Help with megger results of NM-B

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macmikeman said:
What code section is your inspector using to demand such testing from you?
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Im not sure he is siting a code it's just what he wants. It's not even my job. It's a buddy of mine that wired it but the inspector said he had to get an outside agency do the meggering
 
crtemp said:
Im not sure he is siting a code it's just what he wants. It's not even my job. It's a buddy of mine that wired it but the inspector said he had to get an outside agency do the meggering
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Still the same answer, find a NETA testing company, pay the $500 to $1,000 and be glad they didn't make him tear everything out. The inspector is being extremely reasonable in my opinion. I would have to trust the person and have a relationship already before I would do that. It means, no inspection for proper securing, nail plates, stand back from sheetrock etc.
 
Strathead said:
Shoot they won't even allow one-siding in the City limits.
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It is fairly common for exterior sheathing to be on before wiring gets installed - how do they handle that "one-sided" situation, or if it is allowed better ask why not elsewhere?

Sometimes it is nice to drywall one side in a mechanical room before they put furnace/air handler and ductwork in.
 
kwired said:
It is fairly common for exterior sheathing to be on before wiring gets installed - how do they handle that "one-sided" situation, or if it is allowed better ask why not elsewhere?

Sometimes it is nice to drywall one side in a mechanical room before they put furnace/air handler and ductwork in.
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Hopefully my inspector isn't reading this. He feels that it takes too much of his time to have to walk around a wall and look at the wiring from only one side. They have never said anything about exterior walls though. Bottom line you have to work within the whims of your local inspector or fight them and live with the consequences.
 
Strathead said:
Hopefully my inspector isn't reading this. He feels that it takes too much of his time to have to walk around a wall and look at the wiring from only one side. They have never said anything about exterior walls though. Bottom line you have to work within the whims of your local inspector or fight them and live with the consequences.
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I'd rather fight than succumb.
 
480sparky said:
I'd rather fight than succumb.
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I pick my battles, and that certainly isn't one of them, because it isn't a right or wrong one, it is really up to the inspection department. It doesn't cost me money and it is the way it is in this city.

Regarding fighting, my usual strategy is to do as the inspector asks, if the cost isn't high and then respectfully discuss the actual code on his return visit. The honey will often get far better results.
 
iwire said:
Which tells me you don't work on jobs with serious schedules.
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serious schedule may be a reason to cover one side of a wall at times, yet leave other side open for other work and inspections.

All new construction - probably not going to have "one side" all throughout the project, but IMO nothing wrong at all with an occasional "one side" here and there. If inspector is too lazy to walk around to see other side he is an idiot, plain and simple. He has to go up/down stairs to see what is in the floor/ceilings on multi level buildings doesn't he - or does he want floor joists left open on both sides as well?:huh:
 
GoldDigger said:
... But if the insulated ungrounded conductor is touching grounded metal (including a bare EGC) anywhere in the system you will be applying 1000V across only one layer of insulation when you test from ungrounded conductor or neutral to ground.
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Each conductor is rated at 600VRMS so it won't be an issue regardless of whether measuring between L-L or L-G. Like others have mentioned 1kVDC is the standard test voltage for 600V equipment and new wiring will be able to wothstand far more than that.
 
luckylerado said:
Absolutely true. The 1000V test will not harm the 600V wire and in fact is the NETA suggested voltage for testing the wire if the wire is listed as 600V. Testing it as a system IAW NETA standards would allow for a test voltage of 500V for systems of 250V or less.


NETA ATS-09

7.3.2.2.2
Perform insulation-resistance test on each conductor with respect to ground and adjacent
conductors. Applied potential shall be 500 volts dc for 300-volt rated cable and 1000 volts dc
for 600-volt rated cable. Test duration shall be one minute.

T 100.1
View attachment 13218
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Thank you for this :)

What is a typical ok value per 100 feet of NM? Doesn't seem to give wire per x distance.
 
kwired said:
serious schedule may be a reason to cover one side of a wall at times, yet leave other side open for other work and inspections.

