74 RT-SVX36G-EN
Installation
Power Wire Sizing and Protection Device Equations
To correctly size the main power wiring for the unit, use the appropriate calculation(s) listed below.
Read the load definitions that follow and use Calculation #1 for determining the MCA (Minimum
Circuit Ampacity), MOP (Maximum Over current Protection), and RDE (Recommended Dual
Element fuse size) for SAH_ (Cooling Only) units, SXH_ (Extended Casing) units, SLH_ and SSH_
(Cooling with Wet Heat) units, and SFH_ (Cooling with Gas Heat) units Use Calculation #2 for SEH_
(Cooling with electric Heat) units.
Load Definitions:
LOAD 1 = CURRENT OF THE LARGEST MOTOR (COMPRESSOR OR FAN MOTOR)
LOAD 2 = SUM OF THE CURRENTS OF ALL REMAINING MOTORS
LOAD 3 = CURRENT OF ELECTRIC HEATERS
LOAD 4 = ANY OTHER LOAD RATED AT 1 AMP OR MORE
CONTROL POWER TRANSFORMER FOR ALL MODES
20 - 40 Ton Units, add 3 FL Amps
50 - 75 Ton Units, add 6 FL Amps
90 - 130 Ton Units, add 8 FL Amps
CRANKCASE HEATERS FOR HEATING MODE 460/575V ONLY
20 - 30 Ton Units, Add 1 Amp
40 - 60 Ton Units, Add 2 Amps
70 - 75 Ton Units, Add 3 Amps
90 - 130 Ton Units, Add 4 Amps
• Calculation #1 - SAH_, SXH_, SLH_, SSH_, and SFH_ Units.
MCA = (1.25 x LOAD 1) + LOAD 2 + LOAD 4 MOP = (2.25 x LOAD 1) + LOAD 2 + LOAD 4
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 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.
Note: If the selected RDE is greater than the selected MOP value, then select the RDE value to equal
the MOP value.
• Calculation #2 - Rooftop units with Electric Heat (SEH_ 20 - 130 Tons)
a. Single Source Power (380V, 415V, 460V, and 575V)
Table 27. Voltage utilization range
Unit Voltage
Voltage
Utilization Range
200/60/3 180-220
230/60/3 207-253
380/50/3 342-418
415/50/3 373-457
460/60/3 414-506
575/60/3 517-633
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