[masonic82]

I recently purchased an Extech CT80 Electrical tester because I wanted to go above and beyond to determine whether or not the electrical work I'm performing is safe (outside of using a 3 light outlet tester). This lead me to impedance testing on the ground, hot and neutral and I'm having a difficult time determining if my interpretation on limits is correct. I wrote a support email to Extech but I don't know if they'll respond (if they do, i'll report it here). This is the exact e-mail I sent and I'm hoping to hear anyone's opinion on what I said.

Not sure if you can help me or not but I recently purchased the Extech CT80 to test/diagnose wiring issues inside my home and there’s something I confused about regarding the neutral and hot impedance tests. The main reason I purchased this equipment is because the wire I specifically wanted to test was 14 awg and I suspect it to be probably somewhere around ~100 ft long (and about 18 duplex outlets on it). When you calculate the impedance of the cable at that awg and length with a resistance of 2.525 per 1000 ft at 68 degrees, my assumption is with the wire length itself in the best case scenario wouldn’t fall below 0.2525 Ohms at the end of the run (thus setting the floor threshold number).

In the manual under Test 5: (Impedance Measurements) It states that if the voltage drop measurements are too high (greater than 5%) is then to test the hot and neutral impedance to determine the problem (In my case it was 13%). In the suggestions chart it states a limit of 0.048 ohm /ft of 14 awg wire. At 100ft, the limit (correct me if I’m wrong) is 4.8 ohms max (0.048 x Length of Wire) and the likely cause is excessive loading (I understand these are just troubleshooting suggestions). This is where I’m confused.

Since the branch circuit I’m testing has nothing plugged in and the only draw would be from the load test with CT80, what would be the ceiling limit (max safe threshold value) be on a 14awg branch circuit per foot? (I spent days researching this and still can’t find anything useful for hot/neutral impedance testing) The reason I ask is because if I use the resistance of the wire and set the “best possible scenario” @ 0.2525 ohms and my testing results show 0.50 ohms (which is close to double the resistance of the wire), I’m not sure if this is great or really bad. If I use the 10 awg number of 0.01 ohm/ft, then the limit is stating that it’s ok to be up to 1.0 ohm (which is well below what I’m currently seeing but that’s on 10 awg and I would assume 14 awg would be much higher)

Is there any reference on what would min and max should be in best case/worst case scenarios? Right now my assumption is the min value is the resistance of the cable itself without any splices/cuts/etc and the maximum value is 0.048ohms/ft (or 4.8ohms at 100 ft)? Or am I just completely misunderstanding something?

I appreciate any support you can provide!

 

[Know A Little]

What is your intent with the testing you are performing, to verify impedance and voltage drop is acceptable? Or than you made no wiring errors in your splices?

While I have never used the EXTECH unit I have used something similar and found the results to be less that acceptable. I was less than impressed with the meter.

Estimating the length of your branch circuit and doing the math should give you a fairly close guesstimate of what you are looking for.

To test the meter take a extension cord of a known wire gauge and length connect at the panel and take a look at your results. This unit is not just looking at the branch circuit wiring it is looking at all conductors back to the source transformer.

We have performed voltage drop test for several facilities on their branch circuit wiring and for that we used a resistive heater and variable transformer. Though I believe the EU has tighter standards than we do for this type of testing and you might want to try searching their sites.

Typically (and we do a lot of testing) the primary test is an Insulation Resistance Test, commonly called Megger test, this verifies the insulation of the installed wiring. Power must be off Neutral disconnected and you test , Neutral to ground, Neutral to Phase conductor, and Phase conductor to ground.


Not much help I know but just my two cents.

Remember rely on a professional do not take the advice of any old knuckle head on the internet.

 

[rjniles]

I think you are over thinking and playing with a new toy. I have never heard of an electrician doing impedance testing on a branch circuit. What will you do with the results?

 

[masonic82]

Reply

They actually responded really quick and this was their reply. This doesn't necessarily answer my question but it does verify the wire length is too long to pull a full 15 amp's on ~100 ft of 14 gauge wire. If anyone can provide an explanation on what I should expect to see on the neutral and hot impedance test, it would be greatly appreciated. Here's the reply for anyone else's curiosity. (Btw, the CT-80 is very similar to the SureTest)

____

The CT meter calculates the impedance of the wire based on a voltage pulse it generates at the zero crossing point of the voltage sinewave.
This calculation them provides the % number shown on the display.
5% is great, 7% is fine, anything higher like your 13% indicates that the wire is undersized for the length of the wire.

The impedance can be a loose connection on any of the receptacles in the route or connections at the panel or a true wire length issue based on the wire size.

I hope this helps.

 

[Jim Port]

There are online voltage drop calculators available. You could compare your results against those.

