hoist, the minimum ampacity of the power supply conductors shall be the name-plate full-load ampere rating of the largest motor or group of motors for any single crane motion, plus 50 percent of the nameplate full-load ampere rating of the next largest motor or group of motors, using that column of Table 610.14(A) that applies to the longest time-rated motor. Show
(3) Multiple Cranes or Hoists on a Common Conductor System. For multiple cranes, hoists, or both, supplied bya common conductor system, calculate the motor minimum ampacity for each crane as defined in (F) Other Loads. Additional loads, such as heating, light ing, and air conditioning, shall be provided for by application of the appropriate sections of this Code.(G) Nameplate. Each crane, monorail, or hoist shall be provided with a visible nameplate marked with the manufacturer's name, rating in volts, frequency, number of phases, and circuit amperes as calculated in 610.14(E) and (F).Table 610.14(E) Demand FactorsNumber of DemandCranes or Hoists Factor2 0.95 3 0.91 4 0.87 5 0.84 6 0.81 7 0.78 610.15 Common Return. Where a crane or hoist is operated by more than one motor, a common-return conductor of proper ampacity shall be permitted.III. Contact Conductors610.21 Installation of Contact Conductors. Contact conductors shall comply with 610.21(A) through (H).(A) Locating or Guarding Contact Conductors. Runway contact conductors shall be guarded, and bridge contact conductors shall be located or guarded in such a manner that persons cannot inadvertently touch energized current-carrying parts. (B) Contact Wires. Wires that are used as contact conductors shall be secured at the ends by means of approved strain insulators and shall be mounted on approved insulators so that the extreme limit of displacement of the wire does not bring the latter within less than 38 mm (l'/a in.) from the surface wired over. (C) Supports Along Runways. Main contact conductors carried along runways shall be supported on insulating supports placed at intervals not exceeding 6.0 m (20 ft) unless otherwise permitted in 610.21(F). Such conductors shall be separated at not less than 150 mm (6 in.), other than for monorail hoists where a spacing of not less than 75 mm (3 in.) shall be permitted. Where necessary, intervals between insulating supports shall be permitted to be increased up to 12 m (40 ft), the separation between conductors being increased proportionately. (D) Supports on Bridges. Bridge wire contact conductors shall be kept at least 65 mm (2Vi in.) apart, and, where the span exceeds 25 m (80 ft), insulating saddles shall be placed at intervals not exceeding 15 m (50 ft). (E) Supports for Rigid Conductors. Conductors along runways and crane bridges, that are of the rigid type specified in 610.13(B) and not contained within an approved enclosed assembly, shall be carried on insulating supports spaced at intervals of not more than80 times the vertical dimension of the conductor, but in no case greater than 4.5 m (15 ft), and spaced apart sufficiently Page 2
to give a clear electrical separation of conductors or adjacent collectors of not less than 25 mm (1 in.). (F) Track as Circuit Conductor. Monorail, tram rail, or crane runway tracks shall be permitted as a conductor of current for one phase of a 3-phase, ac system furnishing power to the carrier, crane, or trolley, provided all of the following conditions are met:(1) The conductors supplying the other two phases of the power supply are insulated. (2) The power for all phases is obtained from an insulating transformer. (3) The voltage does not exceed 300 volts. (4) The rail serving as a conductor shall be bonded to the equipment grounding conductor at the transformer and also shall be permitted to be grounded by the fittings used for the suspension or attachment of the rail to a building or structure. (G) Electrical Continuity of Contact Conductors. All sections of contact conductors shall be mechanically joined to provide a continuous electrical connection.(H) Not to Supply Other Equipment Contact conductors shall not be used as feeders for any equipment other than the crane(s) or hoist(s) that they are primarily designed to serve.610.22 Collectors. Collectors shall be designed so as to reduce to a minimum sparking between them and the contact conductor; and, where operated in rooms used for the storage of easily ignitible combustible fibers and materials, they shall comply with 503.155.IV. Disconnecting Means610.31 Runway Conductor Disconnecting Means. A dis- connecting means that has a continuous ampere rating not less than that calculated in 610.14(E) and (F) shall be provided between the runway contact conductorsand the power supply. Such disconnecting means shall consist of a motor-circuit switch, circuit breaker, or molded-case switch. This disconnecting means shall be as follows: (1) Readily accessible and operable from the ground or floor level. (2) Capable of being locked in the open position. The provision for locking or adding a lock to the disconnecting means shall be installed on or at the switch or circuit breaker used as the disconnecting means and shall remain in place with or without the lock installed. Portable means for adding a lock to the switch or circuit breaker shall not be permitted as the means required to be installed at and remain with the equipment. (3) Open all ungrounded conductors simultaneously. (4) Placed within view of the runway contact conductors. 610.32 Disconnecting Means for Cranes and Monorail Hoists. A motor-circuit switch, molded-case switch, or circuit breaker shall be provided in the leads from the runway contact conductors or other power supply on all cranes and monorail hoists. The disconnecting means shall be capable of being locked in the open position. The provision for locking or adding a lock to the disconnecting means shall be installed on or at the switch or circuit breaker used as the disconnecting means and shall remain in place with or without the lock installed. Portable means for adding a lock to the switch or circuit breaker shall not be permitted.Where a monorail hoist or hand-propelled crane bridge installation meets all of PagesNobody intends for things to go drastically wrong with a crane or hoist application, but errors still continue to occur — errors that can sometimes be fatal. According to the most recent data from the Bureau of Labor Statistics (BLS), 72 crane-related occupational fatalities occurred in 2006, which was down from an average of 78 fatalities