This is the first time I have ever known an inspector to test for outlet impedance and, as you may have guessed, I didn’t pass the final inspection. As a result, I did not get the final check. This in turn, has caused me to feel a little salty today.
I hope I can get some guidance from you sparkys out there so I can iron this out with my electrician and move on.
Project info
Enclosed a small screem room (120 s.f.)
Used the existing circuit for the new electric. It had a ceiling fan with light, exterior flood lights(one fixture, two bulbs), one outlet maybe two (can’t remember)
Existing circuit is 15A with aluminum wire (probably 12g)
The room now has the same exterior fixture, four 4″ cans on a dimmer, three outlets
New wiring is 14g copper
Device connections are hooked not stabbed
The measured impedance is 45 ohms, max allowed by code is 25 ohms
The inspector seems to thinks the problem is a loose ground somewhere, a likely problem for aluminum. Other than that he did not offer much info.
Any suggestions?
Replies
"Would have to see it" before anyone could even determine the accuracy of any written deficiencies about the flow. Saftey is the good inspector's worries. What was his recomedations out of curiousity?
He asked that I have the electrician come out an tighten all of the grounds from the service to the outlets
CARPEnter DIEM
Does the inspector collect portraits of dead presidents?
You ready for a little search (for the wire path) and destroy (the oxide) mission?
Sand each bare wire down to new metal and (especially on the aluminium wires) apply an anti-oxidation paste. Retighten. It's gotta help. The annoying thing is that it is probably only one or two connections out of many.
...and chances are, you want to find and fix those connections. If they're corroded, you don't want 'em to start arcing...
First time I've ever heard of anybody checking impedance in household wiring...geez, one more thing to check before the inspector gets there!
with aluminum wire this is VERY important.
aluminum residential wire was abandoned in the early '70s for this reason (fires caused by oxidation at the joints).
The safest solution is to have a licensed electrician install air tight jumpers designed to eliminate this at EVERY joint in the house (the jumpers end in copper wire, which is then attached to the device, or nutted together).
I grew up in a house wired in alu (built '67) and my dad checked the joints every time he worked on any circuit, and applied the anti-oxide paste that was introduced as a stop-gap during the '70s. The new owners had the jumpers installed (condition of the sale) so that they would not have to worry about it as we did.
Any undue resistance in an aluminum wire circuit is a sign and only checking the ground wire is NOT enough, even if it is all that the inspector requires.
Hope this helps,
Norm
Tapcon,
Can you give some more detail on how the inspector did the test? Where and how were the measurements made? What sort of instrument?
But I'd first raise the issue of his authority to fail a job based on what I assume is the impedance of the equipment grounding conductor (EGC). The only NEC requirement for grounding impedance is that the impedance of a ground rod to earth be less than 25 ohms. Unless there's some local or state addendum to the NEC, I'd say that the inspector has no legal basis for failing your job based on EGC impedance measurements. That's not to say that making the measurement isn't a good idea. Especially in a branch circuit that includes aluminum wiring.
I use a tester to measure EGC (and also the supply and return conductors) impedance. The thing looks like an outlet polarity checker on steroids. It's the Sure-Test made by Ideal Industries, and it also tests voltage drop under load. These measurements are very helpful in detecting bad connections. But I find it hard to believe that any building dept has included such testing in its regulations.
At any rate, if indeed there's 45 ohms impedance in the EGC, and the test was done right, then the inspector did you a BIG favor by finding it.
The funny thing is, if the EGC is bad (but not completely open), you'll never know it. If there's a ground fault and and the EGC path is poor (and a multimeter resistance measurement is no good at telling you whether a conductor can pass 15 amps or more), the breaker won't open as soon as it should to clear the fault. Meantime, the fault is heating up and the EGC is heating up, and maybe if you're lucky the fault will burn itself open before a fire gets started.
I'd like to know more about the testing and the basis for the red tag. And also how your sparky made the aluminum to copper connections, and whether you have had any work done on the aluminum wiring.
Cliff
Cliff
I was waiting for you or 4lorn1 to jump on this.
I am an electrical engineer by training. But all of my professional work has been in digital controls and how software based controls. 12 volt is "high voltage" to me.
And I was wonder what he was testing because 25 ohns in the not or neutral is way to high.
