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Permanent Moisture Level

Permanent Moisture Level

Permanent Moisture Level


I'm not a PE - I'm involved in the electrical trade. But I'm hoping that a civil engineer can point me in the right direction. There is a code in the National Electrical Code [2014 NEC Article 250.53(A)(1)] that references "permanent moisture level." It reads like this,

"If practicable, rod, pipe, and plate electrodes shall be embedded below permanent moisture level."

My Questions:

1. Is there such a thing as "permanent moisture level?" If so, would it not change due to rainfall, lack thereof, etc.?
2. What document or standard do I go to in order to learn more about permanent moisture level and where to look up the various depths of permanent moisture level for any given area? Or does this depth require some kind of a soil study for that particular piece of property?
3. Can permanent moisture level vary from area to area, or even lot to lot?

Thank you for any help, or if you can at least point me in the right direction.

RE: Permanent Moisture Level

To me this is a meaningless statement. In my experience I don't think I have ever sampled a natural soil zone at zero moisture content well above a ground water table (which also varies). Instead it would seem that the electrical resistance minimum per some unit might mean more. Even then determining that for routine work would seem to be unrealistic. I believe you folks in design of lightning protection do measure resisivity. That can be determined approximately with depth. Geotechnical engineers do have a system for using that in approximately investigating subsoil conditions. Soil pores tend to hold moisture under a number of different modes, such as capillarity. If a moisture condition below some depth is to be specified, one also might bring in a measurement that is common in the agricultural area called tension or suction, how tightly moisture is held by the sample tested.. This is useful in evaluating conditions for plant growth and that may be tied to electrical resistance I would suppose. Maybe there is an agronomy forum that can help. Finally, what works? or what doesn't work might be found locally as an acceptable depth, forgetting about examining permanent moisture conditions. Didn't help much did I?

RE: Permanent Moisture Level

I think you did help somewhat. Knowing nothing of soils and soil investigation, my thought was - that if the water table changes, then how can this "thing" called permanent moisture level even begin to remain constant? How could that even be researched? To me, the more I think of it, it sounds like total BS.

So I'm questioning the validity of that code entirely. I would welcome any further comments.

RE: Permanent Moisture Level

typically, seasonal variation in moisture content is in the upper 4 to 5 ft. In areas of shrink-swell soils (I used to consult in Virginia and am now with the department of transportation), we'd lower footings to 4 ft to avoid such seasonal varation and the likely affect on foundation performance. Frost depth is 2 ft, but that was too shallow for foundations in these soils. Variation in moisture content varies by plant uptake, capillarity, hot and sunny weather and rainfall. Such variations are limited below 4 ft, even if the water table is much lower.


ípapß gordo ainÆt no madre flaca!

RE: Permanent Moisture Level

So how do I locate this zone?

For example, I have a service upgrade that I must do on Hilton Head Island, SC. Where do I go to ask - or what reference manual to I go to, in order to find out how far down my ground rod must be (per NEC)?

Please don't tell me to go to the local building department. They don't have any idea what the "permanent moisture level" is.

Please help me find the unicorn. What do I do?

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