Q. Back installing switches on a metal box, are a arena appendage or self-grounding switches appropriate or are the about-face screws able for accomplishments the about-face to the metal box?
A. The metal about-face ascent screws accommodate the appropriate low impedance ground-fault accepted aisle appropriate to the metal box [404.9(B)(1)]. Therefore, a bonding jumper or “self-grounding” about-face isn’t appropriate (Figure below).
Q. Back adjusting aqueduct ampacity, am I appropriate by 310.15(B)(2) to abject it on the 90°C insulation appraisement (for THHN) or the 75°C temperature appraisement of the accessories terminals?
A. Back adjusting aqueduct ampacity, use the temperature appraisement of the conductor, not the accessories terminals [110.14(C)]. The accessories terminal appraisement limitation of 110.14(C) has annihilation to do with aqueduct ampacity. For example, let’s accept we accept an accession of four current-carrying conductors (THHN) that accumulation a 40A affiliated nonlinear amount that’s to be able by a 50A aegis accessory (terminals rated 75°C).
Step 1. Admeasurement the conductors to ensure:
According to Table 310.16, 75°C column, an 8 AWG aqueduct (rated 50A) will accommodated both these requirements.
Step 2. However, 8 AWG THHN isn’t able for use in this accession because the ampacity of the 8 AWG aqueduct afterwards ampacity acclimation is alone 44A (55A×0.80), which can’t be able by a 50A aegis accessory [240.4(B)]. The able admeasurement aqueduct for this accession would be a 6 AWG THHN, rated 52A afterwards ampacity acclimation (65A×0.80)
Conductor ampacity is adapted in accordance with the acclimation 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 application four 500 kcmil THHN conductors in anniversary approach (all accepted carrying), how abounding alongside runs would be appropriate for a 2,000A feeder?
A. The accession of four current-carrying conductors in a approach requires the use of an ampacity acclimation agency of 80% per Table 310.15(B)(2)(a). The 90°C ampacity of 500-kcmil nut is 430A. The adapted ampacity for 500 THHN is 430A×0.80=344A. This accession will crave the use of six sets of 500 kcmil for a 2,000A ambit (2,000A/344A).
Q. Back EMT is installed aural metal studs and it’s not comatose on the basal of the opening, is added abutment required?
A. Horizontal runs of EMT are advised accurate by openings through framing associates breadth deeply attached aural 3 anxiety of abortion credibility [358.30(B)]. But if the approach isn’t comatose on the framing member, again it’s not supported. This doesn’t beggarly the aqueduct charge blow on every framing member. As continued as it’s comatose on one framing affiliate every 10 anxiety and is deeply attached aural 3 anxiety of the tubing abortion points, the accession is acceptable.
Q. Why does Table 310.16 account ampacities in the 75°C and 90°C columns for THHW and XHHW?
A. Because these conductors are rated 90°C back installed in a dry breadth and 75°C back installed in a wet breadth (like a approach in arena attic slab). If the blazon cardinal has the suffix “-2,” such as THWN-2, again it’s rated for 90°C in both wet and dry locations. See Table 310.13 and its notes.
Q. Inspectors in our breadth crave a disconnecting agency at the point of access to a architecture for architect feeders, alike back the alfresco architect has its own abstract aural afterimage of the building. What’s the deal?
A. If the architect abstract is aural afterimage of the architecture (not added than 50 feet) and readily accessible, again a agriculturalist disconnecting agency isn’t appropriate at the building. However, this alone applies to emergency ability systems [700.11(B)(5)] or accurately appropriate ability systems [701.11(B)(5)]. There is no agnate aphorism for alternative standby ability systems (Art. 702), but this blank will be adapted in the 2005 NEC.
Q. How far afar do cull boxes charge to be amid for low voltage cabling in an underground nonmetallic aqueduct run?
A. The Code doesn’t specify a ambit amid cull credibility because this is a architecture and accession issue. However, acquaintance the cable architect to actuate the best affairs force for the cable in question.
Q. I accept a botheration with a computer allowance that has a aloft attic acclimated for air conditioning. Back we charge ability for accessories amid aloft the aloft floor, a bond is run through the attic into a bowl aperture amid below. My ambassador says the bond isn’t able to be amid in the aloft attic breadth unless it’s DP rated. What is he talking about?
A. Your ambassador is correct. All cords amid in the aloft attic breadth of an advice technology allowance charge be a Blazon DP cable, which agency it has able fire-resistant characteristics able for use beneath aloft floors of an advice technology accessories allowance [645.5(D)(5)]. To accomplish the accession Code-compliant application the absolute cord, you’ll charge to locate the bowl aperture aloft the aloft floor. If the aloft attic isn’t acclimated for air handling, the accession would be Code compliant.
Q. I’m base a two-story building. The abbreviating units are amid on the arena alfresco the architecture in advanced of the nonfused abstract switches. Do the 3-foot approval requirements of Table 110.26(A) administer in advanced of a nonfused disconnect?
A. Yes, bright alive amplitude is still required. Back account technicians activate assignment on nonfunctioning abbreviating units, their aboriginal footfall is oftentimes to analysis for the actual voltage at the assemblage disconnect. This action qualifies as “examination, adjustment, servicing, or aliment while energized.” Therefore, acquiescence with the safe workspace rules in 110.26(A) of the 2002 Code is required.
A. My acquaintance told me a 208V motor could be affiliated to the 208V high-leg aqueduct acquired from a delta-connected agent bank? I can’t accept the amount would assignment if active this way, but if it does, is this able by the Code?
A. The motor will accomplish altogether if active in this configuration. However, a single-pole ambit is rated 120/240V, and it’s alone able to serve a ambit breadth the nominal voltage of any one aqueduct to arena (actually the metal case) doesn’t beat the lower of the two ethics (240.85). So you can see this would be a abuse because the 208V high-leg ambit aqueduct exceeds the single-pole (120V) voltage appraisement of the ambit breaker.
Clashing with an inspector? Got Code questions? Mike Holt has the answers. E-mail him at [email protected].
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