ELECTRICAL STUFF
FOR MECHANICAL
ENGINEERS
ELECTRICAL DATA FOR SYSTEM DESIGN
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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=16.5X1.25+1.3+1.3=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
VOLTAGE
VOLTAGE
120
115
110
208
200
NA
240
230
220
277
277
NA
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 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
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.
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.
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
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
KVA
KVAR

KW
Power Factor (PF) = Cos  = KW / KVA
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.