Conduit as equipment grounding conductor 1

[thinker]

NEC allows to use a rigid metal conduit as equipment grounding conductor. Please advise where I can find information on electrical properties of conduits to estimate, say, 3" trade size conduit body conductivity and determine what would be AWG size of the corresponding copper equipment grounding conductor.The practical situation: at one of installations contractor forgot to pull equipment grounding conductors along with power wires (isolated DC system). It is impossible to add (pull) new conductors, so we are considering bonding both ends of the conduit and using it as a grounding conductor. Normally, we would use,say, AWG # 4/0 wire as equipment grounding conductor. Now we need to evaluate would the conduit be an adequate replacement.

 

[davidbeach]

Under the logic of the NEC that allows the conduit to be the grounding conductor, it is always assumed to be of sufficient size. If the conduit is adequately maintained and remains electrically continuous, most ground fault current will flow via the conduit rather than the contained grounding conductor, but if it isn't electrically continuous the internal grounding conductor will carry the fault current. In all cases the conduit is required to be bonded, so if your installation is otherwise to code there is no need to add bonding at both ends of the conduit. If the bonding isn't already there you have far bigger problems with your contractor than the forgotten wire.

 

[thinker]

Thank you, David. Aside problems with our contractor, assuming the conduit is bonded, there is still a question how to quantify the "suffucient" size of the conduit in terms of comparison with wire gauge of the internal grounding conductor?

 

[waross]

In an old Canadian Electrical Code (1986) the following information is given
Table 16:
MINIMUM SIZE OF CONDUCTORS, METALIC CONDUIT OR ELECTRICAL METALIC TUBING FOR BONDING RACEWAYS AND EQUIPMENT.
(EXCERPT)
Rating or Setting of Overcurrent Device in Circuit Ahead of Equipment, Conduit, Etc.
Not Exceeding 2000 Amperes
copper-0000, aluminum-350 kcmil, Rigid conduit-not listed, E.M.T. not listed.

Rating or Setting of Overcurrent Device in Circuit Ahead of Equipment, Conduit, Etc.
Not Exceeding 1200 Amperes
copper-000, aluminum-250 kcmil, Rigid conduit-6", E.M.T. not listed.

Rating or Setting of Overcurrent Device in Circuit Ahead of Equipment, Conduit, Etc.
Not Exceeding 1000 Amperes
copper-00, aluminum-0000, Rigid conduit-4", E.M.T.-4"

The 2006 Canadian Electrical Code allows conduit to be used for bonding.
Table 16 no longer lists conduit sizes.
The notes on the rules state that when conductors are installed in conduit the conduit shall be deemed adequate for grounding.

This would appear to be in harmony with the NEC as suggested by davidbeach.
respectfully

 

[thinker]

Waross, thank you very much for valuable information.