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RT-SVX34V-EN
Through-the-Base Electrical
(Optional Accessory)
Liquid-tight conduit couplings are secured to the base of
the unit for both power and control wiring. Liquid-tight
conduit must be field installed between the couplings and
the unit control box to prevent water leaks into the building.
Note: If the unit is set on the roof curb and temporary
auxiliary heat is provided in the building, it is
recommended that the electrical and control wiring
conduit opening in the control box be temporarily
sealed to provide a vapor barrier.
Electrical Wire Sizing and
Protection Device Equations
To correctly size electrical service wiring for your 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 your unit type.
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.)
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.
Note: If selected MOP is less than the MCA, then reselect
the lowest standard maximum fuse size which is
equal to or larger than the MCA, provided the
reselected 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.
Note: If the selected RDE is greater than the selected MOP
value, then reselect the RDE value to equal the
MOP value.
Keep in mind when determining LOADS that crankcase
heaters are disabled in the cooling mode.
DSS = 1.15 x (LOAD1 + LOAD2 + LOAD4)
Select a disconnect switch size equal to or larger than the
DSS value calculated.
Set 2. Rooftop Units with Electric Heat
To arrive at the correct MCA, MOP, and RDE values for
these units, you must perform two sets of calculations. 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 was
in the heating mode as follows.
Note: Keep in mind when determining LOADS that the
compressors and condenser fans don’t run while the
unit is in the heating mode and crankcase heaters
are disabled in the cooling mode.
• For units using heaters less than 50 kw: MCA = 1.25 x
(LOAD1 + LOAD2 + LOAD4) + (1.25 x LOAD3)
• For units using heaters equal to or greater than 50 kw:
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.
Note: If selected MOP is less than the MCA, then reselect
the lowest standard maximum fuse size which is
equal to or larger than the MCA, provided the
reselected 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.
Note: If the selected RDE is greater than the selected MOP
value, then reselect the RDE value to equal the
MOP value.
DSS = 1.15 x (LOAD1 + LOAD2 + LOAD3 + LOAD4)
Note: Keep in mind when determining LOADS that the
compressors and condenser fans don’t run while the
unit is in the heating mode.
The selection DSS value will be the larger of the cooling
mode DSS or the heating mode DSS calculated above.
Select a disconnect switch size equal to or larger than the
DSS value calculated.
Installation Electrical
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