Anyway my question is does you tester really test IMPEDENCE or just resistance under load?
Bill,
Well, I don't have your credentials, but I'll have to guess it tests impedance (which would be resistance+inductance+reactance, right?)
It does this by dropping a 12 or 15 amp load across the conductors for 8 cycles to see what the voltage drop is, then calculating the impedance. Because the instrument will report separate values of impedance for the ungrounded, grounded, and grounding conductors without an outside reference, I assume that there's some comparison made among the three, and an algorithm is applied to estimate each conductor's value.
The SureTest is a great tool*. Example: I was testing receptacle outlets in an old house the other day. As best I can recall (and keep in mind that my CRS syndrome is getting worse every day), the voltage drop (hot-neutral) at one was 18 percent at 15 amps. The impedance of the hot conductor was over one ohm; the impedance of the neutral and of the equipment ground were 0.01 and 0.02 respectively, or something close to that.
That's clear evidence of a bad splice or termination on the ungrounded side of the circuit. The surrounding outlets showed Vdrop of 6 percent or so, with impedance (h,n,g) of 0.05 or less. My guess is that it's a poke-in backwire connection that's gone south. I told the homeowner to not use that outlet; it's a bedroom that's not occupied now, anyway, with little downstream load. If it'd been in a circuit that I expected to be heavily loaded, I've have opened things up then and there. But there is that separation between evaluation/inspection and troubleshooting/repair that needs to be maintained unless there's an overriding reason...
Anyway, I'm arranging to return to the house to add a couple of new circuits, and I'll open up the receptacle with the high Vdrop. It'll probably be an easy fix. I'll take and post a digi photo if it's something worth sharing.
Regards,
Cliff
* I have no connection to Ideal Industries, really. I do think they make great tools and instruments, though. Oh, and if you want to see my latest tester, search "KO instruments". They make a circuit breaker tester that performance tests 15 and 20 amp branch circuit breakers, safely. It also does a voltage drop test. It's expensive, but paying it's way pretty nicely. CP
"Oh, and if you want to see my latest tester, search "KO instruments". They make a circuit breaker tester that performance tests 15 and 20 amp branch circuit breakers, safely. It also does a voltage drop test. It's expensive, but paying it's way pretty nicely. CP"
I looked it up, very interesting device. I wonder how many 'bad' back stabs that tester will take out. - "Fault current during breaker test - 120 A to 1500 A"
I did some thinking about the how the a tester like the Idea is probably made. And it mostly like does test for impedance, without reguard to the phase anagle.
But a 25b ohn ground resistance that is all reactive would be worthless for use with an surge protection. But in real life you would never have that.
I was just surprised at the orginal poster use of impedance instead of resistance.
I had to go back and look at the NEC test for grounding electrodes and there they do use resistance.
Edited 5/14/2004 4:04 pm ET by Bill Hartmann
I'm not sure either if he's talking about the impedance to true earth of the ground rod(s), or the impedance of the hot and neutral in the outlet viewed as a source.
Impedance of the ground rods to true earth has nothing to do with tripping breakers. Resistance between the ground at the outlet and the ground and neutral at the service entrance is what would be an issue for that.
As for source impedance of the hot and neutral, well, the traditional half voltage method sure isn't the way to measure that! ;-). I did figure it out years ago in my old apartment when I happened to have a 2,000 watt lamp handy. It was 0.7 Ohms. If you really had a source impedance of 25 Ohms, a single 100 watt bulb would load the circuit down to a tad under 100 volts. A 1000 watt toaster wouldn't work at all. This would be a huge fire hazard.
Aluminum should be replaced if at all possible, and if not, it needs to be pigtailed with copper by somebody who has the special tool and training.
-- J.S.
John , to you and all...isn't a Dimmer a variable resistor? Take the dimmer to full on and the prob. will be lessend ,no?
The dimmer would be my first stab..
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Spheramid Enterprises Architectural Woodworks
Repairs, Remodeling, Restorations.
See the "how hot should a switch be" thread for more than you want to know about dimmers. Modern household dimmers aren't simple resistances.
But if this impedance measurement is being made through the dimmer, all bets are off. That would be a mistake.
-- J.S.
and I'm bettin that IS the mistake
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Spheramid Enterprises Architectural Woodworks
Repairs, Remodeling, Restorations.