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By Mike Holt for EC&M Magazine
Q 1. I have a question on the application of the conductor ampacity adjustment factors contain in 310.15 (B)(2). When adjusting conductor ampacity, do we use the conductor ampacity based on the 90°C insulation rating (for THHN) or the 75°C temperature rating of the equipment terminals?
A 1. When adjusting conductor ampacity, use the temperature rating of the conductor, not the equipment terminals [110.14(C)].
Q 2. If I am using four 500 THHN kcmil conductors in each raceway (all current carrying), how many parallel runs would be required for a 2000A feeder?
A 2. 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 430 amperes. The adjusted ampacity for 500 THHN is 430A x 0.80 = 344A. This installation will require the use of six sets of 500kcmil for a 2000A circuit (2,000A/344A).
Q 3. When EMT is installed within metal studs and it is not resting on the bottom of the opening, is additional support required?
A 3. Horizontal runs of EMT is considered
supported by openings through framing members where securely fastened within 3 ft of termination
points [358.30(B)]. The key word here is "supported". If the raceway is not
resting on the framing member, then it is not supported. This does not mean that the conduit
must rest on every framing member. As long as it is resting on one framing member every
10' and it is securely fastened within 3' of the tubing termination points, the installation
complies with the NEC.
A 4. None, assuming this is a non-dwelling unit installation. This is a design issue based on the intended use of the area.
Q 5. Why does Table 310.16 list ampacities in the 75°C and 90°C columns for THHW and XHHW?
A 5. 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", for example, THWN-2, then it is rated for 90°C in both wet and dry locations. See Table 310.13 and its notes.
Q 6. 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 it's own disconnect within site of the building. What is the deal?
A 6. If the generator disconnect is within sight of the building (not more than 50 ft) and readily accessible, then a feeder disconnecting means is not required at the building. But this only applies to emergency power system [700.11(B)(5)] or legally required power systems [701.11(B)(5)]. There is no similar rule for optional standby power systems (Article 702), but, this omission will be corrected in the 2005 NEC.
Q 7. In an underground nonmetallic conduit run, how far apart do pull boxes need to be located for low voltage cabling?
A 7. The Code does not specify a distance between pull points because this is a design and installation issue. However, contact the cable manufacture to determine the maximum pulling force for the cable in question.
Q 8. I have a problem with a computer room that has a raised floor. When you need 120V power, a cord is run through the raised floor into a receptacle outlet located in the raised floor. Our inspector says that the cords are not permitted in the raised floor area, because they are not rated for this purpose.
A 8. Your inspector is correct, the cords must be listed as Type DP cable having adequate fire-resistant characteristics suitable for use under raised floors of an information technology equipment room. What you need to do is install floor boxes or wall mounted receptacle outlets to serve the loads above the raised floor area.
Q 9. I am wiring a two-story building, the condensing units are located on the ground outside the building in front of the nonfused disconnect switches. Does the 3 ft clearance requirements of Table 110.26(A) apply in front of nonfused disconnects?
A 9. Yes clear working space is required. When a service man come to work on a nonfunctioning condensing unit, often the first step taken is to check for the correct voltage at the unit disconnect. This checking for voltage is "examination, adjustment, servicing, or maintenance while energized" and compliance with the safe workspace rules in 110.26(A) is required.
Q 10. Is GFCI protection required for a 230V, 20A pool pump motor that is hard wired?
A 10. Nope.
Q 11. When installing switches on a metal box, is a ground tail or self-grounding switches required, or are the switch screws suitable for grounding the switch to the metal box?
Q 11. The metal switch mounting screws
provides 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 is not
A 12. The code does not provide for
an exemption to the rule in 210.52(F) that requires a receptacle outlet served by a 20A
branch circuit to be provided in the laundry area of a dwelling unit. This receptacle
outlet circuit would be in addition to the 30A branch circuit required for the combination
washer/dryer. There are other items such as electric irons used in the laundry area and
there must be a provision to provide power for these loads.
A 2. No, a single transfer switch is not permitted to serve emergency and non-emergency loads [700.6(D)]. However, a single generator is permitted to serve emergency and nonemergency loads, provided multiple transfer switches are used, and automatic load shedding ensures adequate power for the emergency loads [700.5(B)].
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