76
RT-SVX24Q-EN
Electrical Service Sizing
To correctly size electrical service wiring for a unit, find
the appropriate calculations listed below. Each type of
unit has its own set of calculations for MCA (Minimum
Circuit Ampacity), MOP (Maximum Overcurrent
Protection), and RDE (Recommended Dual Element
fuse size). Read the load definitions that follow and
then find the appropriate set of calculations based on
unit type.
N
No
otte
e:: Set 1 is for cooling only and cooling with gas
heat units, and set 2 is for cooling with electric
heat units.
Load Definitions: (To determine load values, see the
Electrical Service Sizing Data Tables on the following
page.)
LOAD1 = Current of the largest motor (compressor or
fan motor)
LOAD2 = Sum of the currents of all remaining motors
LOAD3 = Current of electric heaters
LOAD4 = Any other load rated at 1 AMP or more
Set 1: Cooling Only Rooftop Units and
Cooling with Gas Heat Rooftop Units
MCA = (1.25 x LOAD1) + LOAD2 + LOAD4
MOP = (2.25 x LOAD1) + LOAD2 + LOAD4
Select a fuse rating equal to the MOP value. If the MOP
value does not equal a standard fuse size as listed in
NEC 240-6, select the next lower standard fuse rating.
N
No
otte
e:: If selected MOP is less than the MCA, then select
the lowest standard maximum fuse size which is
equal to or larger than the MCA, provided the
selected fuse size does not exceed 800 amps.
RDE = (1.5 x LOAD1) + LOAD2 + LOAD4
Select a fuse rating equal to the RDE value. If the RDE
value does not equal a standard fuse size as listed in
NEC 240-6, select the next higher standard fuse rating.
N
No
otte
e:: If the selected RDE is greater than the selected
MOP value, then select the RDE value to equal
the MOP value.
Set 2: Rooftop units with Electric Heat
To arrive at the correct MCA, MOP, and RDE values for
these units, two sets of calculations must be
performed. First calculate the MCA, MOP, and RDE
values as if the unit was in cooling mode (use the
equations given in Set 1). Then calculate the MCA,
MOP, and RDE values as if the unit were in heating
mode as follows. (Keep in mind when determining
LOADS that the compressors don't run while the unit is
in heating mode).
MCA = 1.25 x (LOAD1 + LOAD2 + LOAD4) + LOAD3
The nameplate MCA value will be the larger of the
cooling mode MCA value or the heating mode MCA
value calculated above.
MOP = (2.25 x LOAD1) + LOAD2 + LOAD3 + LOAD4
The selection MOP value will be the larger of the
cooling mode MOP value or the heating mode MOP
value calculated above.
Select a fuse rating equal to the MOP value. If the MOP
value does not equal a standard fuse size as listed in
NEC 240-6, select the next lower standard fuse rating.
N
No
otte
e:: If selected MOP is less than the MCA, then select
the lowest standard maximum fuse size which is
equal to or larger than the MCA, provided the
selected fuse size does not exceed 800 amps.
RDE = (1.5 x LOAD1) + LOAD2 + LOAD3 + LOAD4
The selection RDE value will be the larger of the
cooling mode RDE value or the heating mode RDE
value calculated above.
Select a fuse rating equal to the RDE value. If the RDE
value does not equal a standard fuse size as listed in
NEC 240-6, select the next higher standard fuse rating.
N
No
otte
ess::
•
If the selected RDE is greater than the
selected MOP value, then select the RDE
value to equal the MOP value.
•
On 90 to 162 ton rooftops, the selected MOP
value is stamped in the MOP field on the
unit nameplate.
Service Sizing Data
Table 31.
Electrical service sizing data — air-cooled and evaporative condensing
Fixed Capacity Compressors
Nom
Tons
AC/EC
Compressor
Nominal Voltage
Size
No per
Unit
460 V
575 V
380 V
RLA (ea.)
LRA (ea.)
RLA (ea.)
LRA (ea.)
RLA (ea.)
LRA (ea.)
90/100
250
4
34.1
215
27.3
175
34
215
105/118
250
2
34.1
215
27.3
175
34
215
315
2
44.7
260
35.8
210
44.6
260
120/128
315
4
44.7
260
35.8
210
44.6
260
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