resistance testing

[Daniel Munoz]

I recently had to test each branch circuit for total resistance and my fluke would show 1 ohm for each circuit. Today i used a amprobe and i was getting .4, .2, .3. The foreman disagreed with my recording and said the lower two floors had 3, 5, 7, and so on and mine didnt seem correct. Can anyone help me with this im a little lost with these results. thanks

 

[480sparky]

Were you just checking for continuity, or were you doing a resistance test ("Megger"ing)?

 

[Jim W in Tampa]

Exactly what were you measuring and what was the load

 

[cadpoint]

I thought there was another table in the NEC on resistance values but...

2011 NFPA Table 9 Page 70-682, note this is 3 phase ... - Three single conductors in conduit.

Depends on what size and length your measuring, I recon... :grin:

 

[Daniel Munoz]

No load and no megger. just resistance testing. what i dont understand is why the other floors were getting readings so high

 

[Jim W in Tampa]

OK lets try this another way.Just what are you putting the probes on ?

 

[Daniel Munoz]

the probes are on the hot conductor and the ground at the panel (not hot) and at the last junction box i have the hot and the ground tied together

 

[480sparky]

the probes are on the hot conductor and the ground at the panel (not hot) and at the last junction box i have the hot and the ground tied together

So you're just checking for continuity of the hot and ground.

 

[Daniel Munoz]

pretty much but i also have to provide the reading from the meter for each circuit as well.

 

[Daniel Munoz]

my question is why did the foreman say my readings of one ohm per circuit were incorrect? When i checked with the amprobe i did get lower readings but he said the lower floors were getting higher reading like seven and even eleven. Does this make sence to anyone because it doesnt make sence to me?

 

[Jim W in Tampa]

I have no idea why you would be doing this but if all the floors are the same then perhaps you have some unintended loads like lights that are on or emerg / exit signs. Without knowing the size of wire and number of feet we can not tell you if your floor or his is wrong.
If we are talking a few hundred feet x2 your reading does sound low. Could you have any conductors touching ground in the boxes ?

 

[Daniel Munoz]

Thanks anyways guys. I appreciate the help.

 

[ELA]

Assuming you are just taking ohm readings with your DVM you are likely to get widely varying numbers. When reading low value resistances with a DVM you have to be sure to subtract the lead resistance from the reading, also make sure each test lead is making a very good mechanical contact with the wires under test.

In addition to this there cannot be any voltage present on the conductors under test. Any voltage present on the conductors will upset the ability of the meter to accurately read ohms. Even induced voltages can be an issue.
Take a reading on the volts scale and record before you take each resistance reading and compare against the readings that do not agree.

 

[don_resqcapt19]

One table I have shows the DC resistance of #12 at 1.6 ohms per 1000'. Numbers below one would sound correct to me for most circuits.