per year from 2003 to 2005. According to the BLS, of the cranes that were specified in the fatality total, mobile, truck, rail-mounted, and overhead cranes represented the type of crane involved in the majority of fatalities. To make sure your customers don't end up as one of these statistics, turn to the National Electrical Code (NEC) for answers. But where do you begin? Because cranes and hoists are motor-driven, Art. 430 is a good place to start. This Article contains Table 430.5, which lists “Other Articles,” based on equipment/occupancy. Article 610 is one of those other Articles, providing the requirements for cranes and hoists. After Chapter 4, the NEC Articles assume you've applied the requirements of the previous four Chapters. They build on those by amending them or providing additional requirements. So Art. 610 is an “amend and append” of Chapters 1-4, as they pertain to cranes and hoists (see SIDEBAR: NEC Flow below). Article 610 has two “Other Article” requirements not covered by Chapters 1-4:
WiringArticle 610 begins its “amend and append” process with wiring methods. You must enclose all conductors in raceways or run them as one of the following [610.11]:
But you can:
Terminal fittingsWhere conductors leave raceways or cables, use a box or terminal fitting that has a separately bushed hole for each conductor. The fitting can't contain taps or splices or be used at luminaire outlets [6102.12]. The exception is that you can use a bushing instead of a box at the end of:
… if the raceway terminates at unenclosed controls or similar equipment. Examples of “similar equipment” include contact conductors, resistors, brakes, power-circuit limit switches, and DC split-frame motors. Conductor ampacityFor ampacity calculations, Art. 610 replaces the Art. 310 tables required by 310.15. If you compare Table 610.14(A) to Table 310.16, you'll notice several differences, such as:
The first two differences make sense because cranes and hoists involve movement. The environments of usage justify the second two differences. Another key difference is that Table 610.14(A) adds the dimension of time to the ampacity tables. Why is it there? Fans, rotary pumps, process conveyors, and most machine tools use NEMA Design B motors (click here to see Fig. 1) above and (click here to see Table), which are classified as “medium torque.” But crane and hoist motors are NEMA Design D and thus are classified as “extra high torque.” These motors must overcome the inertia of starting under high loads. Because of this high-intensity service, Table 610.14(B) provides secondary conductor rating factors starting with 5 sec of operation. The shorter the time between operations, the higher the ampacity you must apply in percent of full-load secondary current. Use Table 610.14(D) when calculating the size of the contact conductors. The minimum conductor size depends on the distance between supports (end strain insulators or clamp-type intermediate supports), but can never be less than 6 AWG. Motor load calculationsApply Art. 430 load calculations, but modify those depending on the number of motors [610.14(E)]. For a single motor, use 100% of the motor nameplate full-load ampere rating. For multiple motors on a single crane or hoist, it gets a bit more dicey. Determine the minimum ampacity of the power supply conductors per 610.14(E)(2) as follows:
If you have multiple cranes or hoists on a common conductor system:
With these multiple motor installations, you can use a common-return conductor if it's of the proper ampacity [610.15]. How do you size conductors for lighting or heating a crane cab? If the conductors don't supply crane motors, size them per the appropriate sections in Chapters 3 and 4 [610.14(F). Contact conductorsWhen looking at wiring and ampacity requirements, we saw that contact conductors got special mention. Article 610 devotes Part III entirely to contact conductors (there are six Parts to this Article). Part III provides eight requirements, A through F:
Disconnecting meansSupply a suitably rated disconnect between the runway contact conductors and the power supply. It can be a motor-circuit switch, circuit-breaker, or molded-case switch. It must:
The continuous ampere rating must be at least 50% of the combined short-term rating of the motors or less than 75% of the sum of the short-time ampere rating of the motors required for any single motion [610.33]. In addition, you must supply a suitably rated disconnect in the leads from the runway contact conductors or other power supply conductors on all cranes and monorail hoists. The requirements are similar to those above [610.32]. Overcurrent protectionWhat size does the overcurrent protection device (OCPD) need to be for the runway supply conductors or the main contact conductors? Its rating can't exceed that of the largest rating (or setting) of any branch-circuit OCPD plus the sum of all the other loads. Don't forget to apply the demand factors from Table 610.14(E). Protect branch circuits with fuses or inverse-time breakers rated per Table 430.52. Overload protectionYou can use the branch circuit OCPD to provide overload protection for a single motor. For multiple motors, you can use:
BondingPart VII is titled “Grounding,” but it has nothing to do with grounding (see Art. 100 definition). It's actually referring to bonding. Basically, you need to establish a ground-fault current path as defined in Art. 250, Part V. Article 610 considers moving parts to be bonded if connected through bearing surfaces. But to avoid burning bearings out, provide flexible bonding jumpers around them wherever possible. Draw the current paths on paper, if necessary. Article 610 compliance is much easier if you keep in mind the “amend and append” system the NEC uses. Remember that hoists and cranes use specialized, high-torque motors, and their conductors are specialized due to the need to move as the crane travels along its rails. Lamendola is an electrical consultant based in Merriam, Kan. He can be reached at . SIDEBAR: NEC FlowNEC requirements “flow forward” from the first four Chapters. Notice that Art. 430 lists Art. 610 among “Other Articles,” but Art. 610 doesn't reciprocate. This is because the NEC starts with the general component (e.g., motor) in Chapters 1-4, then amends or appends requirements for the special case (e.g., crane motor) in Chapters 5-7. |
The power supply to contact conductors of a crane in a class iii location shall ______ .
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