# ELECTRICAL STUFF FOR MECHANICAL ENGINEERS

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ELECTRICAL STUFF FOR MECHANICAL ENGINEERS

ELECTRICAL DATA FOR SYSTEM DESIGN
FLA vs. MCA VOLTAGE DESIGNATIONS VOLTAGE UTILIZATION VOLTAGE AND PHASE SELECTION FIRE ALARM & ELECTRICAL SYSTEM INTERLOCKS MOTOR STARTERS

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.

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. MCA=16.5X =23.3A 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.

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.

VOLTAGE TABLES SYSTEM EQPT OLD VOLTAGE 120 115 110 208 200 NA 240 230
220 277 480 460 440 600 575 550

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.

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.

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.

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.

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 troffer—expensive and requires more coordination.

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.

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.

Exhibit 110.9

Exhibit 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

Exhibit The two distinct indoor installation spaces required by (A) and (F): the working space and the dedicated electrical space.

Exhibit The working space in front of a panelboard required by (A). This illustration supplements the dedicated electrical space shown in Exhibit

Exhibit 110.21 The dedicated electrical space above and below a panelboard required by 110.26(F)(1).

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. MCC’s and VFD’s 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 40°C (104°F). Ventilation with office air is usually adequate.

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.

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.

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.

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.

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.

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.

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 Motors—Similar criteria as electrical enclosures

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   = constant F       = supply frequency (in cycles/sec) P       = number of motor winding poles

Srpm = 120 x F P Srpm = 120 x 60 4 Srpm = 7200 4 Srpm = 1800 rpm
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             P Srpm = 120 x 60             4 Srpm = 7200              4 Srpm = 1800 rpm

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.

POWER FACTOR

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.

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