ELECTRICAL STUFF FOR MECHANICAL ENGINEERS

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ELECTRICAL STUFFFOR MECHANICAL ENGINEERS
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ELECTRICAL DATA FOR SYSTEM DESIGN

• FLA vs. MCA
• VOLTAGE DESIGNATIONS
• VOLTAGE UTILIZATION
• VOLTAGE AND PHASE SELECTION
• FIRE ALARM ELECTRICAL SYSTEM INTERLOCKS
• MOTOR STARTERS

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FLA vs. MCA

• FLA (full load amperes)is the current the devise
  draws when operating under full rated load. This
  is a nameplate rating of a device such as a
  motor.
• MCA (minimum circuit amperes) is the minimum
  allowable current rating of the circuit serving
  the equipment. This equipment usually has more
  than one device using electrical energy such as a
  heat pump.
• The MCA is calculated by multiplying the FLA of
  the largest device by 125 and adding the FLA of
  the other devices.

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EXAMPLE

• An example would be a heat pump or condenser unit
  with a compressor with a FLA of 16.5A and two
  fans at 1.3A each.
• MCA16.5X1.251.31.323.3A
• If available, provide your EE with cut sheets of
  the equipment that lists all of the components
  and the FLA of each device. It will also
  indicate the MCA and which items are 1phase and 3
  phase.

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VOLTAGE DESIGNATIONS

• There are basically two voltage designations you
  will encounter the system voltage and the
  equipment voltage rating. The difference is the
  allowance for voltage drop between the electrical
  service delivery or transformer secondary and the
  point of use. But, just to keep everybody
  confused, we still encounter the old obsolete
  voltage designations. The following is a table
  listing the most common voltage designations for
  motor driven equipment. THIS DOES NOT APPLY TO
  RESISTANCE HEATING ELEMENTS. Resistance heating
  elements MUST be rated at system voltage.

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VOLTAGE TABLES
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VOLTAGE UTILIZATION

• Can a 240 v heater be operated at 208 v? Yes
  with a reduction of heat output. A 10 KW, 240
  volt heater operated at 208 volts will only
  provide 8.67 KW worth of heat. Can a 240 v
  heater be operated at 277 v? NO!
• Can a 240 volt motor be operated at 208 volts?
  No! Will it run? Maybe, but with a shorten life
  expectance and in violation of the NEC.

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VOLTAGE AND PHASE SELECTION

• Residential Very Small Commercial
• 120/240 volts, 1 phase
• Small Commercial
• 208Y/120 volts, 3 phase
• Large Commercial Industrial
• 480Y/277 volts, 3 phase
• Very large machinery may require higher voltages.
• For mechanical equipment rated at ½ HP or 0.5 KW
  it is generally better to serve these devices at
  the higher voltage and 3 phase rather than 1
  phase.
• Here again, review this with your EE.

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FIRE ALARM ELECTRICAL SYSTEM INTERLOCKS

• Depending on the CFM rating of an AHU, Supply Fan
  (2000 CFM), Return Fan or Exhaust Fan (15,000
  CFM), these devices require interlocking with the
  Fire Alarm System to shut down the fans in the
  event the FAS is activated. These fans may also
  require smoke detectors installed within the the
  duct work to both shut down the fan and activate
  the FAS.
• Kitchen hoods with fire suppression systems will
  require interlocks to de-energize the energy
  sources of all equipment under the hood, both gas
  and electric.

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MOTOR STARTERS

• Coordinate with your EE as to who is providing
  motor starters. For motors, 5 to 50 HP and for
  most industrial projects the EE may want to
  provide the motor starters and maybe the motors.
• For Chillers with reduced inrush starters, it is
  generally a good idea to have the chiller MFG
  provide the starter.
• For motors over 50 HP and for special equipment,
  review with your EE. The Utility may require
  reduced inrush starters.
• If starters are provided under HVAC
  specifications, let your EE review the specs.

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LIGHTING FIXTURES

• Vented vs Non-Vented
• Vented fixtures release a large amount of heat
  into the ceiling cavity and less into the room.
  This may be more efficient if the ceiling cavity
  is used as a return air plenum.
• If a ducted return is used non-vented fixtures
  would not be a good choice.
• Light-Diffuser coordination, who has the R/W?
• The Architect!
• Air handling trofferexpensive and requires more
  coordination.

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LIGHTING FIXTURES (cont)

• Interlock With Exhaust Fan
• In toilets, it is a good idea to interlock the
  light switch with the exhaust fan. Relays are
  available with a time delay that will allow the
  fan to run for a designated period after the
  lights are extinguished.
• Light fixture-duct work coordination
• To install a lay-in troffer takes a clearance of
  approx. 1 foot in the ceiling cavity. For large
  ducts in tight ceiling cavities, coordination is
  a must.

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ELECTRICAL EQUIPMENT

• Working Clearances
• Minimum Width 30 or the width of the
  equipment which ever is greater.
• Minimum Depth 240 or 208 v 3 ft.
• 480 v 3 1/2 to 4 ft.
• Dedicated Space
• No ducts, pipes, etc.to structure above or a
  maximum of 6 ft.

