fail (based on % drop) if the voltage starts off at 124.

Right.

 

[hbiss]

The voltage should be irrelevant, all we're talking about is %VD at the end of whatever is there at the beginning. Which brings up a good point. How does that meter work? Hopefully it doesn't assume a nominal voltage but instead have you measure the actual voltage (under load) at the panel or first receptacle, input it, then calculate at the last. If it doesn't do that it's worthless IMO.

ETA: After reading the instruction manual it looks like (but they don't say) that it ASSumes 120V. So that thing is worthless.
-Hal

 

[mtfallsmikey]

No pic to post, but I have an old school RCA power line monitor, just a large meter.

 

[hbiss]

I remember those!

-Hal

 

[mtfallsmikey]

I'll take a pic (if I can remember to) and see if I can post it here, last time I tried to do that I couldn't.
Maybe K8MHZ has one in his ham shack too.

 

[gar]

190514-2115 EDT

kwired:

It was a few days back when I looked thru the manual on the referenced meter. It looked quite impressive, and by deduction I concluded that they did use a substantial load for the test. Meaning probably at least 5 A. Note that they specify a cool down period of 30 seconds between measurements. This was suffucuent information to indicate that a large load is used and that it is of short duration.

Suppose they use 24 ohms for the load. At 120 V that is 600 W, and if the load is applied for 1/60 second that is 10 watt-seconds. I believe they suggested 30 seconds cool down between measurements. This would be an average power of 1/3 W. Easily tolerated in their package size. So possibly they test at a higher current, and/or longer time. That instrument size can not tolerate too much added average power dissipation. I would guess not 10 W. 10 W is a lot of heat. 8 ohms at 120 is 1800 W or pulsed for 1/60 second or averaged over 30 seconds 1 W. This might be tolerable in this package.

Anyone designing a decent instrument would not use an assumed 120 V. The voltage would be measured before application of the load, and then with the load. The change in voltage between these two measurements would be the primary item in calculating voltage drop.

One can use the instrument at the main panel to determine the power company contribution to voltage drop.

This instrument can be a very good tool for troubleshooting circuits with a problem. You just have to understand circuit theory and the instrument.

One does not want any other load changes during the test measurement time, but one can assume this time is short, possibly 2/60 second.

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[kwired]

The voltage should be irrelevant, all we're talking about is %VD at the end of whatever is there at the beginning. Which brings up a good point. How does that meter work? Hopefully it doesn't assume a nominal voltage but instead have you measure the actual voltage (under load) at the panel or first receptacle, input it, then calculate at the last. If it doesn't do that it's worthless IMO.

ETA: After reading the instruction manual it looks like (but they don't say) that it ASSumes 120V. So that thing is worthless.
-Hal

I don't know much details about such testers. From what I have seen from simpler testers, mostly data sheets or instructions, it appears you plug it in and it gives you some status indications, like correct polarity as well as voltage level indications (may not give you an actual voltage but rather a go/no go for an acceptable range) you then press a button to do testing under load. Might even be GFCI test function included on some.

I would think it is either comparing before and after pressing that button for any VD results or has an already designed nominal voltage it is using for an assumed base to calculate acceptable VD from. May even be both designs available. Either way it won't know what VD is already present at the service disconnect, upstream feeders, etc. it can only respond to conditions present at its connection point.

 

[growler]

Either way it won't know what VD is already present at the service disconnect, upstream feeders, etc. it can only respond to conditions present at its connection point.

The voltage drop given by those circuit analyzers is the total amount of drop.

210.19 (A) Informational note. Conductors for branch circuit should be sized to prevent a voltage drop of 3%. This leaves the other 2% for feeders and service cable.

I had a house with a higher than normal voltage drop and noticed that it was a long way from the transformer. I kind of wondered if there could also be some connection issues. I called the power company and they sent a tech (lineman) out with the beast to do a check. He tested it and he agreed that it was high but still within acceptable limits as far as the power company was concerned ( more than 2%).

That 5% suggestion is for efficiency and not safety. That makes it more of a design issue than a code issue.

There is not much you can do about the incoming power problem.


Just out of curiosity I tested voltage drop in several newer homes and found that there are likely to be a few receptacles that are over 5% in most of them. These will be the ones with the longest home runs, just as you would expect.

We could use #12 GA wire on the longer runs but I don't think it's going to happen unless it's specified that way. As far as I know it's not been a problem.

 

[kwired]

The voltage drop given by those circuit analyzers is the total amount of drop.

Based on what though? It can't know what drop from source to a disconnecting means is unless either a no load measurement is taken first or a reading at the source is taken first. It can assume a nominal accepted value is the base, or it can test before and after effects of it's own test load, but differences in whatever is upstream can affect the results one test vs another if that is what it is doing.

 

[gar]

190515-1630 EDT

I believe what the instrument manual is saying is ---
To make a voltage drop measurement two voltages are measured, one is before the incremental load from the instrument is applied, and the second is while the instrument load is on. The difference between these two measurements is divided by probably the voltage before application of the load to provide a percentage change. I suspect this is done with only a single load resistor value. Then percentage is adjusted for whatever is the wire rating.

