Nobody 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:
Apply Art. 668 if the crane or hoist is part of an electrolytic line [610.3(C)].
Any crane or hoist operating in a hazardous location must conform to Art. 500 [610.3]. In Class I, Class II, and Class III locations, it must conform to Art. 501, 502, and 503, respectively.
Article 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]:
Type AC cable with insulated grounding conductor
Type MC cable
Type MI cable
But you can:
Run contact conductors without enclosing them in raceways.
Have short lengths of exposed conductors at resistors, collectors, and similar equipment.
Use various flexible raceways for connections to motors and similar equipment, if you use flexible stranded conductors (to avoid breakage).
Use a multiconductor cable for pendant controls and similar push-button stations, if you provide the conductors with strain support.
Use a flexible cord suitable to the application wherever you need to supply control or power to moving parts.
Where 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:
Rigid metal conduit
Intermediate metal conduit
Electrical metallic tubing
… 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.
For 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:
Table 610.14(A) stops with 500 MCM, while Table 310.16 goes up to 2,000 MCM.
Fewer conductor types appear in 610.14(A).
There's no 60°F column in 610.14(A).
Unlike Table 310.16, 610.14(A) has a 125°F column.
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.
Apply 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:
Start with the nameplate full-load ampere rating of the largest motor (or group of motors) for any single crane motion.
Add 50% of the next largest motor (or group of motors).
Use the column of Table 610.14(A) that applies to the longest time-rated motor.
If you have multiple cranes or hoists on a common conductor system:
Run the 610.14(E)(2) calculations for each crane or hoist.
Add them together.
Multiply this sum by the appropriate demand factor from Table 610.14(E).
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).
When 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:
Guard runway conductors, and locate bridge contact conductors, so people can't inadvertently touch energized parts.
Secure contact conductors at their ends. Mount them on approved insulators so the extreme limit of wire displacement brings the wire within at least 1.5 in. of the surface it's routed over.
Where main contact conductors are carried along runways, support them on insulating supports at intervals of 20 ft or less. The exception is where the track is used as a circuit conductor [requirement (F)].
On bridges, keep contact conductors at least 2.5 in. apart. Use insulating saddles each 50 ft for spans of 80 ft or more.
If you have any rigid conductors not contained within an approved enclosed assembly, put them on insulating supports. Space the supports at intervals that are, at most, 80 times the vertical dimension of the conductor. The maximum distance, regardless of conductor size, is 15 ft. You also must ensure the spacing results in a clear separation of conductors or adjacent collectors of at least 1 in.
You can use monorail, tram rail, or crane runway tracks as a conductor of current for one phase of a 3-phase system providing power to carrier, crane, or trolley — but only if the voltage is less than 300V, the other two conductors are insulated, the supply is an insulating transformer, and the rail is properly bonded.
Join all sections of contact conductors to form a continuous electrical connection.
Don't use contact conductors to supply other equipment.
Supply 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:
Be within view of the runway conductors and be readily operable from the floor level.
Open all ungrounded conductors simultaneously.
Be capable of being locked in the open position.
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].
What 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.
You can use the branch circuit OCPD to provide overload protection for a single motor. For multiple motors, you can use:
Overload relay elements.
Thermal sensors that are in contact with the motor windings.
Part 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 comments@mindconnection.com.
NEC 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.