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Service Maintenance Company, Inc. Electrical Contractors |
| NEC Questions | 1/15/2004 |
| By Mike Holt for EC&M Magazine Q1. I have heard that unused wires must be removed unless they are tagged for future use. My plant has lots of unused conduit runs with the old feed wires taped up in the panel. Is there some where in the code where it says these runs should be removed? It is starting to look like a spider web in some spots. A1. The NEC does not require unused power and lighting conductors be removed, except for cellular and underfloor raceways [372.13, 374.7, and 390.7]. However, the accessible portions of abandoned low-voltage and limited-energy cables shall be removed. According to Chapters 7 and 8, abandoned cables are those that do not terminate at equipment and are not identified for future use. Q2. Is it illegal to affix a permanent lock on an electrical panel? I want to be able to quickly shut off breakers or the whole panel in case of an emergency. A2. The NEC does not prohibit a permanent lock on a panelboard or disconnect switch. This means that a switch could be locked open or locked closed. I realize that in some cases either of these two conditions could be a hazard, but this issue is not within the scope of the NEC. Q3. Is there a code requirement as to the height of a receptacle outlet? A3. No, unless the receptacle is specifically required by the NEC. For example, the required wall receptacle outlets for a dwelling unit shall not be located above 5.5 ft [210.52]. Required dwelling unit kitchen counter top receptacles shall not be located more than 20 in. above the countertop [210.52(C)(5)], and required outdoor receptacles for one and two family dwellings shall not be located more than 6.5 ft above grade [210.52(E)]. Q4. How close to a shower stall or hydromassage bathtub can I install a switch? A4. Switches are not permitted to be located within the wet location of a tub, shower space, or hydromassage tub [404.4 and 680.72]. Which means it can be located "next" to the tub or shower space. However, switches must be located at least 5 ft from pools, spa's and hot tubs [680.22(C)]. Q5. Two times in the last couple of months I have been instructed by the inspector to install a second ground rod. I took ohm readings both times and explained that the ohm readings were less than 25 ohms. To top it off, one inspector wanted it less than six feet, the second inspector wanted it over six feet. A5. If a rod electrode does not have a resistance to ground of 25 ohms or less, it must be supplemented by a second electrode located not less than 6' away.[250.56] Q6. In your book "Understanding the NEC," you show service conductors feeding through the first disconnect to the second disconnect. You make it sound like this is okay even though you refer us back to 230.7. A6. Section 230.7 states that "Conductors other than service conductors shall not be installed in the same service raceway or service cable." Service conductors are permitted in the same raceway. Q7. I hear that the NEC no longer allows receptacles to be located above a suspended ceiling. Frequently there is the desire to have a receptacle above the ceiling to plug in a ceiling-mounted LCD video projector, or small transformer for security installations. I can't find the NEC rule that prohibits locating receptacles above a suspended ceiling. A7. Receptacles are permitted above a suspended ceiling, however cords are not permitted to be concealed by walls, floors, or ceilings nor are they permitted to be located above suspended or dropped ceilings [400.8(5)]. For the example in your question, a receptacle would have to be mounted in the ceiling, accessible from below. However, a receptacle above the suspended ceiling might be required or desirable for use for portable equipment and/or tools. Example: Section 210.63 requires a 125V, single-phase, 15- or 20-ampere-rated receptacle outlet at an accessible location for the servicing of heating, air-conditioning, and refrigeration equipment. Q8. I designed a chiller plant based on a shop drawing that listed: · Rated-load current (RLC) as 147A · Minimum circuit ampere as 184A · Maximum overcurrent protection at 250A I specified a 250A circuit breaker with 4/0 AWG conductors (184A x 1.25 = 230A). However, a couple of engineers in our office say that 3/0 AWG, rated 200A at 75C, could be used, because you don't multiply the minimum circuit ampere by 1.25. They also say if I use 3/0 AWG, then the circuit protection device must be sized not greater than 200A A8. Your buddies are almost correct. Conductor Size. It is true that the minimum circuit conductors size is based on the "minimum circuit ampacity" as identified on the equipment nameplate, which is 184A. This is because the rated minimum circuit ampacity for motor-compressor already includes 125 percent of the motor-compressor rated-load current [440.32]. 147A x 1.25 = 184A Table 310.16 identifies 3/0 AWG conductor at 75C as having an ampacity of 200A, which is acceptable for this minimum circuit ampacity. Circuit/Equipment Protection. The branch-circuit conductors, control apparatus, and motors in circuits supplying hermetic refrigerant motor-compressors must be protected against overcurrent due to short circuits and grounds by a protective device having a rating or setting not exceeding 175 percent of the motor-compressor rated-load current [440.22]. 147A x 1.75 = 257A So a 250A circuit breaker is fine to protect the 3/0 AWG conductor for hermetic refrigerant motor-compressors. Q9. I would like your thoughts on a situation where my inspector is requiring the rebar within a building's footing to be bonded to the service grounding electrode. Section 250.50 indicates that if this item is available on the premises then it is to be bonded so that it's part of the building's grounding electrode system. What if the concrete has already been poured? I remember reading somewhere that the NEC does not require the concrete to be chipped out to gain access to the footing steel. It seems to me that chipping out the footing creates other problems, and that there are other reasonable methods to establish an earth ground. A9. According to the NFPA Formal Interpretation 78-4, it is not the intent that reinforcing steel be made available for grounding. See the NEC Handbook, page 272 for specifics. Q10. What is the maximum conduit run length for rigid nonmetallic conduit, before a junction or pull box is required? A10. The only NEC limitations on conduit run length include splices not permitted in a raceway [300.13(A)] and no more than 360 degrees of conduit bending radius [352.26]. Copyright © 2004 Mike Holt Enterprises,Inc. |
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