Measure voltage drop with no load at all?

[ggunn]

I use 3 circuits, this way I'm guaranteed to double-up on a phase (in case only one service conductor is acting up)



He gave me one, and it seems to be the same method iwire has seen an inspector use: With no load on the branch, test the panel lugs, then test the farthest point on branch (DMM) and that's your official drop.

With no current flow those voltage readings will be the same. Current flowing through a resistance is what creates voltage drop; no current, no drop. EE101.

 

[MD84]

If voltage drop is the issue under consideration, then you can use a procedure not unlike what the local inspector tried to describe. But he described it incorrectly. With no load (and indeed with the power off), you can measure the resistance of the conductor(s) from the breaker to the receptacle.

What instrument would be used for this test and how would the test be setup? Would it be a DLRO and a jumper? I can't see a typical DMM giving any useful results.

Also for the simple test of plugging in a load and checking voltage what would be a suggested load? Anything compact for the tool kit?

 

[gar]

160730-2426 EDT

escop:

We need an exact written description from the inspector on how you are to measure voltage drop or resistance without any current flow in the conductor(s) in question.

If your circuit has no load anywhere, then the output voltage for the pair of wires will exactly equal the input voltage to the wire pair for wire lengths much shorter than the wavelength of the frequency of measurement. Residual factors will produce only very small differences. Possibly 1/1,000,000 of the input voltage for a loop resistance of 10 ohms with a 10 Megohm meter. At 100 V input the difference is about 100/1,000,000 V or 0.1 millivolt. A rather controlled experiment would be required to perform this test.

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[escopf]

160730-2426 EDT

escop:

We need an exact written description from the inspector on how you are to measure voltage drop or resistance without any current flow in the conductor(s) in question.

If your circuit has no load anywhere, then the output voltage for the pair of wires will exactly equal the input voltage to the wire pair for wire lengths much shorter than the wavelength of the frequency of measurement. Residual factors will produce only very small differences. Possibly 1/1,000,000 of the input voltage for a loop resistance of 10 ohms with a 10 Megohm meter. At 100 V input the difference is about 100/1,000,000 V or 0.1 millivolt. A rather controlled experiment would be required to perform this test.

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thanks for the info; I didn't know about the wavelength constriction, very interesting... Just to check: an RMS meter would have frequency, but a cheaper one (that measures peak and assumes sine) wouldn't? Or some harmonic of 60Hz?

Well, all that stuff would go over the inspectors head though; he does not know how to measure drop. Everyone here, me included, does. My concern now is getting the situation fixed, and it's going to be a wacky Wednesday if everybody in town measures like this.

I agree that I need a written description from the AHJ, I'll try to get one. I also need to map the system more thoroughly. Will post back here in a few weeks with all that.

thanks for your help

 

[K8MHZ]

thanks for the info; I didn't know about the wavelength constriction, very interesting... Just to check: an RMS meter would have frequency, but a cheaper one (that measures peak and assumes sine) wouldn't? Or some harmonic of 60Hz?

Pretty much all, if not all, meters will display RMS. Even the cheapest ones.

Well, all that stuff would go over the inspectors head though; he does not know how to measure drop. Everyone here, me included, does. My concern now is getting the situation fixed, and it's going to be a wacky Wednesday if everybody in town measures like this.

You posted a link to the FBC and pointed out they require testing under a 'design load'. I would make a copy of that and bring that in, with the requirements for loading highlighted. I would then ask why they are purposely ignoring that part of the requirement. Good luck trying to get the reason in writing, but I would try that, too.

 

[K8MHZ]

Hello and thanks, always, for your time.

A homeowner of a new-construction house (Florida) has been having her TV's power off intermittently, lights dim sometimes etc. So my first step is to see what branch-circuit drop is. At the outlets farthest from the panel I measure 123v@no load, 121v@minimal load, 114v@7A load and 101v@15A . The line is 15A OCPD so at 12A it would be dropping well over 10%.

My first step would be to monitor the voltages at the outlets that exhibit the problem. If the problem is one of short duration, an analog or fast DVOM will be needed unless you have a recorder.

If you do find a problematic drop, then the reason must be figured out, and one reason could be a high resistance neutral. Just checking the hot wires may not locate the problem.

No matter what the codes say, or what the inspector thinks about acceptable voltage drop, voltage drop that shuts off TV's and makes lights dim to the point of being annoying needs to be corrected.

 

[iwire]

voltage drop that shuts off TV's and makes lights dim to the point of being annoying needs to be corrected.

I do not think anyone can yet say this is a conductor voltage drop issue.

It seems to me the OP has latched onto voltage drop being the issue without considering more likely causes.

 

[escopf]

I do not think anyone can yet say this is a conductor voltage drop issue.

It seems to me the OP has latched onto voltage drop being the issue without considering more likely causes.

Agreed. I'll post back here once I collect more info.

 

[K8MHZ]

I do not think anyone can yet say this is a conductor voltage drop issue.

It seems to me the OP has latched onto voltage drop being the issue without considering more likely causes.

Good point. Actually checking the voltage where the problem is would lead one to pursue it or not. I guess that was really my point. If the TV shuts off and the voltage stays above 110 volts (for example) I doubt the problem is voltage drop. It could be the TV for all we know.

However, if the voltage drops to 98 volts (for example), that would be a voltage drop issue that needs to be addressed, no matter if it's a code violation or not. There may be a bad connection somewhere, and that is a hazard in itself sometimes.

 

[JFletcher]

Hello and thanks, always, for your time.

A homeowner of a new-construction house (Florida) has been having her TV's power off intermittently, lights dim sometimes etc. [/URL].

thanks as always

Are the TVs on a sleep timer? Lights dimming for a few moments is pretty typical this time of year in high heat areas. They would not be on the same circuit.

Possible that an animal or kid is playing with the remote? Do the TVs cut back on right away? How many TVs, how old are they? Is it even the TV powering off, or the cable box?