This kind of measurement is of course all the way back to the power plant. But most of the percentage is likely from your main panel to the test point. To approximately separate out the power company portion you make a measurement at the main panel.

As I said before it does not matter what loading is on the system when the measurement is made. The only requirement is that the non-test loading is constant.

To understand this instrument you need to know electrical circuit theory, read the manual, and play with the instrument.

I would find it interesting to play with this instrument, but I don't need to spend the money on it. I can do the same measurement with one of my meters and a space heater.

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[hbiss]

To understand this instrument you need to know electrical circuit theory...

Which brings us to an ignorant homeowner buying it on the internet and using it to "bust" the EC. There needs to be some interpretation and understanding of how this works. Not taking it face value.

-Hal

 

[growler]

I would find it interesting to play with this instrument, but I don't need to spend the money on it. I can do the same measurement with one of my meters and a space heater.

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Which brings us to an ignorant homeowner buying it on the internet and using it to "bust" the EC. There needs to be some interpretation and understanding of how this works. Not taking it face value.

-Hal

As gar suggest these are neat little gadgets to play with and can give some useful information quickly. Much more convenient in the field than using a meter and space heater.

The regular little plug in testers that are commonly used to test for polarity and make sure the circuit is live and even test protected GFCI circuits can be fooled by such things as "bootleg" grounds but not these testers. Most of the time they will notice if the neutral and ground are connected before the panel. These testers don't just give a couple of amber lights they actually test the voltage and make sure it is at an acceptable level.

I doubt that the homeowner actually intended to bust the EC. I still remember when I got my first plug in tester to check for polarity. I found several receptacles at work that had reversed polarity so my boss who was an electrical engineer borrowed it and tested his receptacles at home and found several that had reversed polarity. Both at work and at his home we corrected these receptacles with reversed polarity. They would work without it but were not as safe.

Who knows, if the homeowner found a voltage drop of over 14% he may have found a real problem. A problem that could be a hazard and start a fire.

 

[kwired]

Which brings us to an ignorant homeowner buying it on the internet and using it to "bust" the EC. There needs to be some interpretation and understanding of how this works. Not taking it face value.

-Hal

That is where the EC is supposed to present the theory and challenge the HO to disprove it.

 

[ptonsparky]

Which takes us back to page 1.

 

[growler]

Does a voltage drop of 14.8% on the last outlet in a 15 amp branch circuit using 14-2 wire, with 6 outlets, concern you?

That is where the EC is supposed to present the theory and challenge the HO to disprove it.

If a homeowner finds a voltage drop of 14.8% I think I would want to go and check it out for a bad connection.

I find bad connections all the time but it's normally after they have heated up and melted a wire nut or burned a receptacle. One loose back stab connection could give this type of reading or even a loose wire nut.

 

[oldsparky52]

Both at work and at his home we corrected these receptacles with reversed polarity. They would work without it but were not as safe.

While those receptacles were not wired code compliant, what was the safety issue? It's AC, so there is no real polarity.

Please do not interpret what I'm asking as if I think it's okay to wire it that way.

Also, did you find that the receptacle was wired backwards or was the white hot and black neutral (leaving the panel)? The reason I ask, when I first started, just 2 weeks into being a helper, the boss had a circuit that was tripping (new apartment) when the breaker was turned on. He just swapped the hot and neutral and let it go. He did this on more than one unit. I guess the ground wire was touching a hot lead or a staple was overdriven. Yea, I had to work at untraining myself over the years.

 

[LarryFine]

While those receptacles were not wired code compliant, what was the safety issue? It's AC, so there is no real polarity.

It's not a matter of polarity, it's a matter of reducing shock risk. For example, by making sure the neutral is the conductor supplying the screw shell of a lamp or fixture socket, the risk of shock while changing a bulb is greatly reduced.

A partially-unscrewed bulb's screw-shell is exposed while it's still in contact with the socket's screw-shell. The ribbed conductor of the lamp cord is connected to the screw-shell, so it will be connected to the neutral via the wide plug blade.

 

[growler]

Both at work and at his home we corrected these receptacles with reversed polarity. They would work without it but were not as safe.

While those receptacles were not wired code compliant, what was the safety issue? It's AC, so there is no real polarity.

Yes, I remember those days well. A lot of the engineers that worked for the company would make the same argument, "no real polarity".

Even though I did a lot of electrical work in those days I wasn't working as an electrician. I would have been considered more of an electronics technician. We did a lot of bench work and sometimes with the equipment hot (for testing ) and sometimes with equipment cold (change out parts). Many of the techs would just turn off the switch on the equipment and think that it was safe.

Think about it for a second. If your switch only breaks the neutral ( which is really just a return path for an AC circuit) what do you still have that's live in the equipment ? Answer is everything.

After burning up an expensive circuit board one day my boss decided that I was right and the switch really should break the hot conductor to prevent accidents.


It's really best to unplug equipment or turn of a breaker to make sure the power is fully disconnected. But remember that 40 years ago people didn't even wear their seat belts and some even thought a helmet for riding was sissy.