Code Quandaries

Aug 1, 2004 12:00 PM, By Mike Holt, NEC Consultant

Stumped by the Code?

Q. When installing switches on a metal box, are a ground tail or self-grounding switches required or are the switch screws suitable for grounding the switch to the metal box?

A. The metal switch mounting screws provide the required low impedance ground-fault current path required to the metal box [404.9(B)(1)]. Therefore, a bonding jumper or “self-grounding” switch isn't required (Figure below).

Q. When adjusting conductor ampacity, am I required by 310.15(B)(2) to base it on the 90°C insulation rating (for THHN) or the 75°C temperature rating of the equipment terminals?

Snap switches, dimmers, and similar control switches must be effectively grounded to provide a means to ground metal faceplates, whether or not one is installed [404.9(B)].

A. When adjusting conductor ampacity, use the temperature rating of the conductor, not the equipment terminals [110.14(C)]. The equipment terminal rating limitation of 110.14(C) has nothing to do with conductor ampacity. For example, let's assume we have an installation of four current-carrying conductors (THHN) that supply a 40A continuous nonlinear load that's to be protected by a 50A protection device (terminals rated 75°C).

Step 1. Size the conductors to ensure:

They have the capacity to carry 40A continuously (40A×1.25=50A) [210.19(A)(1)].
They're sized in accordance with 75°C terminal rating [110.14(C)(1)].

According to Table 310.16, 75°C column, an 8 AWG conductor (rated 50A) will meet both these requirements.

Step 2. However, 8 AWG THHN isn't permitted for use in this installation because the ampacity of the 8 AWG conductor after ampacity adjustment is only 44A (55A×0.80), which can't be protected by a 50A protection device [240.4(B)]. The proper size conductor for this installation would be a 6 AWG THHN, rated 52A after ampacity adjustment (65A×0.80)

Conductor ampacity is adjusted in accordance with the adjustment factors listed in Table 310.15(B)(2)(a) based on the 90°C ampacity as listed in Table 310.16, not the 75°C terminal rating.

Q. If I'm using four 500 kcmil THHN conductors in each raceway (all current carrying), how many parallel runs would be required for a 2,000A feeder?

A. The installation of four current-carrying conductors in a raceway requires the use of an ampacity adjustment factor of 80% per Table 310.15(B)(2)(a). The 90°C ampacity of 500-kcmil copper is 430A. The adjusted ampacity for 500 THHN is 430A×0.80=344A. This installation will require the use of six sets of 500 kcmil for a 2,000A circuit (2,000A/344A).

Q. When EMT is installed within metal studs and it's not resting on the bottom of the opening, is additional support required?

A. Horizontal runs of EMT are considered supported by openings through framing members where securely fastened within 3 feet of termination points [358.30(B)]. But if the raceway isn't resting on the framing member, then it's not supported. This doesn't mean the conduit must rest on every framing member. As long as it's resting on one framing member every 10 feet and is securely fastened within 3 feet of the tubing termination points, the installation is acceptable.

Q. Why does Table 310.16 list ampacities in the 75°C and 90°C columns for THHW and XHHW?

A. Because these conductors are rated 90°C when installed in a dry location and 75°C when installed in a wet location (like a raceway in ground floor slab). If the type number has the suffix “-2,” such as THWN-2, then it's rated for 90°C in both wet and dry locations. See Table 310.13 and its notes.

Q. Inspectors in our area require a disconnecting means at the point of entrance to a building for generator feeders, even when the outside generator has its own disconnect within sight of the building. What's the deal?

A. If the generator disconnect is within sight of the building (not more than 50 feet) and readily accessible, then a feeder disconnecting means isn't required at the building. However, this only applies to emergency power systems [700.11(B)(5)] or legally required power systems [701.11(B)(5)]. There is no similar rule for optional standby power systems (Art. 702), but this omission will be corrected in the 2005 NEC.

Q. How far apart do pull boxes need to be located for low voltage cabling in an underground nonmetallic conduit run?

A. The Code doesn't specify a distance between pull points because this is a design and installation issue. However, contact the cable manufacturer to determine the maximum pulling force for the cable in question.

Q. I have a problem with a computer room that has a raised floor used for air conditioning. When we need power for equipment located above the raised floor, a cord is run through the floor into a receptacle outlet located below. My inspector says the cord isn't permitted to be located in the raised floor area unless it's DP rated. What is he talking about?

A. Your inspector is correct. All cords located in the raised floor area of an information technology room must be a Type DP cable, which means it has adequate fire-resistant characteristics suitable for use under raised floors of an information technology equipment room [645.5(D)(5)]. To make the installation Code-compliant using the existing cord, you'll need to locate the receptacle outlet above the raised floor. If the raised floor isn't used for air handling, the installation would be Code compliant.

Q. I'm wiring a two-story building. The condensing units are located on the ground outside the building in front of the nonfused disconnect switches. Do the 3-foot clearance requirements of Table 110.26(A) apply in front of a nonfused disconnect?

A. Yes, clear working space is still required. When service technicians begin work on nonfunctioning condensing units, their first step is oftentimes to check for the correct voltage at the unit disconnect. This process qualifies as “examination, adjustment, servicing, or maintenance while energized.” Therefore, compliance with the safe workspace rules in 110.26(A) of the 2002 Code is required.

A. My friend told me a 208V motor could be connected to the 208V high-leg conductor derived from a delta-connected transformer bank? I can't believe the load would work if wired this way, but if it does, is this permitted by the Code?

A. The motor will operate perfectly if wired in this configuration. However, a single-pole circuit is rated 120/240V, and it's only permitted to serve a circuit where the nominal voltage of any one conductor to ground (actually the metal case) doesn't exceed the lower of the two values (240.85). So you can see this would be a violation because the 208V high-leg circuit conductor exceeds the single-pole (120V) voltage rating of the circuit breaker.

Clashing with an inspector? Got Code questions? Mike Holt has the answers. E-mail him at mike@mikeholt.com.