MIL-HDBK-1038
5.5.5.18 Raceways. For both rigid metal conduit and liquid-tight flexible metal
conduits, fill is required to be limited to 40 percent of the cross-section of the
conduit. The cross-sectional areas of conductors having various types of
insulation are listed in Chapter 9 of the National Electrical Code. For a cable,
either shielded and unshielded, or a conductor with stranding not included in NEC
Chapter 9, run in conduit, its actual cross-sectional area is required to be used
in the calculation of percentage of fill. The cross-sectional area of equipment
grounding conductors is required to be included in the calculation.
5.5.5.19 Electrifications. Bridge crane runway and bridge-to-trolley
electrification system conductors are required to be selected so as to be of the
longest length without splices. Conductors are required to be fabricated from
copper. Conductors are required to be sized for simultaneous operation of bridge,
trolley, and hoist drives. The crane is required to be grounded through the
runway electrification system.
A festooned type electrification system is required to use flat cables
suspended from carriers riding on an I-beam; for conductors larger than #2 AWG,
round cable is to be used. Conductors are required to be extra-flexible type with
ethylene propylene rubber insulation in a neoprene jacketed flat cable. Type G or
type W portable power cables are to be used for round cable applications. It is
required that the cable loops do not extend low enough to come into contact with
any obstructions. A voltage drop of less than 5 percent from the supply to the
drive motor and of less than 3 percent in either the feeders and mains or in the
branch circuit will provide reasonably efficient operation. Effectively
minimizing the contribution of the voltage drop along the length of the
electrification conductor to the overall voltage drop being considered is required
to be a criterion in electrification conductor selection.
The rings in collector ring assemblies on portal and floating cranes are
required to be fabricated from a copper alloy; silver plated rings are required to
be utilized in communication circuits. Collector ring assemblies are required to
be selected based upon their non-rotating, continuous duty current rating. Spare
rings, two of each size and construction used, are required to be provided on new
cranes; however, there are to be three spare collector ring assemblies of the
largest size used.
A thermostatically controlled anti-condensation heater is required to be installed
in the collector ring assembly enclosure. It is required to be selected to
provide 7 watts per cubic foot. All heaters are required to be fed from a
separate panel dedicated to equipment heater and battery charger circuits.
Redundant, full capacity, sintered copper-alloy/graphite collectors are
required to be used with rigid conductor systems on bridge cranes and collector
ring assemblies on portal and floating cranes.
5.5.5.20 Grounding/Bonding/Lightning Protection. An equipment grounding conductor
in the form of a wire, sized in accordance with NEC Table 250-95, is required to
be routed with all ungrounded conductors. Only one equipment grounding conductor
needs to be run in each conduit. It would be the largest size required for any
circuit routed in that conduit. Regardless of the type of equipment grounding
conductor used elsewhere on a crane, an equipment grounding conductor in the form
of a wire is required to be included in all bridge crane festooned conductor and
cable reel
201
Previous Page
