Resistance test on cores of buried cable (Ed.)

[forumscout]

Okay, so here is my situation. I am pulling a 20 amp circuit from my garage, 150' to pillars at the end of my driveway. A buddy had 4 runs of 150' of 8 gauge wire he had pulled from underground, because he needed more amperage to his workshop. Like an idiot, because I was looking at today's problem, not tomorrow's, I ran 1.25" PVC for the run, 24" underground (per code here), pulled three wires through, and buried it. Unfortunately, work (real job) interfered, and it was 2 weeks before I could get back to the project and run my connections. Because I am slow and an amateur, it didn't occur to me to check continuity on my lines until this morning. Still haven't attached any of the 3 wires that are underground, but used 4th wire connected to each in sequence to check resistance. Each wire showed ~ 0.1 ohm resistance over the full run (300') which should be about right, as I should be getting 0.18 ohms resistance for this length of 8 gauge. The problem is that I accidentally tested known good to one of the unconnected wires, and instead of OL (open line), my ohm meter showed 2 M ohms resistance... from my reading and very basic university level physics, I get that I may show some resistance because of magnetic induction in the unconnected wire, but, before I connect these wire to the home power, I wanted to ask some experts if this is indicative of a short underground, or if this is perfectly normal.

In order to anticipate the info you may need, I am using a 400a 600V Kobalt DT-9180A digital multi-meter and it is multi-strand copper wire 8 gauge, run through 1.25" PVC. Any other info you may need to help me out, let me know. As of now, I still haven't attached either end of the wires, either to the house power or to a plug at the gate pillar.

Best regards,
Dan

 

[forumscout]

Ooops, while to me it is self-evident (simply because I grew up with it) I suppose I should mention that this is a 110V line, not 220/240... not sure if this needed saying or if it is relevant at this time, but wanted to give all data I could think of.

 

[Justin Passing]

OL is/wasn't ever "open line", it's just out of range for the meter to measure, more like over load.

Any hint of water or of course you touching something, might give a reading of 2M ohm.
You do have a long aerial there so could be picking something up which is making the meter show a reading.
Only answer really is to get an insulation tester (aka "Megger" if you're old enough) which tests with a lot of volts instead of the very low voltage and very sensitive resistance meter.

 

[FrodoOne]

Okay, so here is my situation. I am pulling a 20 amp circuit from my garage, 150' to pillars at the end of my driveway. A buddy had 4 runs of 150' of 8 gauge wire he had pulled from underground, because he needed more amperage to his workshop. Like an idiot, because I was looking at today's problem, not tomorrow's, I ran 1.25" PVC for the run, 24" underground (per code here), pulled three wires through, and buried it. Unfortunately, work (real job) interfered, and it was 2 weeks before I could get back to the project and run my connections. Because I am slow and an amateur, it didn't occur to me to check continuity on my lines until this morning. Still haven't attached any of the 3 wires that are underground, but used 4th wire connected to each in sequence to check resistance. Each wire showed ~ 0.1 ohm resistance over the full run (300') which should be about right, as I should be getting 0.18 ohms resistance for this length of 8 gauge. The problem is that I accidentally tested known good to one of the unconnected wires, and instead of OL (open line), my ohm meter showed 2 M ohms resistance... from my reading and very basic university level physics, I get that I may show some resistance because of magnetic induction in the unconnected wire, but, before I connect these wire to the home power, I wanted to ask some experts if this is indicative of a short underground, or if this is perfectly normal.

In order to anticipate the info you may need, I am using a 400a 600V Kobalt DT-9180A digital multi-meter and it is multi-strand copper wire 8 gauge, run through 1.25" PVC. Any other info you may need to help me out, let me know. As of now, I still haven't attached either end of the wires, either to the house power or to a plug at the gate pillar.

Best regards,
Dan

While I really do not have much of a clue about that which you are asking,
I suggest that this question would be better posted on the North American site ofv

https://www.diychatroom.com/forums/electrical.18/

 

[securespark]

If the cable is:

undamaged and

has nothing connected to either end and

the conductors are well isolated from each other

then I would expect a very very much higher insulation resistance reading.

 

[Ragnar_AT]

However, connecting a DMM to 150 feet of wire could cause all sorts of strange readings, mainly from capacitive coupling I suspect. As others have said, you really need a proper insulation resistance tester with 250 or 500 V DC test voltage. Finding one in the US will be tricky though, they have nothing like the testing and inspection we‘re used to. I don’t thin most electricians own anything beyond a plug-in socket tester and a multimeter.

 

[bernardgreen]

The resistance of a length of a conductor can be measured by passing a known current through the conductor and meauring the voltage created by the current. Then calculate R = V / I ( I being the test current ),

The test current should be about an Amp to swamp any induced current and the Voltmeter should be protected against over voltage.

 

[Ragnar_AT]

You can test the resistance of each conductor (which was the initial question) but not insulation resistance between two conductors. That requires a significant DC voltage.