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Exhibit 110.9
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Exhibit 110.12 The 30 in. wide front working
space, which is not required to be directly
centered on the electrical equipment if space is
sufficient for safe operation and maintenance of
such equipment
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Exhibit 110.19 The two distinct indoor
installation spaces required by 110.26(A) and
110.26(F) the working space and the dedicated
electrical space.
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Exhibit 110.20 The working space in front of a
panelboard required by 110.26(A). This
illustration supplements the dedicated electrical
space shown in Exhibit 110.19.
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Exhibit 110.21 The dedicated electrical space
above and below a panelboard required by
110.26(F)(1).
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ELECTRICAL EQUIPMENT (cont)

• Heat Rejection
• With the exception of transformers and motor
  controls, the heat rejection from electrical
  equipment is minor. For transformers, the KVA
  rating X Z will give an approx. KW value. Z
  approx 5. MCCs and VFDs are a bit more
  complex due to the varying size of motor starters
  and other factors. Ask your EE.
• Environment
• Most electrical equipment and wiring is rated to
  operate in an environment of of 40C (104F).
  Ventilation with office air is usually adequate.

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NEMA ENCLOSURE DESIGNATION

• NEMA 1 Enclosures are intended for indoor use
  primarily to provide protection against contact
  with the enclosed equipment or locations where
  unusual service conditions do not exist.
• NEMA 2 Enclosures are intended for indoor use
  primarily to provide degree of protection against
  limited amounts of falling water and dirt.
• NEMA 3 Enclosures are intended for outdoor use
  primarily to provide degree of protection against
  windblown dust, rain and sleet. Undamaged by the
  formation of ice on the enclosure.

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NEMA ENCLOSURE DESIGNATION (cont)

• NEMA 3R Enclosures are intended for outdoor use
  primarily to provide degree of protection against
  falling rain and sleet. Undamaged by the
  formation of ice on the enclosure.
• NEMA 4 Enclosures are intended for indoor or
  outdoor use primarily to provide degree of
  protection against windblown dust and rain,
  splashed water and hose directed water.
  Undamaged by the formation of ice on the
  enclosure.
• NEMA 4X Same as NEMA 4 with corrosion
  resistance. Usually stainless steel.

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NEMA ENCLOSURE DESIGNATION (cont)

• NEMA 6 Enclosures are intended for indoor or
  outdoor use where occasional submersion at
  limited depth may occur. Undamaged by the
  formation of ice on the enclosure.
• NEMA 12 Enclosures are intended for indoor use
  primarily to provide protection against dust,
  falling dirt and dripping non-corrosive liquids.
• NEMA 13 Enclosures are intended for indoor use
  primarily to provide protection against dust and
  spraying of water, oil and non-corrosive
  coolants.
• None of these enclosures are explosion proof.

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EXPLOSION PROOF ENCLOSURES

• Explosion proof enclosures are designated by
  Class, Division and Group environment.
• Class I Flammable liquid or gases.
• Class II Combustible dust.
• Class III Ignitable fibers or flying.
• Division I Flammable, combustible or ignitable
  products are normally present.
• Division II Flammable, combustible or ignitable
  products are normally contained or are only
  present due to abnormal operations.
• Group A, B, C, D, E, F G based upon the degree
  of flammability, combustibility or ignitability
  of the product.

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NEMA MOTOR ENCLOSURES

• OPEN---Motor housing is open with slots. For
  clean dry areas.
• ODP (Open Drip proof) -- Ventilation openings in
  shield and/or frame prevents drops of liquid from
  falling into motor within up to 15 degree angle
  from vertical. Designed for reasonably dry,
  clean, and well ventilated (usually indoors)
  areas. Outdoors installation require the motor to
  be protected with a cover that does not restrict
  the flow of air to the motor.

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• TENV (Totally Enclosed Non-Ventilated)--No
  ventilation openings, enclosed to prevent free
  exchange of air (not airtight). No external
  cooling fan, relies on convection cooling.
  Suitable where the motor is exposed to dirt or
  dampness. Not suited in very moist humid or
  hazardous (explosive) air.
• TEFC (Totally Enclosed Fan Cooled)--Same as TENV
  with an external fan as an integral part of the
  motor. The fan provides cooling by blowing air on
  the outside of the motor.
• TEAO (Totally Enclosed Air Over)--Dust-tight fan
  and blower motors for shaft mounted fans or belt
  driven fans. The motors mounted within the
  airflow of the fan.

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• Totally Enclosed, Hostile and Severe
  Environment--Designed for use in extreme
  conditions - moist and/or chemical environments.
  Not for hazardous locations.
• Totally Enclosed Blower Cooled--Same as TEFC with
  external fan on a power supply independent of the
  inverter output. Full cooling even at lower motor
  speeds.
• Explosion-Proof MotorsSimilar criteria as
  electrical enclosures

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MOTOR SPEED

• Calculating Motor Speed
• To Calculate the speed of a induction motor,
  apply this formula
• Srpm 120 x F            P
• Srpm synchronous revolutions per minute.120  
  constantF       supply frequency (in
  cycles/sec)P       number of motor winding
  poles

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MOTOR SPEED (cont)

• Example What is the synchronous of a motor
  having 4 poles connected to a 60 hz power supply?
• Srpm 120 x F            PSrpm 120 x
  60            4Srpm 7200             4Srpm
  1800 rpm

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MOTOR SPEED (cont)

• A squirrel cage induction motor is a constant
  speed device. It cannot operate for any length of
  time at speeds below those shown on the nameplate
  without danger of burning out.
• Variable Frequency Drives (VFD)
• Specify a motor rated for a VFD.

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POWER FACTOR
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MECHANICAL EQUIPMENT DATA

• Provide the EE with your best guess as to your
  equipment loads ASAP. This lets him establish
  his equipment size and space requirements. Data
  can be revised as you proceed with your design.
• Likewise the EE should be able to provide you
  with his expected lighting loads in watts/SF by
  areas very early.