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Anyone measures voltage drop...


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Do many of you measures voltage drop at the outlets? For example, I use Ideal's 61-165 Suretest and scan the 12A, 15A and 20A load tests. It's very common for me to find a couple outlets exceeding 10%-15%...more recently in new homes where they've installed TOO MANY outlets on one breaker.

If you do test for it, what do you write if you find a failing test? Below is what I wrote for the most recent report (with attached pics in the report illustrating the exact outlets):

"A few areas of the home measured a high impedance level, at the receptacles. The impedance levels exceeded 10%, which indicates either an excessively long run of wire or too many junctions leading back to the distribution panel. An impedance (resistance) reading of 5% or lower is acceptable. A product of electrical resistance is heat; excessive heat may result in fire. If plans are to operate high power appliances in said room, first recommend an electrician further evaluate said receptacle wiring. "



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This topic has been discussed several times on this board.

I recommend going to the grey bar at the top of the screen, put your cursor over the "Forums link, drop down to the search link, click on it and type in volt drop in the search box. you will find several discussions and some great comments.

I have no problem with you opening this topic again but there is a wealth of information in the archives for those that know where to look for it.

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Is that the instrument that UL has been complaining about because it is misrepresented as an AFCI tester? I hope it comes with instructions that provide a better explanation about what they mean by 12A, 15A, and 20A load tests. Impedance is not exactly the same thing as resistance (resistance is a component of overall impedance) and it sure isn't measured in percentages of anything. I don't understand what the statement in quotes is trying to say, other than the word "fire" catching my eye.


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Wel I guess we are going to do this again.

Here is what I put in my reports, I stole 99.5% of it from the wise Mr. Katen.

A voltage drop greater than normal was detected on one or more branch circuits in the electrical system. WHAT IS VOLTAGE DROP?

When current passes through a wire, a certain amount of voltage is lost. The result is that the voltage at the point of use will be less than that at the power supply. This loss is called voltage drop. The amount of voltage drop in a circuit is dependent on the magnitude of the current, the diameter and length of the wire feeding the circuit as well as the integrity of any connections along the way. (The larger and thicker the wire, the less the voltage drop will be.)

Voltage drop is important for several reasons:

1. Sensitive electronic equipment such as computers, laser printers and audio equipment may lock-up, shut down or otherwise behave erratically. Certain elements may overheat causing early failure.

2. Inductive loads, such as motors and ballasts, can overheat, resulting in shorter equipment life, and higher operating costs.

3. Resistive loads, such as heaters and incandescent lighting, will operate with surprising inefficiency. For instance, at 10% voltage drop, an incandescent light only produces 70% of its normal light.

4. It is wasteful. The lost voltage in the circuit is converted to heat that is dissipated into unconditioned spaces.

5. It may cause incandescent lights to dim or flicker when other loads cycle on. For instance, lights may flicker when the air conditioner or washing machine start up. Though annoying, this is not dangerous and does not violate any code or regulation.


The National Electric Code does not limit voltage drop in branch circuits. There is a fine print note at 210-19(a) FPN 4, that recommends branch circuit conductors be sized to prevent a voltage drop in excess of 3%, or a total drop of 5%, including feeder losses. Fine print notes are for informational purposes only and are not enforceable. However, Section 110-3(b) says that equipment must be installed in accordance with its listing or labeling. Most electrical equipment is designed to operate at no more than 5% and no less than 10% of its voltage rating. Therefore, I consider voltage drop in excess of 10% to be deficient.


When inspecting homes, I use an Ideal Suretest Circuit Analyzer model ST-1D. This tester applies a 15-amp load to the circuit while displaying the corresponding voltage drop. It then extrapolates the voltage drop for a 20-amp load. In my experience, most homes have at least one circuit that exceeds the NEC's 5% recommendation. However, very few homes have circuits that exceed 10% voltage drop.

Two conditions may be causing this voltage drop. If the cable runs to these circuits are overly long, the resistance of the cable alone will cause the drop. In this case, the remedy will require replacing the cable with one containing larger wires. The other possible cause is poor connections along the way. In particular, the stab-back connections on the back of receptacle outlets have high resistance. If a given circuit passes through enough of these connections, the cumulative resistance can cause the voltage drop. In this case, the remedy will require abandoning the stab-back connections in favor of wire nut connections that have a lower resistance.


I recommend that you have your electrician calculate the cable runs to these areas to determine if the cables are adequately sized. If they are not, have the cables replaced with ones that are appropriately sized. If they are now adequately sized, investigate the integrity of the splices at the outlets, improving them as necessary. Strive for a voltage drop of less than 5%.

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It's way simpler to not test for voltage drop; there's no good test, results would be inconclusive anyway, the NEC doesn't provide useful guidelines (it's only a FPN), etc.

I did it for about 4 jobs; haven't done it in a year. Worry about stuff that matters instead.

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My Suretest tests for drop automatically. The only choice I have is whether or not to report it.

A while back, with about ten students in tow, I tested a receptacle and heard a strange buzzing. I removed the Suretest and the face plate. The screws on the wires were improperly tightened, there were signs of significant arcing, the insulation was melted on the hot and neutral all the way out of the box and the receptacle was crumbly.

The three light tester showed an intermittent neutral when you wiggled it around like a worn receptacle would behave.

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Same here; the big question is whether to report it. I used to report anything >10%; now, I'm not sure what percentage I'd report, but it would have to be a LOT.

The buzzing outlet thing is easy; it's a horribly dangerous condition. That's not about voltage drop, that's about nasty electrical components. And, it's another good reason 3 light testers don't work so good for lots of stuff.

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