Topics discussed in our “Code Forum” column involve complicated issues requiring extensive analysis. However, not every Code question warrants such in-depth treatment. Here are the latest short answers to questions posed on our Web site. Coverage includes topics in: Sec. 110-26(a)(3), 110-26(f)(1)(a), 240-3(b)(1), 250-20(d) (FPN No. 2), 250-30(a)(1), 250-54, and 445-5, Art. 530 Part G, and grandfathered applications under Art. 550.
Q. I keep running into problems consulting on power systems for permanent audio installations in new construction. Our company hired an electrical engineer to design a scheme involving a single-phase transformer. After passing this drawing on to project engineers, we received a variety of responses from “Okay, sure” to “you can’t do that!” We’re trying to eliminate “system hum” or “buzz” caused by a compromised ground system. My understanding is that a single phase system is desirable in this regard but causes an unbalanced situation for the electrical system. So, do any articles or drawings contain a viable solution?
A. You need to wire your system as a center-tapped separately derived 120V system operating at 60V to ground in accordance with Art. 530, Part G. This 1996 NEC revision ended up in the Code after we did a full Code Forum Department analysis in the February 1992 issue. Refer also to our Illustrated Changes in the 1996 NEC analysis in the November 1995 issue, or also in my book of the same title. Although those articles only envisioned one transformer, and although a proposal envisioning three such transformers joined at their star points failed to get all the way through the 1999 NEC process, there still isn’t any reason why you couldn’t run three such transformers on separate connections to spread the load across all three phases.
Q. Is it permissible to install a 20-in.-deep transformer directly above a panelboard as this would be equipment “associated with the electrical installation?” If so, does it need to be at least 6 ft above the panelboard?
A. It can go directly above (i.e., within the projected footprint), because of the electrical association. As you indicate, this comports with Sec. 110-26(f)(1)(a). However, it also needs to stay out of the workspace, which per Sec. 110-26(a)(3) extends to 6½ ft above the floor or working platform. Although this paragraph now has a 6-in. leeway built into it, that allowance probably won’t accommodate your 20-in.-deep transformer.
Q. Evidently some mobile homes in my area were built together to make what used to be a “motel.” The place is in dire need of rehab work. One problem is that the mobile homes are the old ones with aluminum wiring in them. The inspector is wondering if he can do anything about it.
A. There’s little to be done about the aluminum wiring other than vigilance, unless your jurisdiction has something unusual in the way of restrictions on grandfathering. This is an administrative law issue, not a Code issue.
More intriguing is the report that the mobile homes were put together to make a motel of sorts. You could probably find violations of Art. 550 in this if you’re careful to go back to the actual Code in effect at the time of the original construction. When there were plainly violations on day one, most grandfathering rules get much tougher.
Q. Should a ground rod be driven and bonded to the frame of a standby emergency generator? All I require is an equipment grounding conductor from the MDP to the ATS to the generator. These installations have a solidly connected neutral at the ATS and the generator neutral is removed from the frame of the generator. If your answer is no and a ground rod is driven, is this a Code violation?
A. You don’t need a grounding electrode, since the system isn’t separately derived. However, you have to size the neutral to carry fault return current, per Sec. 445-5. See also Sec. 250-20(d) (FPN No. 2).
With respect to whether driving the rod anyway creates other violations, it depends on where you make the connection. If it’s connected to the neutral, the arrangement might violate Sec. 250-30(a)(1) (See also Ex. 1). You worry about this if the second connection creates a parallel path for neutral current, although if the only such path runs through the earth you can disregard it. If it’s connected to the generator frame only, it’s a supplementary grounding electrode, and perfectly allowable per Sec. 250-54.