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Code Quandaries

Nov 1, 1999 12:00 PM, By Fred Hartwell, Hartwell Electrical Services, Inc.

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. 250-2(d), 250-8, 250-50 Ex., 300-22(b), 300-22(c), 410-31 Ex. 2, Art. 352, and a brief discussion of battery room area classifications.

Q. Is it acceptable for a builder to attach the ground for a house to the outside water faucet?

A. Probably not. Sec. 250-8 requires installers to use listed grounding connectors. Listings usually run in terms of various trade sizes but not the irregular shape of a faucet. Also, is the attachment within 5 ft of the water entrance to the building? You can’t use Sec. 250-50 Ex. in this case, given the residential use.

Instead of relying on listing restrictions, some inspectors question the small pipe sizes likely to serve a hose bib. I can remember seeing an 800A service grounded to a 3/8-in. cold-water supply to a utility sink (with all conditions of Sec. 250-50 Ex. fully satisfied and an appropriately sized and listed connector). There isn’t any minimum water pipe size in the NEC for the qualification of a water pipe electrode, so you’d have to revert to the performance-based language in Sec. 250-2(d) if you took this less objective approach.

Q. We are installing a UPS system with the batteries in a separate room with only a small air blower in one wall. I think the room should be classified Class I Div. 1 Group B. The owner, however, insists on wiring the room as an unclassified area. What does the Code say about this?

A. First, the NEC doesn’t make these area classifications; you always need on-site engineering for this. Hydrogen is relatively difficult to confine to the point of rising to the lower explosive level (LEL) because of its extreme low density and low molecular weight. The blower makes it even less likely to be problematic. However, you need to consult a qualified engineer. The answer depends on the exact chemical nature of the batteries, and their sizes, quantities, charging rates, and other variables unique to your installation.

Usually, it’s far better to increase ventilation than to actually wire one of these rooms Div. 1 Group B.

Q. I know you can pass wire through a cold air space between floor joist or stud spaces, not to exceed 4 ft. Is that a total length of wire involved in multiple through-runs, or just the space between divisions? Here’s an example:

In an eight-family building, Units 2-7 have a return cold air plenum adjacent to the street-side exterior door. We’re supposed to replace the existing receptacle and switch wiring in these cavities. This means we’re rewiring these devices directly in the cold air return path. I don’t see how you can install Type NM cable (per the contract) and still meet the wiring method restrictions in Sec. 300-22(c)(1).

A. First, the 4-ft parameter, presumably from the allowable length of liquidtight flexible metal conduit in a duct or plenum per Sec. 300-22(b), doesn’t apply. Your location wasn’t “specifically fabricated” for environmental air transport. Instead, you need to stay within Sec. 300-22(c) as you suggested. Indeed, Type NM cable isn’t generally allowable for this purpose.

If you can’t change wiring methods, consider Sec. 300-22(c) Ex. This allows noncompliant wiring in dwelling units to pass through such cavities at right angles to the long dimension to such spaces. This doesn’t apply to switch and receptacle terminations, etc., only transverse runs. You might consider adding another stud to isolate the switch and receptacle wiring from the return air cavity, if the ventilation design would allow for such reduction.

Q. Can you feed an additional stem-mounted fluorescent by coming out of the end of an existing row of fixtures through a mounting stem, mounting a surface metal raceway box on the ceiling, and traveling across the ceiling to a similar box for the additional fixture?

A. Yes, per Sec. 410-31 Ex. 2. Just be sure the circuit feeding through is the 2-wire or a leg of the multiwire circuit feeding the first line of fixtures. The exception includes interconnections using “recognized wiring methods” (as contained in Chapter 3), and Art. 352 is one such method.

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