RES Technical Corner by Brett Eliasz, PE

Technical Corner

RES - Technical Corner

For the article this month we will take a look at bonding the grounded conductor (i.e. Neutral) to the equipment grounding system. Where is the best place to do that and why?

NEC Article 250.30(A)(1) states that the system bonding jumper shall be made at any single point on the separately derived system from the source to the first disconnecting means or overcurrent device. It also notes that if the source is located outside the building, a grounding electrode connection shall be installed. Therefore, it appears that we have 2 options for system bonding, either at the source or at the first disconnecting means. It is important to note that the main bonding jumper is a system bonding jumper, but specifically used to bond the main service entrance grounded conductor to the equipment grounding system.

The intent of the equipment grounding system is to create a low impedance path back to source that would generate a quick response to a ground fault and trip a circuit breaker before someone could touch the faulted equipment and get shocked. The equipment grounding system is made up of the usually green equipment grounding conductors (EGCs), metal equipment enclosures, metal boxes, metal raceways, panelboards, and disconnect switches. Should a current carrying conductor disconnect from a terminal point and land on an equipment enclosure, the enclosure should be tied to the equipment grounding network, which is continuous using metal raceways and EGCs all the way back to the separately derived system or building source.

The building source is typically a pole or pad mounted transformer that is outside the building. A separately derived system could be a dry type transformer inside the building; it could also be a generator where the neutral is switched. If you were to run an EGC all the way out to your transformer enclosure and ground bar and then bond the ground to the low potential of the transformer windings (i.e. the neutral terminal), you would have made your system bond at the source. In this scenario, there would never be any normal current on your equipment grounding system, it is completely separated from the normal power system from the point of the fault to the neutral on the building transformer. From the research that I have done, this is your most reliable, tried, and true method for ensuring a low impedance path back to the transformer and therefore fastest circuit breaker tripping and fault detection. Figure 1 (right) is an example of grounding a separately derived system, commonly a dry type transformer in a building, this is not a main bond, but rather a system bond. Notice how the equipment ground is brought all the way into the transformer before it is bonded to the neutral.

I find the image to the right nicely defines the various conductors that you will see as part of a grounding system. Notice that in addition to the neutral conductor and the system bonding jumper, there is also a grounding electrode conductor tied to the lowest potential on the transformer secondary. The grounding electrode conductor is key in keeping the neutral of the transformer at ~ 0V. It is essentially holding the lowest potential of the transformer at the potential of earth.

Figure 1: Source website: https://www.ecmweb.com/national-electricalcode/qa/article/20900937/stumped-by-the-code-nec-rules-for-groundingand-bonding-transformers

Often, the main bond will occur at the first disconnecting means. In other words, in your main panel, the equipment ground bar and enclosure are bonded to the neutral bar and therefore the grounded conductor from the service entrance transformer needs to be sized to handle its normal current load as well as any fault current, given that faults will jump onto the neutral at this point and take the neutral back to source. The reason this is done, as I understand it is two-fold. Typically, the raceway from the service entrance transformer to the main panel is not metallic and therefore cannot be used as part of the equipment ground system. Therefore, the electrician would have to run a supply side bonding jumper from the main panel to the transformer enclosure and then perform the main bond at the transformer as shown in the image above. To save costs, the main bond is done at the first disconnecting means and the neutral, which must be run anyways, is used as the return path back to source.

Hopefully this article finds you well and can be used as areference for your project needs.

If anyone would like to contribute to the RES magazine and add an article or would like to request information on a specific topic (not limited to Electrical) just email me at jdoores@bergmannpc.com or beliasz@ bergmannpc.com. As always, any comments are appreciated…! Thank you for reading.

Brett Eliasz, P.E., LEED AP BD+C , RES Director

Joshua Doores, ElectricalEngineer, ColliersEngineering & Design