Non-Electrical Equipment Bonding Conductor Size

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BlameTheIntern

Member
Location
Houston
Occupation
Electrical Engineer
Hello

I'm wrapping up my first electrical engineering internship and I have a question about bonding conductor sizing for non-electrical equipment like steel columns, cable trays, tanks, and vessels.

I was reviewing some grounding plans and I noticed that we generally use #2 AWG to bond steel columns to the ground grid. However, sometimes I see #2/0 AWG used to bond a tank or a vessel to the ground grid. I asked a senior designer about this, and he said that #2/0 AWG is typically used for large equipment. I then asked him what defines "large" equipment, and he said that's up to the engineer to decide. My lead directed me to NEC 250, but in this case, the equipment doesn't have an upstream breaker / protective device.

My questions are, in the absence of client specifications, when should we use #2 AWG vs. #2/0 AWG? Is there a calculation or NEC article that covers this?
 
The only NEC requirements that have anything to do with sizing bonding conductors are 250.122 for Equipment Grounding Conductors which have to do with clearing a fault and 250.102 for Grounding Electrode Conductors which have to do with connecting the electrical service to the earth. Your application does not fall under either so there are no rules to follow in the NEC. Most of these specifications are all about what feels good and not performance based. I suspect 4 AWG would be just as effective as 2/0.
 
Welcome to the forum.

The general rule is that metallic structures are considered to be grounded by the grounding conductor of the equipment likely to energize it.
 
Thank you all for the replies. I guess it's one of those things that people have been doing forever and the "why" was lost.

It makes me wonder if people will notice if I started replacing all the #2 AWG with #12 AWG, hahaha!
 
If the only purpose is 'static grounding', then #12 would be more than sufficient to conduct any charges.

The larger physical size might be for mechanical reasons. We see this in electrical code where the minimum size grounding electrical conductor would require mechanical protection, but larger conductors get used because they can be run exposed.

If you are bonding large vessels out of doors, perhaps lightning protection requirements are a factor.
 
Following up on what Winnie said, for static see NFPA 77 and API 2003, if it is for lightning, see NFPA 780 and API 545. If it is for portable pumping, drum pumping or such, bonding everything is critical. Not really in the wheelhouse of the NEC, but may be interesting.
 
Different kinds of buildings might have standards (not codes) for guidance that impact wire sizes for those type of auxiliary bonding connections. For example, many data centers or Telecom buildings use ANSI / TIA 607 Table 1 based on the length.
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