RES Technical Corner by Brett Eliasz, RES Director

Technical Corner

This month’s article focuses on one of the most important and often misunderstood concepts: Grounding in the electrical system. Some nice illustrations shown below dictate the ground fault current path back to the source. You will notice that the ground path gets back to the source transformer through the ground conductor when there is a neutral to ground bond at the transformer. If the neutral to ground bond occurs at the main distribution panel, which is most often the case, the ground current flows back to the source through the neutral conductor eliminating the need for an equipment ground in most service entrances. It is a very important concept to understand that the ground conductor is there to provide a low impedance path back to the source, thus resulting in increased current which is required to trip a breaker. You will see in the TC curve below that low-level faults can occur and seemingly “sit there” and not trip the upstream breaker resulting in a dangerous touch hazard.

Example:

The main switchboard or MDP in a high-rise building is rated at 4000A with a 4000A main circuit breaker. The new tenant in the building will require a 400A circuit out of this switchboard that has no physical space to mount a breaker/fuse. Therefore, a tap must be made within this MDP and shall feed a 400A fused disconnect switch. The tap conductor length will not exceed 10’.

What size wiring and equipment grounding conductor should be provided for this 3PH, 4W connection?

The phase and neutral conductors may be the same-#600kcmil and not #500kcmil. This is due to NEC Section 240.21(B). When utilizing a tap rule, using the next size up rule depicted in NEC 240.4(B) does not apply, so using #500kcmil, which are rated for 380A per 75-degree column cannot be used. #600kcmil are rated for 420A per 75-degree column which is greater than the 400A fuse in the disconnect. Alternatively, 350A fuses could be placed in the fused disconnect (next std. size lower per NEC 240.6(A)) if circuit loading allows it, which would permit the #500kcmil cabling.

The equipment grounding conductor must be sized per NEC table 250.122 as the overcurrent protection upstream is 4000A. This equates to a #500kcmil. Please note that under no circumstance shall the equipment grounding conductor be larger than the phase/neutral conductors serving the equipment as per NEC 250.122(A) and 250.122(G).

Conclusion:

We need to make sure that we are adequately providing the appropriate ground conductors in the right places at the right size in order to trip the upstream breaker to avoid any potential touch hazards in an electrical system.

Hopefully this article finds you well and can be used as a reference 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 beliasz@bergmannpc.com. As always, any comments are appreciated…! Thank you for reading.

Images Credit: Mike Holt

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

res - technical corner APRIL 2019 The ROCHESTER ENGINEER | 7