[TowerTalk] Resistance in house neutral

[Jim Brown]

On Mon, 26 Jul 2010 11:19:53 -0400, Pete Smith wrote:

>I just measured the resistance between the neutral wire on one outlet in 
>my shack and the SPG entry panel, which is grounded through 35 feet of 
>#2 wire to the same ground stake used by the breaker panel.  To my 
>surprise, it measured (DC) 88 ohms.  Isn't that far too high?  I expect 
>I have some trouble-shooting to do.

Maybe, or maybe not. I suggest that you make a simple spreadsheet for the 
expected resistance based on the length of wire from outlet to panel, from 
panel to the ground rod(s), and your shack to the ground rods. Assume #14 
for 15A circuits, #12 for 20A circuits. #12 is 1.588 ohms/1,000 ft, #14 is 
2.525 ohms/1,000 ft. #2 is 0.156 ohms/1,000. 

The ONLY connection between neutral and ground must be at the breaker 
panel where power comes into the house, and it must be a VERY good bond. 

One thing that could muddy the water is the presence of current on those 
conductors, which is highly distorted. Distortion can also result in 
imbalance -- that is, DC -- and that DC could be giving you a false 
resistance reading. I would look into that before chasing your tail too 
far. :) To get rid of that distortion, you'll need to kill power to the 
building. 

One good way to get at least a rough handle on proper bonding is to look 
at the AC voltage between neutral and ground with a meter that reads low 
AC voltages well. The only place it should be zero is at the panel, or 
very close to the panel (in terms of wire length). Anything up to a volt 
or so is normal at outlets that are drawing current. The voltage is mostly 
the result of IR drop on the neutral, but can also be caused by leakage 
currents. In a steel structure with other big power draws, the leakage 
currents can be significant (a volt or more). If you see zero volts at an 
outlet with something plugged into it, there's probably a bonded neutral 
in that equipment. 

Don't rule out mis-wired outlets. They get wired by humans, so the law of 
averages says that almost every building will have at least one, and 
that's all it takes to make a mess. A simple outlet tester is a great 
investment. Ecos and Tasco are the best brands, but hard to find. Ideal 
also makes one that is less useful.  

73, Jim Brown K9YC