All new construction - probably not going to have "one side" all throughout the project, but IMO nothing wrong at all with an occasional "one side" here and there. If inspector is too lazy to walk around to see other side he is an idiot, plain and simple. He has to go up/down stairs to see what is in the floor/ceilings on multi level buildings doesn't he - or does he want floor joists left open on both sides as well?:huh:
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What you describe as an occasional one side here and there is different that the blanket "one-siding" that I was referring to. When it is necessary or makes the job far easier, the inspector I refer to would not have a problem with it. And in defense and agreement with iwire, if you are in a rush it would be less time consuming to get the inspector there than to one side, and have to tear it back off again, of even fail for a day, which unless you are far more perfect than my guys an inspector can do on every job. Fighting an inspector on little can never work out well.
 
mbrooke said:
Thank you for this :)

What is a typical ok value per 100 feet of NM? Doesn't seem to give wire per x distance.
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It does not work that way. You are testing the insulation to ensure it is intact. It is pretty much a pass fail scenario at these voltages. Greater than 2KΩ is all you can ask for and is essentially infinite ohms.

Less than that and there is likely something nicked or touching somewhere or there is a staple overdriven or a smashed wire from a hammer miss or at 1000V even a conductor that has exceeded the max bend radius if it is extreme.
 
luckylerado said:
It does not work that way. You are testing the insulation to ensure it is intact. It is pretty much a pass fail scenario at these voltages. Greater than 2KΩ is all you can ask for and is essentially infinite ohms.

Less than that and there is likely something nicked or touching somewhere or there is a staple overdriven or a smashed wire from a hammer miss or at 1000V even a conductor that has exceeded the max bend radius if it is extreme.
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But doesn't the extra length lower the total readings? 2000 feet on NM would meg lower then say 25 feet?
 
mbrooke said:
But doesn't the extra length lower the total readings? 2000 feet on NM would meg lower then say 25 feet?
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No. If you hold the leads apart shoulder width you should measure the same as if the leads were a mile apart. There should be no measurable resistance between the leads at these voltages. This is a relatively low voltage test when compared to say a hi-pot and the idea is to prove that the insulation is insulating. The meter in this case is not actually measuring 2KΩ rather it is bottoming out at 2KΩ. We cannot call it infinite because we know at some voltage the insulation will breakdown and allow electron flow so we say greater than somemaxΩ at some testvoltage
 
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luckylerado said:
No. If you hold the leads apart shoulder width you should measure the same as if the leads were a mile apart. There should be no measurable resistance between the leads at these voltages. This is a relatively low voltage test when compared to say a hi-pot and the idea is to prove that the insulation is insulating. The meter in this case is not actually measuring 2KΩ rather it is bottoming out at 2KΩ. We cannot call it infinite because we know at some voltage the insulation will breakdown and allow electron flow so we say greater than somemaxΩ at some testvoltage
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Got it, thanks :)
 
As I read this thread, I can't help but wonder 2 things (honestly):

1) How do you become an electrician without knowing how to test insulation?

2) Why wouldn't you megger all the wire you run? It's cheap insurance against a lot of headache later, should there be a nicked wire, hammered wire, or sloppy connection.

Keep in mind, I didn't go through a fancy union apprenticeship, so I'm honestly perplexed. I was taught never to make a connection you hadn't tested. If I have wire nuts in one hand, I have my megger in another. I'm a lot happier re-running a 50' section of NM in open framework than trying to troubleshoot a tripped breaker after all the drywall has gone up.
 
YourElectricianOC said:
As I read this thread, I can't help but wonder 2 things (honestly):

1) How do you become an electrician without knowing how to test insulation?

2) Why wouldn't you megger all the wire you run? It's cheap insurance against a lot of headache later, should there be a nicked wire, hammered wire, or sloppy connection.

Keep in mind, I didn't go through a fancy union apprenticeship, so I'm honestly perplexed. I was taught never to make a connection you hadn't tested. If I have wire nuts in one hand, I have my megger in another. I'm a lot happier re-running a 50' section of NM in open framework than trying to troubleshoot a tripped breaker after all the drywall has gone up.
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In fairness, I am an electrician like you, sort of. I understand the value of testing as you describe etc, but dispute that it has as much benefit as you place on it. for one thing, most of the deficiencies you describe are much more likely after sheetrock goes on. That said, being in construction for many years, there are far more people out there who can wire things and follow instruction, check to make sure something is dead, but don't really have a clue what electricity does, than there are who understand the nuance of megging. I have learned to accept that and use people (and pay them) to the level that they are. As a smart one, you generally get to avoid things like crawling in an attic or digging a ditch. those who don't know how to megger spend their time in a ditch. The person who asked this question shows a desire and willingness to improve and a lack of arrogance that makes him willing to ask what he doesn't know. Ask around, you may find that your coworkers know and I mean really know, less than you think.
 
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