Article 250

[jar546]

Resistance. Buried or driven electrodes shall, if practicable, have resistance to ground not to exceed 25 ohms. If the resistance is not as low as 25 ohms, two or more electrodes in parallel shall be used.

Installation. ...................... Each electrode shall be separated at least 6 feet from any other electrode,...................

1947 Edition of National Electrical Code

 

[rshuey]

Common sense never changes.

 

[Bryan Holland]

The first edition of the code to contain the 25-ohm section was the 1918 edition. Also in the 1918 edition, the first allowance for pipe electrodes became code. The original requirement permitted a minimum of a 1-in pipe with no less than 4-ft² of surface to exterior soil. The 1923 edition added the driven rod.

 

[jar546]

Great info Bryan!

 

[rshuey]

that's outstanding.

 

[GHRoberts]

Since measuring the resistance is impractical, it is a poorly written code section. Much better to remove the resistance provision.

 

[jar546]

There are plenty of instruments available to check resistance rather easily.