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Equipment Frame Grounding Conductor Sizing

04/16/2018 2:47 AM

Which code governs the sizing of frame grounding of the motor to the grounding grid?(this is in addition to the equipment grounding conductor)

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#1

Re: Equipment frame grounding Conductor Sizing

04/16/2018 5:43 AM

Simple. The national electrical code applicable to the country of installation, of course. For example:

  • If the installation were in he UK: British Standard 7671.
  • If the installation were in India: Indian Standard 3053.
  • Etc., etc.

If in doubt, consult a qualified local Electrician.

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#2

Re: Equipment Frame Grounding Conductor Sizing

04/16/2018 10:23 AM

The use of a search engine can greatly increase the apparent knowledge of those without answers to questions whose answers they should already have known.

NEC or IEC? Electrical Standards Listed by Country - E&S Grounding

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#3

Re: Equipment Frame Grounding Conductor Sizing

04/17/2018 6:05 AM
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#4

Re: Equipment Frame Grounding Conductor Sizing

04/17/2018 1:25 PM

Selk,

Your drawing references NEC table 250.122, so I assume you are in an area under its jurisdiction. You also mention IEEE 80, for which I have no unpaid access. However, I believe that in the revisions and writing of the NEC, if there were a significant discrepancy between these two standards it would have been addressed in the NEC.

You are in part VI of article 250, which discusses "Equipment Grounding and Equipment Grounding Conductors". Section 250.112(C) refers you to 430.242 (motors), wherein the text there tells when grounding is required, and then is followed in 430.245 with a reference back to part VI of article 250. As long as you are talking about grounding, then table 250.122 would be the one I would use. Also, note in 250.102(D), where it talks about "Equipment Bonding Jumper on Load Side of an Overcurrent Device" that it takes you right back to 250.122. Other sections address the integrity of the bonding/grounding connection and conductor.... and need to be heeded.

If I were your AHJ I would say to apply table 250.122, as a minimum specification.

-JMM

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#5

Re: Equipment Frame Grounding Conductor Sizing

04/20/2018 1:22 AM

In my opinion, at first, if an equipment grounding conductor is provided that means the system [neutral] is grounded. Then according to 250.32 B (1): “ Any installed grounded conductor shall not be connected to the equipment grounding conductor or to the grounding electrode(s).”. That means the motor frame shall not be connected directly to any grounding electrode [earth]. The equipment grounding conductor has to be grounded only close to the source to the grounding electrode.

The logic of this policy it is to concentrate all the fault current to the point of supply in order to facilitate ground fault protection. [See art.250.32 for permitted exceptions.]

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