 

[masonic82]

Thank you for all your replies! And yes it's a new toy but it's just my nature to try to truly understand what the test is and what the expectations should be. The simple idea of something that can alert you of a condition (while I'm testing everything else) in one unit seemed like a ideal tool to use.
In reality, I just want to make sure the splices are not causing any major type of resistance and I read many articles how many people use the impedance testing to track down faulty splices, connections without having to open up all the sockets. The curiosity caught me and I'm just trying to learn as much and as best as I can. (Instead of just assuming my confidence is correct). It could of just been a waste of money but that's part of the learning process.

 

[masonic82]

Thanks Jim!

I was aware of the voltage calculations and actually played with the math prior to even purchasing this instrument. So would you say it would be safe to assume that in an ideal situation, the voltage drop calculation should be very close to the actual drop (I can do a load test manually if needed to) if all the splices and connections are fine? I read that the splices will contribute to some loss but it's so negligible that it's not even worth to take into consideration.
My other thought is, I don't actually know the actual length since the walls are closed and I just purchased the home 2 months ago. My assumption in the best case scenario is around 95ft but in reality I feel its probably closer to 120ft but it's obviously not something easy to figure out without actually seeing the path the cables run (I know the sequence of the runs for MOST of the run).
And thanks again for your response! It's very much appreciated!

 

[Oso954]

I would pull a few of those receptacles and check how they are wired, condition, etc. If those 18 receptacles are all daisy chained you have about 35 connections in the hot wire. You are not testing a 100 ft run with a single receptacle at the end.

The connections would add some resistance, more if the wiring/screws are older and a bit oxidized, or if some of them are loose. If you find the receptacles to be back wired, I'd move them to the screws and to see if that improved anything.

 

[masonic82]

Hey Oso,
The reason I started this was due to the fact that there were a few things about the electrical work that I was highly uncomfortable with and it was obvious one of the previous owners did their own work (skipped grounds on some, backstabbed, daisy chained, GFCI's in every outlet on the same circuit, etc.). So I started replacing EVERYTHING and made sure I pigtailed what we needed to be pigtailed, restriped wires and secured to the screws (in the direction of the turn) on the outlets, etc. I only have two light switches left on this circuit and two fixtures (which with the fixtures, I'm going to verify the wiring but not replace them).
So in reality, I'm curious if the 0.50 impedance is what I should be expecting or if I should expect it to continue to drop if I replaced the remaining components on the circuit.

 

[Know A Little]

There are other factors than just the wire length of the branch circuit, the impedance is of the wiring back to the utility transformer.

http://www.csgnetwork.com/voltagedropcalc.html

 

[masonic82]

Thank you Know! I do get the concept that the impedance goes back the utility transformer but I did run this tests on all the outlets on this circuit. The very first outlet starts at .01 (Maybe 5 ft. from panel) and then the 16th outlet (Maybe ~100+ ft.) comes in about 0.50. Along the way from the first to the last there's an incremental increase in each outlet. With knowing that, would the delta between the first and last mean anything to you? Conditions would be considered normal (home is about 68 degrees and with zero draw) and its just typical romex 14/2 with ground on a 15 amp breaker.

 

[TimPa]

you didn't mention the setup you are using - where are you placing the probes? assuming the circuit is dead (open CB), or is a live tester of some sort? did you install a shunt at the end of the run? are you measuring the hot wire out and the white wire back (which would double the length/resistance)?

if it is in fact 14 awg, make sure it is on a 15a CB. oops, I see you have that!

for clarification, impedance is a term which used to express all current opposition, to include - resistance, capacitance, and inductance. with no capacitive or inductive loads involved, resistance = impedance.

 

[ddawg16]

Mason....I know where you are coming from,.......I'm sort of like you in that respect.

I think there are some good things accomplished by doing a test like that. Given how most houses are wired, it's a good way to find potential problems before it becomes a big problem.

Step 1....you need to make sure you have all of your outlets mapped out and which breaker they go to. I typically write the breaker number on the outlet real small using a fine point sharpie.

Next....I do a load test...

It take the wife's iron...I plug it in to one out outlet with it off. I plug my meter into the other outlet at the recpt. I measure the voltage.

I then turn on the iron. Assuming it's a 1200W iron, it means you are pulling about 10A. Measure the voltage.

5% if a fair ball park figure for voltage drop. The closer you are to the load center, the smaller the drop should be.

If you start at the outlet closest to the load center and then work your way down the line, if you have a bad connection in an box....it will be pretty obvious.

I'm a real fan of pig tails for outlets.

 

[Jump-start]

I think you are over thinking and playing with a new toy. I have never heard of an electrician doing impedance testing on a branch circuit. What will you do with the results?

Its actually very common over seas often mandated by foreign code such as BS7671. At minimum its to ensure the EGC is fully intact from the receptacle to the main panel.



http://fluke.informationstore.net/efulfillment.asp?publication=10642-eng

http://www.seaward.co.uk/downloads/17th Edition Testing Guide.pdf