Electrical service sizing
The minimum circuit ampacity (MCA) and maximum
overcurrent protection (MOP) ratings for each unit
can be found on the unit nameplate, from the unit
factory order form/submittal data, or by using the
•
MCA -
This rating is based on 125% of the
rated load amps for the largest motor plus
100% of the rated load amps for all other
loads included in the circuit, per NFPA 70
Article 440-34. The minimum recommended
disconnect switch is based on 115% of the
rated load amps included in the circuit per
NFPA 70.
•
MOP -
This rating is based on 225% of the
rated load amps for the largest load plus
100% of the rated load amps for all other
loads included in the circuit per NEC Article
440-22.
Supply power voltage limitations are shown in Table
36.
In order to determine the electrical service required
for the rooftop unit, use the appropriate calculations
listed below from U.L. 60335-2-40. Based on the
configuration of the unit, the calculations yield
different MCA and MOP.
Using the following load definitions and
calculations, determine the correct electrical sizing
for the unit. All concurrent load conditions must be
considered in the calculations, and the highest value
for any combination of loads must be used.
Load definitions:
•
LOAD1
is the current of the largest motor –
compressor or fan motor.
•
LOAD2
is the sum of the remaining motor
currents that may run concurrently with
LOAD1.
•
LOAD3
is the current of the electric heaters –
zero for cooling only units.
•
LOAD4
is the sum of any remaining currents
greater than or equal to 1.0 amp.
Use the following calculations to determine MCA
and MOP for units supplied with a single-point
power connection.
Note:
For electric heat applications, the
nameplate MCA/MOP value is the larger of
the cooling mode or the heating mode value
calculated.
MCA = (1.25 × Load1) + (1.25 × Load3) + Load2 +
Load4
Exception:
Load3 may be multiplied by 1 if the
electric heater is greater than 50kW
MOP = (2.25 × Load1) + Load2 + Load3 + Load4
Use the following calculations to determine MCA
and MOP for units supplied with a dual-point power
connection.
Note:
For electric heat applications, the
nameplate MCA 1/MOP 1 value is the larger of
the cooling mode or the heating mode value
calculated.
Electrical circuit 1: compressors and crankcase
heaters, condenser fan motors, electric heat (MCA
1 / MOP 1)
Electrical circuit 2: supply fan, return/exhaust fan,
energy recovery wheel, 120 V transformer for lights/
UV lights/convenience outlet, and 24 VAC control
transformers (MCA 2 / MOP 2)
Use the previous formulas to calculate MCA and
MOP for electrical circuits 1 and 2.
For single/dual point power connection, if the MOP
does not equal a standard current rating of an
overcurrent protective device, then the marked
maximum rating is to be the next lower standard
rating. However, if the device selected for MOP is
less than the MCA, then select the lowest standard
maximum fuse size greater than or equal to the
MCA.
Note:
All wiring must conform to the NEC and
all local codes that may be in addition to the
NEC.
Table 36: Power supply voltage limits
Power supply
Minimum voltage
Maximum voltage
208 to 230 V / 3 Ph / 60 Hz
187
253
460 V / 3 Ph / 60 Hz
414
506
575 V / 3 Ph / 60 Hz
518
632
Tempmaster
®
Omni™ Premier 25 Ton to 80 Ton Rooftop Units Installation and Maintenance Guide
62
Tempmaster